Nipun's Writings

Installing Docker CLI on Ubuntu (Step-by-Step Guide)

2026-04-21T11:45:00.000-07:00

If you're working with containers, Docker is one of the most essential tools you’ll need. In this guide, I’ll walk you through installing the Docker CLI (along with the Docker Engine) on an Ubuntu system in a simple and practical way.

Prerequisites

Before you start, make sure:

You’re using an Ubuntu system (18.04 or later recommended)
You have a user account with sudo privileges
Your system is connected to the internet

Steps

Step 1: Update Package Index

Start by updating your system’s package list:

sudo apt update

Install required dependencies:

sudo apt install -y apt-transport-https ca-certificates curl software-properties-common

Step 2: Add Docker’s Official GPG Key

This ensures packages are verified and trusted:

curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg

Step 3: Add Docker Repository

echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

Step 4: Install Docker Engine & CLI

Update package list again:

sudo apt update

Now install Docker:

sudo apt install -y docker-ce docker-ce-cli containerd.io

Step 5: Start and Enable Docker

Start Docker service:

sudo systemctl start docker

Enable Docker to start on boot:

sudo systemctl enable docker

Step 6: Run Docker Without sudo (Optional but Recommended)

By default, Docker requires sudo. To avoid that, add your user to the Docker group:

sudo usermod -aG docker $USER

Apply the changes immediately:

newgrp docker

Step 7: Verify Installation

Check Docker system info:

docker info

List running containers:

docker ps

Quick Note

docker-ce → Docker Engine
docker-ce-cli → Docker CLI
containerd.io → Container runtime

Conclusion

You now have Docker CLI installed and ready to use on Ubuntu. This setup is ideal for development, testing, and learning containerization.

From here, you can start running containers, pulling images, or even building your own.

2017-12-09T21:40:00.000-08:00

An inspirational quote by Henry Ford.

Singleton Design Pattern In C++

2017-02-18T20:31:00.001-08:00

Singleton Design Pattern is used to keep one instance of a class in the system. To implement this pattern in C++, language features like access modifiers (public, protected, private), class functions and variables by using static, pointer and dynamic memory is used.

To keep single instance of particular class, developers must be prevented from creating new objects. In order to achieve that constructors of that particular class must be private. Doing so, new object of that class cannot be created. Object creating is facilitated well control manner to keep one instance of that class in the system by a public class method. The created single instance also is kept in the class variable. So when program request an instance, if that variable has the instance that instance is returned. Otherwise new instance is created and returned. All subsequent calls to object creation method returns the single instance that is kept in the class variable.

Below implementation use static ThreadPool *getInstance() method to create or get single instance.

Implementation

#include <iostream>

using std::cout;

using std::endl;

class ThreadPool {

private:

static ThreadPool *tPSingleton;

ThreadPool ();

public:

static ThreadPool *getInstance();

bool startThreadPool();

bool stopThreadPool();

~ThreadPool ();

};

ThreadPool* ThreadPool::tPSingleton = NULL;

ThreadPool::ThreadPool (){

cout<<"Thread Pool is created"<<endl;

}

ThreadPool::~ThreadPool (){

cout<<"Thread Pool is destroyed"<<endl;

}

ThreadPool *ThreadPool::getInstance(){

if(ThreadPool::tPSingleton == NULL){

ThreadPool::tPSingleton = new ThreadPool;

}

return ThreadPool::tPSingleton;

}

bool ThreadPool::startThreadPool(){

cout<<"Thread pool is being started"<<endl;

return true;

}

bool ThreadPool::stopThreadPool(){

cout<<"Thread pool is being stoped"<<endl;

return true;

}

int main() {

ThreadPool *threadPool1;

ThreadPool *threadPool2;

threadPool1 = new ThreadPool;

Produce below error with Error No 1

threadPool1 = ThreadPool::getInstance();

threadPool1->startThreadPool();

threadPool1->stopThreadPool();

threadPool2 = ThreadPool::getInstance();

threadPool2->startThreadPool();

threadPool2->stopThreadPool();

return 0;

}

Error No 1

singleton.cpp: In function ‘int main()’:

singleton.cpp:23:1: error: ‘ThreadPool::ThreadPool()’ is private

ThreadPool::ThreadPool (){

singleton.cpp:52:22: error: within this context

threadPool1 = new ThreadPool;

Output

Thread Pool is created

Thread pool is being started

Thread pool is being stoped

Thread pool is being started

Thread pool is being stoped

Android App To Understand Sticky Broadcasts

2016-10-30T02:58:00.003-07:00

This blog post is about an application that can be used to understand what is the difference between sticky broadcasts and ordinary broadcasts of Android system. App name is StickyBCast. App has below package name.

com.blogspot.nipunswritings.stickybcast.

UI of the app is shown below.

Above UI shows what sticky broadcast and ordinary broadcast sent. This app send broadcast at two events. One is when onStop() is called and other one is when user touch on send button. Content that is sent with sticky broadcast is displayed below "Sticky Broadcast Content" and Content that is sent with ordinary broadcast is displayed below "Ordinary Broadcast Content". The content that is sent with broadcast are shown in TextViews.

To show content that is sent with sticky broadcast TextView with Id "sticky_content_tv" is used. TextView with id ordinary_content_tv is used to show the content that is sent with normal broadcast.

Class (MainActivity) below is used to show difference between two types of broadcasts.

public class MainActivity extends AppCompatActivity {

    private TextView oContentTv;
    private TextView sContentTv;

    private String oContent;
    private String sContent;

    private IntentFilter oContentFilter;
    private IntentFilter sContentFilter;

    private BroadcastReceiver oReceiver = new BroadcastReceiver() {
        @Override        public void onReceive(Context context, Intent intent) {
            oContent = intent.getStringExtra(Constants.ORDINARY_EXTRA);
            setTextViews();
        }
    };

    private BroadcastReceiver sReceiver = new BroadcastReceiver() {
        @Override        public void onReceive(Context context, Intent intent) {
            sContent = intent.getStringExtra(Constants.STICKY_EXTRA);
            setTextViews();
        }
    };


    @Override    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        oContentTv = (TextView) findViewById(R.id.ordinary_content_tv);
        sContentTv = (TextView) findViewById(R.id.sticky_content_tv);

        oContentFilter = new IntentFilter(Constants.ORDINARY_ACTION);
        sContentFilter = new IntentFilter(Constants.STICKY_ACTION);
    }

    @Override    protected void onStart() {
        super.onStart();

        oContent = "";
        sContent = "";

        Intent oIntent = registerReceiver(oReceiver, oContentFilter);
        Intent sIntent = registerReceiver(sReceiver, sContentFilter);

        if (oIntent != null) {
            oContent = oIntent.getStringExtra(Constants.ORDINARY_EXTRA);
        }

        if (sIntent != null) {
            sContent = sIntent.getStringExtra(Constants.STICKY_EXTRA);
        }

        setTextViews();
    }


    @Override    protected void onStop() {
        super.onStop();
        sendBroadcasts("Ordinay Value 2", "Sticky Value 2");
    }

    private void setTextViews() {
        oContentTv.setText(this.oContent);
        sContentTv.setText(this.sContent);
    }

    public void sendBroadcasts(String oExtrVal, String sExtrVal) {
        Intent oIntent = new Intent(Constants.ORDINARY_ACTION);
        Intent sIntent = new Intent(Constants.STICKY_ACTION);

        oIntent.putExtra(Constants.ORDINARY_EXTRA, oExtrVal);
        sIntent.putExtra(Constants.STICKY_EXTRA, sExtrVal);


        sendBroadcast(oIntent);
        sendStickyBroadcast(sIntent);
    }

    public void sendBCastWithClick(View view) {
        sendBroadcasts("Ordinay Value 1", "Sticky Value 1");
    }
}

Some of the constants that are needed for this application has been defined in a class called Constants. This is the definition of that class.

public class Constants {
    public static String STICKY_ACTION = "com.blogspot.nipunswritings.s_action";
    public static String ORDINARY_ACTION = "com.blogspot.nipunswritings.o_action";

    public static String STICKY_EXTRA = "com.blogspot.nipunswritings.s_extra";
    public static String ORDINARY_EXTRA = "com.blogspot.nipunswritings.o_extra";
}

In MainActivity, public void sendBroadcasts(String oExtrVal, String sExtrVal) method is called at two event. One event is in the onStop() method of the activity and other one is in the button click listener of send button with the method public void sendBCastWithClick(View view). The arguments that are passed to method sendBroadcasts(String, String) are different in two events.

Code segment that is useful to understand difference between sticky broadcasts and normal broadcasts is this.

        Intent oIntent = registerReceiver(oReceiver, oContentFilter);
        Intent sIntent = registerReceiver(sReceiver, sContentFilter);

here when registration is done if sticky broadcast has been sent earlier that is returned by the function registerReceiver(Intent) for sticky broadcasts. But for normal broadcasts it returns null. If you run the app and click send button both text boxes will display text values that have been sent by broadcasts. In that case there is no difference between those two types of broadcast.

Then do something that cause to call onStop() and come back to app. The TextView for sticky broadcast text has been set with the "Sticky Value 2" but other one is empty. Because system doesn't provide normal broadcast intents that was sent before registration. So for below code oIntent is null.

        if (oIntent != null) {
            oContent = oIntent.getStringExtra(Constants.ORDINARY_EXTRA);
        }

        if (sIntent != null) {
            sContent = sIntent.getStringExtra(Constants.STICKY_EXTRA);
        }

To send broadcasts below function is used. Method sendStickyBroadcast(sIntent) is deprecated even though that is used for this example.

    public void sendBroadcasts(String oExtrVal, String sExtrVal) {
        Intent oIntent = new Intent(Constants.ORDINARY_ACTION);
        Intent sIntent = new Intent(Constants.STICKY_ACTION);

        oIntent.putExtra(Constants.ORDINARY_EXTRA, oExtrVal);
        sIntent.putExtra(Constants.STICKY_EXTRA, sExtrVal);


        sendBroadcast(oIntent);
        sendStickyBroadcast(sIntent);
    }

Reference:

https://developer.android.com/reference/android/content/Context.html#sendStickyBroadcast(android.content.Intent)

sendBroadcast Android Example

2016-08-14T06:07:00.001-07:00

This is an example of using method sendBroadcast() to send broadcasts in Android. App has one BroadcastReceiver. It display broadcast action if there is one else display nothing. It also display the addition of two numbers that were sent to it.

BroadcastReceiver has been implemented with the class NumberReceiver. So it must be registered by using the manifest file.

<receiver

    android:name=".NumberReceiver"

    android:enabled="true"

    android:exported="true">

    <intent-filter>

       <action android:name="com.blogspot.nipunswritings.demosendbcast.add" />

    </intent-filter>

</receiver>

It can receive intents with the below action

com.blogspot.nipunswritings.demosendbcast.add

Project has class Constants that has been defined key for intent actions and extras.

public class Constants {

    public static String ACTION_ADD = "com.blogspot.nipunswritings.demosendbcast.add";

    public static String EXTRA_X = "com.blogspot.nipunswritings.demosendbcast.x";

    public static String EXTRA_Y = "com.blogspot.nipunswritings.demosendbcast.y";

In the activity that invoke sendBroadcast() has Two EditTexts and a Button. When user click the button numbers in two EditTexts are send to BroadcastReceiver after converting them to integers. This is the button onClick method.

public void sendToAddition (View view) {

    boolean withAction = false;

    int x = Integer.parseInt(xEt.getText().toString());
    int y = Integer.parseInt(yEt.getText().toString());

    Intent intent;

    if (withAction) {
        intent = new Intent(Constants.ACTION_ADD);
    } else {
        intent = new Intent(MainActivity.this, NumberReceiver.class);
    }
    intent.putExtra(Constants.EXTRA_X, x);
    intent.putExtra(Constants.EXTRA_Y, y);

    sendBroadcast(intent);
}

Important code snippet of above method is this.

    Intent intent;

    if (withAction) {
        intent = new Intent(Constants.ACTION_ADD);
    } else {
        intent = new Intent(MainActivity.this, NumberReceiver.class);
    }
    intent.putExtra(Constants.EXTRA_X, x);
    intent.putExtra(Constants.EXTRA_Y, y);

    sendBroadcast(intent);

If Boolean variable withAction is true Intent instance is created with the ACTION_ADD in constants file. If it is not true since both Receiver and Activity in the same class Intent instance is created with the class. Then integer values which are got by casting String that are provided by EditTexts are put into Intent instance. Finally sendBroadcast() is called.

Method onReceive(Context, Intent) of NumberReceiver. has been written below.

@Overridepublic void onReceive(Context context, Intent intent) {
    int x = intent.getIntExtra(Constants.EXTRA_X, 0);
    int y = intent.getIntExtra(Constants.EXTRA_Y, 0);

    String action = intent.getAction();
    if (action != null) {
        Toast.makeText(context, "Action = "+action, Toast.LENGTH_LONG).
                show();
    } else {
        Toast.makeText(context, "Action isn't available", Toast.LENGTH_LONG).
                show();
    }


    Toast.makeText(context, "x + y = "+Calculator.performAddition(x,y), Toast.LENGTH_LONG).
            show();
}

It get extras and actions then display them with a Toast.

Calculations is done by using the Calculator class. Calculator class implementation is this.

public class Calculator {
    public static int performAddition (int x, int y) {
        return x+y;
    }
}

Android LocalBroadcastManager Example

2016-07-17T08:04:00.002-07:00

Using broadcasts in Android applications sometimes introduce problems as broadcasts can be received by the other applications too. To prevent these problems Android system has LocalBroadcastManager that can be used to implement secure communication mechanism within app components.

This example shows how to use LocalBroadcastManager to send and receive broadcasts. Though this application has only one activity it can LocalBroadcastManager is capable doing more than that. For example It can be used to send broadcasts from service to another activity, from One BroadcastReceiver to activity like use cases.

The app which is developed for demonstration has one EditText and Button. When user type something on EditText and click the Button typed content send to BroadcastReceiver that has been registered by using LocalBroadcastManager. After it received that Text it shows as a Toast.

Above image shows what this application does. Implementation of this application is on MainActivity class which is shown below.

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.support.v4.content.LocalBroadcastManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    private final String TEXT_SENT_ACTION =

       "com.blogspot.nipunswritings.bcsender.action.TEXT_SENT";
    private final String TEXT_EXTRA =

       "com.blogspot.nipunswritings.bcsender.extra.TEXT";

    private BroadcastReceiver textSentReceier = new BroadcastReceiver() {
        @Override        public void onReceive(Context context, Intent intent) {
            String textExtra = intent.getStringExtra(TEXT_EXTRA);
            Toast.makeText(context, textExtra, Toast.LENGTH_LONG).show();
        }
    };

    @Override    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override    protected void onResume() {
        super.onResume();

        IntentFilter textSentIntentFilter = new IntentFilter(TEXT_SENT_ACTION);
        LocalBroadcastManager
                .getInstance(MainActivity.this)
                .registerReceiver(textSentReceier, textSentIntentFilter);
    }

    @Override    protected void onPause() {
        LocalBroadcastManager.getInstance(MainActivity.this)

            .unregisterReceiver(textSentReceier);

        super.onPause();

    }

    public void sendLocalTestBc(View view) {
        EditText sendingTextEt = (EditText) findViewById(R.id.activity_main_sent_text_et);
        String sendingText = sendingTextEt.getText().toString();

        Intent textSentIntent = new Intent(TEXT_SENT_ACTION);
        textSentIntent.putExtra(TEXT_EXTRA, sendingText);
        LocalBroadcastManager.getInstance(MainActivity.this).sendBroadcast(textSentIntent);
    }
}

As Broadcasts and associated data are needed to be identified at the beginning of above class unique identifiers for above class has been defined like this.

    private final String TEXT_SENT_ACTION =

       "com.blogspot.nipunswritings.bcsender.action.TEXT_SENT";
    private final String TEXT_EXTRA =

       "com.blogspot.nipunswritings.bcsender.extra.TEXT";

Receive Local Broadcasts

To receive broadcasts this activity must have a BroadcastReceiver implementation, that is shown below.

    private BroadcastReceiver textSentReceier = new BroadcastReceiver() {
        @Override        public void onReceive(Context context, Intent intent) {
            String textExtra = intent.getStringExtra(TEXT_EXTRA);
            Toast.makeText(context, textExtra, Toast.LENGTH_LONG).show();
        }
    };

Register Receiver with LocalBroadcastManager

In the onResume() IntentFilter is created to register receiver for designated action. IntentFilter constructor take that action as the parameter so it can receive broadcasts with this action.

Next line is about registering receiver for the IntentFilter by using LocalBroadcastManager and registerReceiver(). After that receiver has ability to get broadcasts with TEXT_SENT_ACTION.

    @Override    protected void onResume() {
        super.onResume();

        IntentFilter textSentIntentFilter = new IntentFilter(TEXT_SENT_ACTION);
        LocalBroadcastManager
                .getInstance(MainActivity.this)
                .registerReceiver(textSentReceier, textSentIntentFilter);
    }

Unregister Receiver with LocalBroadcastManager

    @Override    protected void onPause() {
        LocalBroadcastManager.getInstance(MainActivity.this)

            .unregisterReceiver(textSentReceier);

        super.onPause();

    }

All registered BroadcastReceivers must be unregistered so it has done in onPause() by using unregisterReceiver() and LocalBroadcastManager.

Sending LocalBroadCasts

    public void sendLocalTestBc(View view) {
        EditText sendingTextEt = (EditText) findViewById(R.id.activity_main_sent_text_et);
        String sendingText = sendingTextEt.getText().toString();

        Intent textSentIntent = new Intent(TEXT_SENT_ACTION);
        textSentIntent.putExtra(TEXT_EXTRA, sendingText);
        LocalBroadcastManager.getInstance(MainActivity.this).sendBroadcast(textSentIntent);
    }

Above method is onClick method of the button. When user touch that button a Intent is crated with previously defined action and data key. After that it is sent with sendBroadcast() and LocalBroadcastManager.

References:

https://developer.android.com/reference/android/support/v4/content/LocalBroadcastManager.html

registerReceiver (BroadcastReceiver, IntentFilter) Android Example

2016-06-26T11:11:00.001-07:00

This example is about using registerReceiver and unregisterReceiver methods to use BroadcastReceiver programmatically. When BroadcastReceiver is registered in this way it's not declared in the AndroidManifest.xml file.

This application project has one Activity file and it register BroadcastReceiver to obtain battery level change information. Broadcast action ACTION_BATTERY_CHANGED is used by Android system to broadcast battery level changing information. Therefore that is used in this project.

Below source code is about implementation of the class BatteryChangedReceiver that extends from BroadcastReceiver. It receive battery change information and extra data on that intent are got by using the class BatteryManager. BatteryManager class has definitions of extra data that are sent with aforementioned broadcast action. Once onReceive(Context, Intent) method receives that data they are shown as Toast.

BatteryChangedReceiver Implementation

import android.content.BroadcastReceiver;

import android.content.Context;

import android.content.Intent;

import android.os.BatteryManager;

import android.widget.Toast;

public class BatteryChangedReceiver extends BroadcastReceiver {

   public BatteryChangedReceiver() {

   @Override

   public void onReceive(Context context, Intent intent) {

      if (intent.getAction().equals(Intent.ACTION_BATTERY_CHANGED)) {

         int level = intent.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);

         int scale = intent.getIntExtra(BatteryManager.EXTRA_SCALE, -1);

         Toast.makeText(context, "Level = "+level+",

                        Scale = "+scale, Toast.LENGTH_LONG).show();

Above BroadcastReceiver is registered in the class MainActivity. That source code is this.

MainActivity Implementation

public class MainActivity extends AppCompatActivity {

   private BatteryChangedReceiver batteryChangedReceiver;

   private IntentFilter intentFilter;

   @Override

   protected void onCreate(Bundle savedInstanceState) {

      super.onCreate(savedInstanceState);

      setContentView(R.layout.activity_main);

      batteryChangedReceiver = new BatteryChangedReceiver();

      intentFilter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);

   @Override

   protected void onResume() {

      super.onResume();

      registerReceiver(batteryChangedReceiver, intentFilter);

   @Override

   protected void onPause() {

      unregisterReceiver(batteryChangedReceiver);
      super.onPause();

An instance of the BatteryChangedReceiver is created to be used as first argument to registerReceiver() method. This method needs IntentFilter object for second argument to register BroadcastReceiver provided with first argument for a specific Intent. So both objects for first and second arguments are created in onCreate() method.

   batteryChangedReceiver = new BatteryChangedReceiver();

   intentFilter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);

The method registerReceiver() is called in onResume() to register BroadcastReceiver.

protected void onResume() {

   super.onResume();

   registerReceiver(batteryChangedReceiver, intentFilter);

As the BroadcastReceiver is registered in the onResume() it must be unregistered onPause(). Otherwise error will be occurred for leaking BroadcastReceiver. A BroadcastReceiver must be unregistered as many as it is registered.

@Override

protected void onPause() {

   unregisterReceiver(batteryChangedReceiver);

   super.onPause();

If onStop() is used to register BroadcastReceiver it must be unregistered in onStop().

Reference

https://developer.android.com/reference/android/content/Context.html#registerReceiver(android.content.BroadcastReceiver,%20android.content.IntentFilter)
https://developer.android.com/reference/android/content/Context.html#unregisterReceiver(android.content.BroadcastReceiver)
https://developer.android.com/reference/android/content/Intent.html#ACTION_BATTERY_CHANGED
https://developer.android.com/reference/android/os/BatteryManager.html

Android BroadcastReceiver

2016-06-19T10:29:00.003-07:00

Android applications need some mechanism to know when some event is occurred. For example some applications need to perform tasks when Android OS booting is completed. Android SDK provide BroadcastReceiver class for that purpose. This blog post content is about using BroadcastReceiver to get notification about connecting and disconnecting Android OS running device from external power source.

Name of this application is ConDisConInfo. So it has the main package with name com.blogspot.nipunswritings.ConDisConInfo. To create a BroadcastReceiver follow these steps.

Right Click on Main package
New
Other
Broadcast Receiver
Give name and click Finish

Once Finish is clicked a new java file can be seen in main package with the name PowerConnInfoReceiver.java. It has a PowerConnInfoReceiver class which extends from super class BroadcastReceiver. PowerConnInfoReceiver has below method which receive power connection/disconnection information which is sent by system.

@Overridepublic void onReceive(Context context, Intent intent) {
    
}

After PowerConnInfoReceiver is created AndroidManifest.xml file has been updated with receiver element. That is registering a Broadcast Receiver. The receiver element in AndroidManifest.xml file is this.

<receiver

     android:name=".PowerConnInfoReceiver"

     android:enabled="true"

     android:exported="true">

</receiver>

Though that element is in the AndroidManifest.xml file PowerConnInfoReceiver will not receive any broadcast. A BroadcastReceiver to receive broadcasts it must be specified what broadcasts it must be received through the intent-filter element. Therefore receiver element is updated like below to receive broadcasts as mentioned above.

<receiver

    android:name=".PowerConnInfoReceiver"

    android:enabled="true"

    android:exported="true">
    <intent-filter>
        <action android:name="android.intent.action.ACTION_POWER_CONNECTED" />
        <action android:name="android.intent.action.ACTION_POWER_DISCONNECTED" />
    </intent-filter>
</receiver>

Notice the actions inside intent-filter. Without those actions this application doesn't work as expected.

Method onReceive(Context, Intent) is updated to filter received information via Intent object and to display information to user as a Toast. So updated onReceive(Context, Intent) method is this.

@Overridepublic void onReceive(Context context, Intent intent) {
    if (intent.getAction().equals(Intent.ACTION_POWER_CONNECTED)){
        Toast.makeText(context, "Power Connected", Toast.LENGTH_LONG).show();
    } else if (intent.getAction().equals(Intent.ACTION_POWER_DISCONNECTED)) {
        Toast.makeText(context, "Power Disconnected", Toast.LENGTH_LONG).show();
    }
}

Run this app on a emulator. This application can be tested with battery section in extended controls of emulator. When charger connection is toggled between AC Charger and None corresponding Toast is displayed.

Reference:

https://developer.android.com/reference/android/content/BroadcastReceiver.html
https://developer.android.com/reference/android/content/Intent.html#ACTION_POWER_CONNECTED
https://developer.android.com/reference/android/content/Intent.html#ACTION_POWER_DISCONNECTED

HelloCharts for Android Example

2016-06-12T11:05:00.000-07:00

This example is using HelloCharts for Android to plot line chart for function |sin(x)|. The value range for x axis is from 0 to 360 degrees. The x values are given like 0, 15, 30, 45 ... 360. So there is difference of 15 degrees between two points. But X axis is drawn with 30 degree scale. Therefore it goes like 0, 30, 60, 90, ..., 180, ...360. Y axis is drawn with scale of 0.25 from 0 to 1.0 to make Y axis range 0, 0.25, 0.5, 1.

At the end of this example developed application creates the below chart.

Add hello-charts library to app by adding below line to dependencies block of build.gradle file of app module. Then sync the project by using Android Studio.

compile 'com.github.lecho:hellocharts-library:1.5.8@aar'

Project has one Activity file with the name MainActivity.java. It's layout resource file is activity_main.xml.

This is the content of activity_main.xml. It has RelativeLayout as root element and RelativeLayout contain TextView and chart definition.

<TextView

 android:id="@+id/chartLbl"

 android:gravity="center"

 android:layout_width="match_parent"

 android:layout_height="wrap_content"

 android:text="|sin(x)|"

 android:layout_alignParentTop="true"

 android:layout_alignParentStart="true"

 android:layout_alignParentLeft="true"

 android:layout_margin="5dp"

 android:textStyle="bold" />

<lecho.lib.hellocharts.view.LineChartView

 android:id="@+id/chart"

 android:layout_width="match_parent"

 android:layout_height="match_parent"

 android:padding="10dp"

 android:layout_below="@+id/chartLbl"/>

The element lecho.lib.hellocharts.view.LineChartView is xml definition of the line chart.

Now reference to this LineChartView can be made with findViewById(int) method. To hold reference to that view, field of type LineChartView has been defined in the MainActivity.java like this.

private LineChartView lineChartView;

Line chart has been created with below method. That is called in onCreate(Bundle) method.

public void drawSinAbsChart()

Method definition of drawSinAbsChart()

public void drawSinAbsChart() {
    String decimalPattern = "#.##";
    DecimalFormat decimalFormat = new DecimalFormat(decimalPattern);

    lineChartView = (LineChartView) findViewById(R.id.chart);

    List<PointValue> values = new ArrayList<PointValue>();

    PointValue tempPointValue;
    for (float i = 0; i <= 360.0; i+= 15.0f) {
        tempPointValue = new PointValue(i, Math.abs((float)Math.sin(Math.toRadians(i))));
        tempPointValue.setLabel(decimalFormat

                   .format(Math.abs((float)Math.sin(Math.toRadians(i)))));
        values.add(tempPointValue);
    }

    Line line = new Line(values)

                   .setColor(Color.BLUE)

                   .setCubic(false)

                   .setHasPoints(true).setHasLabels(true);
    List<Line> lines = new ArrayList<Line>();
    lines.add(line);

    LineChartData data = new LineChartData();
    data.setLines(lines);

    List<AxisValue> axisValuesForX = new ArrayList<>();
    List<AxisValue> axisValuesForY = new ArrayList<>();
    AxisValue tempAxisValue;
    for (float i = 0; i <= 360.0f; i += 30.0f){
        tempAxisValue = new AxisValue(i);
        tempAxisValue.setLabel(i+"\u00b0");
        axisValuesForX.add(tempAxisValue);
    }

    for (float i = 0.0f; i <= 1.00f; i += 0.25f){
        tempAxisValue = new AxisValue(i);
        tempAxisValue.setLabel(""+i);
        axisValuesForY.add(tempAxisValue);
    }

    Axis xAxis = new Axis(axisValuesForX);
    Axis yAxis = new Axis(axisValuesForY);
    data.setAxisXBottom(xAxis);
    data.setAxisYLeft(yAxis);


    lineChartView.setLineChartData(data);


}

Code Explanation

    String decimalPattern = "#.##";
    DecimalFormat decimalFormat = new DecimalFormat(decimalPattern);

Values that are obtained must be shown only up to two decimal places in chart point labels. Object decimalFormat is created for that purpose.

    lineChartView = (LineChartView) findViewById(R.id.chart);

Above jave statement is to obtain the LineChartView object that is declared in the layout xml resource.

    List<PointValue> values = new ArrayList<PointValue>();

    PointValue tempPointValue;
    for (float i = 0; i <= 360.0; i+= 15.0f) {
        tempPointValue = new PointValue(i, Math.abs((float)Math.sin(Math.toRadians(i))));
        tempPointValue.setLabel(decimalFormat

                   .format(Math.abs((float)Math.sin(Math.toRadians(i)))));
        values.add(tempPointValue);
    }

Tha values that must be displayed in the line chart must be in the List of type List<PointValue>. That is doing here. That list contains points which are drawn in the chart. Point is represented with PointValue Class. for loop create those points and set it label that are shown at the points.

    Line line = new Line(values)

                   .setColor(Color.BLUE)

                   .setCubic(false)

                   .setHasPoints(true).setHasLabels(true);
    List<Line> lines = new ArrayList<Line>();
    lines.add(line);

    LineChartData data = new LineChartData();
    data.setLines(lines);

Line chart can contain many lines, Above code shows how to create new line with the values which was generated function |sin(x)|. Method setColor() is used to define color of the line. This chart has straight line as setCubic() has been provided false value. If setHasPoints() called with argument false value points wouldn't be displayed.When setHasLabels() is provided with argument true value labels that was set during PointValue object are created is shown. The lines variable of type List<Line> holds all the lines that is drawn in the chart. This example chart has only one line so it has been added. Finally that lines object is added to LineChartData object with setLines().

    List<AxisValue> axisValuesForX = new ArrayList<>();
    List<AxisValue> axisValuesForY = new ArrayList<>();
    AxisValue tempAxisValue;
    for (float i = 0; i <= 360.0f; i += 30.0f){
        tempAxisValue = new AxisValue(i);
        tempAxisValue.setLabel(i+"\u00b0");
        axisValuesForX.add(tempAxisValue);
    }

    for (float i = 0.0f; i <= 1.00f; i += 0.25f){
        tempAxisValue = new AxisValue(i);
        tempAxisValue.setLabel(""+i);
        axisValuesForY.add(tempAxisValue);
    }

    Axis xAxis = new Axis(axisValuesForX);
    Axis yAxis = new Axis(axisValuesForY);
    data.setAxisXBottom(xAxis);
    data.setAxisYLeft(yAxis);

Java statements that are shown creating X and Y axis of the chart. First loop creates X axis which is started from 0 and goes upto 360 with increment 15.0 from previous x value. Values of axis is represented with object of type List<AxisValue> and AxisValue objects are used represent a location in the axis. AxisValue has setLabel() to display developer preferred label in the chart. In same way Y axis also has been created. After thata x & y axis is created as Axis objects. X axis is set to bottom with setAxisXBottom() and Y Axis is set to left with setAxisYLeft().

Chart data is provided to LineChartView with below statement.

    lineChartView.setLineChartData(data);

If axis values is cut off this link might be helpful. I had similar problem but I could resolve it without that link by adding 5dp padding around LineChartView.

Reference:

https://github.com/lecho/hellocharts-android

Renaming Android Studio Project and Main Package

2016-06-04T04:59:00.002-07:00

Content of this blog post has been tested with Android Studio 2.0.

Changing Project Name

Open Project Window (View → Tool Windows → Project or Alt + 1)
By default Project window shows the Android view change it to Project view with the drop down menu at top left.

Right click on project root → Show in Explorer.
Close Android Studio.
Go into project folder.
Change .iml file to new name that you want to have for the project.
Change the project folder name to new name which is same name of .iml file.
Start Android Studio
Click on “Open an existing Android Studio project”
Find & select the renamed project, then “Ok”

Changing Package name

Right Click on package name (I tried this on main package)
Refactor
Rename
Rename Package
Type new name & click Refactor
Click on Do Refactor

After doing this I have observed changing the main package name successfully renamed other two packages (androidTest & Test) during refactoring process. Although Generated files like AndroidManifest.xml also had correct values strings.xml file had old project name for the string element with attribute “app_name”.
Above experiment was done on a very small project.

iOS Auto Layout Notes - 1

2016-02-14T10:20:00.000-08:00

This post has written about

Adding auto layout constraints to UI object by using interface builder
Changing auto layout constraints by using swift language

Auto layout constraints can be defined to work with different Size classes. Size classes that is applied to different devices and orientations can be change with the below Pop over in the interface builder.

Since UI components of this application is needed to be fit to all the UIs height and width size class of Any is selected here.

There is a UIImageView object on the UIViewController of this class. To center it horizontally on the screen select the UIImageView click on the align button to display Add New Alignment Constraints pop up.

On the pop up menu Select Horizontally in Container and then click on the button Add 1 Constraint.

UIImageView needs to be 10 point below from the top layout guide. And it need to have width and height of 200 points. To do that click on the pin button to show pop up that can be used to add constraints for above requirements.

Once the constraints were set by using pin pop up like above click on Add 3 Constraints to add them to UIImageView.

To see changes on the storyboard. Click on the Resolve Auto Layout Issues button that shows a menu like below .Then click on Update Frames.

That will remove orange color lines if there are them and adjust UI dimensions to reflect constraints.

Change Auto layout Constraints with Swift Code

Create IBOutlet to UIImageView width and height constraints.

@IBOutlet weak var imageHeight: NSLayoutConstraint!
@IBOutlet weak var imageWidth: NSLayoutConstraint!

Write a method like below to change Auto layout constraints which is referred through outlets by using swift code.

func resizeImage(size: Int){
imageHeight.constant = CGFloat(size)
imageWidth.constant = CGFloat(size)
}

Value of instance of NSLayoutConstraint can be changed by assigning new CGFloat value to that instance's constant property.

How To Install Boost C++ Library On Ubuntu 14.04 LTS

2016-02-07T02:56:00.000-08:00

This is how boost C++ library can be installed into a location that is needed to be put. Here I have installed into a folder in my home home folder. All the below command executed from a folder that contain boost_1_59_0.tar.bz2 file. Boost has been installed without ICU.

$ lsb_release -a

No LSB modules are available.
Distributor ID:    Ubuntu
Description:    Ubuntu 14.04.2 LTS
Release:    14.04
Codename:    trusty

$ python -V

Python 2.7.6

$ sudo apt-get install g++

$ sudo apt-get install python2.7-dev

$ sudo apt-get install libicu-dev

$ sudo apt-get install libbz2-dev

$ sudo apt-get install build-essential

$ sudo apt-get install autotools-dev

$ tar --bzip2 -xf boost_1_59_0.tar.bz2

$ cd boost_1_59_0/

$ ./bootstrap.sh --prefix=/home/nipun/BoostLibs --without-icu

$ ./b2 install --prefix=/home/nipun/BoostLibs

Securing Google Cloud Endpoints API for Android App With HMAC

2015-07-25T04:05:00.002-07:00

Google cloud endpoints can be secured by using Oauth protocol provided via Google accounts. Although that feature is provided by the Google cloud endpoints developers don't want to use Oauth sometimes. It's also possible to protect Google Cloud Endpoint APIs with the HMAC authentication. This blog post demonstrate how to protect APIs with HMAC. Example backend APIs have been written by using Java programming language.

Following implementation details will be more described in paragraphs below.

How to access HttpServletRequest instance within the API methods
How to calculate HMAC in java
How to read http authorization header by using HttpServletRequest instance
How to set HMAC in authorization header by using Google Cloud Endpoint client library for Android

System Requirement

Android app send a name of a user. Server should reply with Hi, <name of user>. For example, if app send nipun it should reply with Hi, Nipun. Aforementioned requirement already implemented when Google Cloud Endpoint module is added with Android Studio. Codes from Reference [1] is on content of this post.

User Interface

Android client UI has a Button and a EditText. EditText has the id nameUiValue. nameUiValue has been assigned to

private EditText nameEditText;

When a user click on the button it calls the method.

public void sendName(View view)

Before securing the APIs Endpoint class and client AsyncTask class is show below.

Cloud

MyEndpoint.java

/*   For step-by-step instructions on connecting your Android application to this backend module,

   see "App Engine Java Endpoints Module" template documentation at

   https://github.com/GoogleCloudPlatform/gradle-appengine-templates/tree/master/HelloEndpoints*/
package com.example.nipun.myapplication.backend;
import com.google.api.server.spi.config.Api;

import com.google.api.server.spi.config.ApiMethod;

import com.google.api.server.spi.config.ApiNamespace;

import javax.inject.Named;

import javax.servlet.http.HttpServletRequest;
/** * An endpoint class we are exposing */@Api(name = "myApi", version = "v1",

        namespace = @ApiNamespace(ownerDomain = "backend.myapplication.nipun.example.com",

                   ownerName = "backend.myapplication.nipun.example.com", packagePath = ""))
public class MyEndpoint {

    /**     * A simple endpoint method that takes a name and says Hi back     */


    @ApiMethod(name = "sayHi")
    public MyBean sayHi(HttpServletRequest request, @Named("name") String name) {
        MyBean response = new MyBean();        response.setData("Hi, " + name);
        return response;    }

}

Android Client AsyncTask

class EndpointsAsyncTask extends AsyncTask<Pair<Context, String>, Void, String> {
    private MyApi myApiService = null;    private Context context;
    @Override    protected String doInBackground(Pair<Context, String>... params) {
        if(myApiService == null) {
            MyApi.Builder builder = new MyApi.Builder(AndroidHttp.newCompatibleTransport(),

                new AndroidJsonFactory(), null)

                    .setRootUrl("http://10.0.2.2:8080/_ah/api/")
                    .setGoogleClientRequestInitializer(new GoogleClientRequestInitializer() {
                        @Override

                        public void initialize(AbstractGoogleClientRequest<?>

                                            abstractGoogleClientRequest) throws IOException {
                            abstractGoogleClientRequest.setDisableGZipContent(true);

                        }
                    });

            myApiService = builder.build();        }

        context = params[0].first;        String name = params[0].second;
        try {
            return myApiService.sayHi(name).execute().getData();        } catch (IOException e) {
            return e.getMessage();        }
    }

    @Override    protected void onPostExecute(String result) {
        Toast.makeText(context, result, Toast.LENGTH_LONG).show();    }
}

Securing Google Cloud Endpoint With HMAC

How to access HttpServletRequest instance within the API methods

In order to get the HMAC value from http authorization header HttpServletRequest must be accessed from the API method. It can be done by using following way.

public MyBean sayHi(HttpServletRequest request, @Named("name") String name)

ApiMethod has included HttpServletRequest as an parameter.

How to calculate HMAC in java

Hmac has been generated according to reference 3 and 4. A new hmac is calculated by using at the endpoint by using AuthManager.java class.

package com.example.nipun.myapplication.backend.auth;
import java.io.UnsupportedEncodingException;import java.security.InvalidKeyException;


import java.security.NoSuchAlgorithmException;
import javax.crypto.Mac;import javax.crypto.spec.SecretKeySpec;
/** * Created by nipun on 7/25/15. */public class AuthManager {
    private String KEY = "E34swDN1Fk1G44zmw9bL2N5B8R85w290";    private String ALGO = "HmacSHA1";
    public boolean authenticationSuccessful(String dataForHmac, String authHeader){

        if (authHeader == null)
            return false;
        String tempHmacParts[] = authHeader.split(" ");        if (tempHmacParts.length != 2)
            return false;
        tempHmacParts = tempHmacParts[1].split(":");
        if (tempHmacParts.length != 2)
            return false;
        if (tempHmacParts[1].equals(hmacDigest(dataForHmac, KEY, ALGO))){
            return true;        }else {
            return false;        }
    }

    /*    *    * This method is taken from

    * http://www.supermind.org/blog/1102/generating-hmac-md5-sha1-sha256-etc-in-java

*/

    private String hmacDigest(String msg, String keyString, String algo) {
        String digest = null;        try {
            SecretKeySpec key = new SecretKeySpec((keyString).getBytes("UTF-8"), algo);

            Mac mac = Mac.getInstance(algo);            mac.init(key);
            byte[] bytes = mac.doFinal(msg.getBytes("ASCII"));
            StringBuffer hash = new StringBuffer();

            for (int i = 0; i < bytes.length; i++) {
                String hex = Integer.toHexString(0xFF & bytes[i]);

                if (hex.length() == 1) {
                    hash.append('0');                }
                hash.append(hex);            }
            digest = hash.toString();        } catch (UnsupportedEncodingException e) {
        } catch (InvalidKeyException e) {
        } catch (NoSuchAlgorithmException e) {
        }
        return digest;    }


}

The method

private String hmacDigest(String msg, String keyString, String algo)

is used to calculate hmac. That method is used to check whether authentication is successful by using this method.

public boolean authenticationSuccessful(String dataForHmac, String authHeader)

Client has class HmacUtil.java to calculate hmac before send it to the server.

package com.blogspot.nipunswritings.hiname;
import java.io.UnsupportedEncodingException;import java.security.InvalidKeyException;

import java.security.NoSuchAlgorithmException;
import javax.crypto.Mac;import javax.crypto.spec.SecretKeySpec;
/** * Created by nipun on 7/25/15. */


public class HmacUtil {

    private static String KEY = "E34swDN1Fk1G44zmw9bL2N5B8R85w290";

    private static String ALGO = "HmacSHA1";


    /*    *    * This method is taken from

    * http://www.supermind.org/blog/1102/generating-hmac-md5-sha1-sha256-etc-in-java

    * but parameters have change here

    * */

     public static String hmacDigest(String msg) {
        String digest = null;        try {
            SecretKeySpec key = new SecretKeySpec((KEY).getBytes("UTF-8"), ALGO);

            Mac mac = Mac.getInstance(ALGO);

            mac.init(key);
            byte[] bytes = mac.doFinal(msg.getBytes("ASCII"));
            StringBuffer hash = new StringBuffer();

            for (int i = 0; i < bytes.length; i++) {
                String hex = Integer.toHexString(0xFF & bytes[i]);

                if (hex.length() == 1) {
                    hash.append('0');                }
                hash.append(hex);            }
            digest = hash.toString();        } catch (UnsupportedEncodingException e) {
        } catch (InvalidKeyException e) {
        } catch (NoSuchAlgorithmException e) {
        }
        return digest;    }

}

Android client use below method to calculate hmac

public static String hmacDigest(String msg)

How to read http authorization header by using HttpServletRequest instance

@ApiMethod(name = "sayHi")
public MyBean sayHi(HttpServletRequest request, @Named("name") String name) throws

                                                                        HmacMismatchException{

    String authHeader = request.getHeader("Authorization");

    AuthManager authManager = new AuthManager();

    String digestData = request.getMethod()+"+/myApi/v1/sayHi/"+name;

    if (!authManager.authenticationSuccessful(digestData, authHeader)){
        throw new HmacMismatchException();    }

    MyBean response = new MyBean();    response.setData("Hi, " + name);
    return response;}

It has added new parameter with the type HttpServletRequest and it has name request. Developers can use getHeader() to get http header. After getting Authorization header data for hmac generation is assigned to digestData. Then it is passed with Authorization header value to

public boolean authenticationSuccessful(String dataForHmac, String authHeader)

of AuthManager. If the authentication is not successful Endpoint send HmacMismatchException to client. HmacMismatchException.java is shown below.

package com.example.nipun.myapplication.backend.auth;
/** * Created by nipun on 7/25/15. */


public class HmacMismatchException extends Exception {

    public HmacMismatchException(){
        super("Can't produce received hmac");    }
}

How to set HMAC to authorization header by using Google Cloud Endpoint client library for Android

Android client in this blog post send a string,a name, to endpoint. Android user interface use EditText and Button to get user data. When button is clicked EndpointsAsyncTask is executed. It's provided name of the user.

Below method is called when a user touch the button.

public void sendName(View view){
    String name = nameEditText.getText().toString();
    new EndpointsAsyncTask().execute(new Pair<Context, String>(this, name));
}

Code for EndpointsAsyncTask with http headers setting

class EndpointsAsyncTask extends AsyncTask<Pair<Context, String>, Void, String> {
    private MyApi myApiService = null;    private Context context;
    @Override    protected String doInBackground(Pair<Context, String>... params) {

        context = params[0].first;        final String name = params[0].second;
        if(myApiService == null) {  
            MyApi.Builder builder = new MyApi.Builder(AndroidHttp.newCompatibleTransport(),

                new AndroidJsonFactory(), null)

                    .setRootUrl("http://10.0.2.2:8080/_ah/api/")
                    .setGoogleClientRequestInitializer(new GoogleClientRequestInitializer() {
                        @Override

                        public void initialize(AbstractGoogleClientRequest<?>

                                           abstractGoogleClientRequest) throws IOException {
                            abstractGoogleClientRequest.setDisableGZipContent(true);

                            HttpHeaders httpHeaders =

                                         abstractGoogleClientRequest.getRequestHeaders();

                            httpHeaders.setAuthorization("hmac "

                                +"public:"+HmacUtil.hmacDigest("POST+/myApi/v1/sayHi/"+name));

                            abstractGoogleClientRequest.setRequestHeaders(httpHeaders);

                        }
                    });
            myApiService = builder.build();        }

        try {
            return myApiService.sayHi(name).execute().getData();

        } catch (IOException e) {
            return e.getMessage();        }
    }

    @Override    protected void onPostExecute(String result) {
        Toast.makeText(context, result, Toast.LENGTH_LONG).show();    }
}

Code to set Authorization header is this.

public void initialize(AbstractGoogleClientRequest<?>

                                  abstractGoogleClientRequest) throws IOException {
    abstractGoogleClientRequest.setDisableGZipContent(true);

    HttpHeaders httpHeaders = abstractGoogleClientRequest.getRequestHeaders();


    httpHeaders.setAuthorization("hmac "

                      +"public:"+HmacUtil.hmacDigest("POST+/myApi/v1/sayHi/"+name));

    abstractGoogleClientRequest.setRequestHeaders(httpHeaders);}

References

https://github.com/GoogleCloudPlatform/gradle-appengine-templates/tree/master/HelloEndpoints
http://stackoverflow.com/questions/15056830/getting-raw-http-data-headers-cookies-etc-in-google-cloud-endpoints
http://www.supermind.org/blog/1102/generating-hmac-md5-sha1-sha256-etc-in-java
http://restcookbook.com/Basics/loggingin/

Include HttpHeaders to Google Cloud API call request made from Android

2015-07-21T13:40:00.000-07:00

When making Cloud Endpoint API call request from the Android client sometimes it's needed to add Http Headers to that request. This blog post say how to add Date http header to request that calls myApi.sayHi() method which is described here. Read the code of EndpointsAsyncTask class. The code where http headers must be set is shown below.

MyApi.Builder builder = new MyApi.Builder(AndroidHttp.newCompatibleTransport(),
                    new AndroidJsonFactory(), null)
                // options for running against local devappserver
                // - 10.0.2.2 is localhost's IP address in Android emulator
                // - turn off compression when running against local devappserver
                .setRootUrl("http://10.0.2.2:8080/_ah/api/")
                .setGoogleClientRequestInitializer(new GoogleClientRequestInitializer() {
                    @Override
                    public void initialize(AbstractGoogleClientRequest<?> abstractGoogleClientRequest) throws IOException {
                        abstractGoogleClientRequest.setDisableGZipContent(true);
                    }
                });
                // end options for devappserver

            myApiService = builder.build();

Now add the Date http header as below. The bold text are the code that is used to add http header to request.

                MyApi.Builder builder = new MyApi.Builder(AndroidHttp.newCompatibleTransport(),
                        new AndroidJsonFactory(), null)
                        // options for running against local devappserver
                        // - 10.0.2.2 is localhost's IP address in Android emulator
                        // - turn off compression when running against local devappserver
                        .setRootUrl("http://10.0.2.2:8080/_ah/api/")
                        .setGoogleClientRequestInitializer(new GoogleClientRequestInitializer() {
                            @Override
                            public void initialize(AbstractGoogleClientRequest<?> abstractGoogleClientRequest) throws IOException {
                                abstractGoogleClientRequest.setDisableGZipContent(true);

                                Calendar calendar = Calendar.getInstance();
                                SimpleDateFormat dateFormat = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss z");

                                HttpHeaders headers = abstractGoogleClientRequest.getRequestHeaders();                                Log.d(TAG, "Date: "+dateFormat.format(calendar.getTime()));
                                headers.setDate(dateFormat.format(calendar.getTime()));
                                abstractGoogleClientRequest.setRequestHeaders(headers);                            }
                        });

                myApiService = builder.build();

Google Cloud Endpoints: Creating New Endpoint

2015-06-21T11:27:00.000-07:00

This blog post is about using Google cloud endpoint and Adding new endpoint to existing backend project by using android studio. In order to learn more about Google cloud endpoints with Android use the references.

Integrate Cloud Endpoint Module To Project

File > New Module
Under Choose Module Type select Google Cloud Module and click Next
In next screen, select App Engine Java Endpoint Module for the Module type, give a Module Name and a Package name.
Click Finish

After Gradle sync finish new module can be seen with the name that has been given as Module Name. Google Cloud Endpoint module has example with MyBean.java, MyEndpoint.java classes. Select Cloud endpoint module in run configuration and click run button. This endpoint can be seen in API Explorer.

After Android Studio Console display “INFO: Dev App Server is now running”.

Go to this http://localhost:8080/
Click on Google Cloud Endpoints API Explorer, It ll direct you to this link

https://apis-explorer.appspot.com/apis-explorer/?base=http%3A%2F%2Flocalhost%3A8080%2F_ah%2Fapi#p/

Above link is the address of API Explorer for the services that is running on the localhost

If services are not displayed click on the shield on Chromes address bar
It ll show a pop up with message “This page is trying to load scripts from unauthorized sources”, click on load unsafe scripts.
After that myApi API can be seen under services

API can be tested by clicking on the API.

Adding A New Endpoint

New endpoint is used to calculate sum of two floats. So This API need two java classes they are CalculatorEndpoint.java and ResultBean.Java. Both are under same package of Cloud Module.

ResultBean.java

package com.example.nipun.myapplication.backend;

/**

* Created by nipun on 6/21/15.

public class ResultBean {

float result;

public float getResult() {

return result;

}

public void setResult(float result) {

this.result = result;

}

CalculatorEndpoint.java

package com.example.nipun.myapplication.backend;

/**

* Created by nipun on 6/21/15.

import com.google.api.server.spi.config.Api;

import com.google.api.server.spi.config.ApiMethod;

import com.google.api.server.spi.config.ApiNamespace;

import javax.inject.Named;

@Api(name = "calculatorApi",

version = "v1",

namespace = @ApiNamespace(

ownerDomain = "backend.myapplication.nipun.example.com",

ownerName = "backend.myapplication.nipun.example.com",

packagePath = ""))

public class CalculatorEndpoint {

@ApiMethod(name = "addTwoNumbers")

public ResultBean addTwoNumbers(@Named("num1") float num1, @Named("num2") float num2){

float result = num1 + num2;

ResultBean resultBean = new ResultBean();

resultBean.setResult(result);

return resultBean;

}

In order to use calculatorApi add fully qualified class name of CalculatorEndpoint.java to the web.xml file of backend module as below. web.xml can be located in <backend_module_name>/src/main/webapp/WEB-INF/web.xml. backend_module_name is the name of the Google Cloud Module.

<servlet-name>SystemServiceServlet</servlet-name>

<servlet-class>com.google.api.server.spi.SystemServiceServlet</servlet-class>

<init-param>

<param-name>services</param-name>

<param-value>

com.example.nipun.myapplication.backend.MyEndpoint,

com.example.nipun.myapplication.backend.CalculatorEndpoint

</param-value>

</init-param>

</servlet>

Run the cloud backend module again and go to API explorer. Now two numbers can be added with calculatorApi.

References

https://github.com/GoogleCloudPlatform/gradle-appengine-templates/tree/master/HelloEndpoints

Rendering Problems: The following class could not found

2015-06-20T02:46:00.000-07:00

Problem

In android studio, after a project was created a message was displayed with following content.

The following class could not found: android.support.v7.internal.widget.actionbaroverlaylayout

Solution

Buld > Rebuild Project

Go to build menu then rebuild the project.

NAND: could not write file Error

2015-04-22T03:18:00.001-07:00

Error:

NAND: could not write file /tmp/android-nipun/emulator-kOjnli, File exists

Solution:

The hard-disk partition which has /tmp folder had not enough space when the Android virtual device (AVD) was started. Making free space that is more than needed by AVD resolve this error.

How To Hide Status Bar In iOS 7

2014-07-23T03:12:00.000-07:00

- (BOOL)prefersStatusBarHidden
{
return YES;
}

Write above method on every UIViewController class which has a UIView to show without status bar.

A Code Snippet For Converting NSDate To NSString With NSDateFormatter

2014-06-10T22:04:00.003-07:00

When working on programs that manipulate Dates sometimes those days need to be store & printed in particular format. iOS has given NSDate for date and time related data handling and NSDateFormatter can be used to give a date of NSDate object a visual form.

Code

Current Date and Time is obtained by init method of NSDate.

NSDate *date = [[NSDate alloc] init];

Create a NSDateFormatter for set the date format that is needed to print and get a NSString object from the date.

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];

Use setDateFormat of NSDateFormatter to get a date format that is needed to display. This method get the required date format by using NSString parameter and returns nothing. The method has been called on the object dateFormatter.

[dateFormatter setDateFormat:@"dd-MM-YYYY HH:mm:ss"];

Method signature for setDateFormat

- (void)setDateFormat:(NSString *)string

NSString *dateString = [dateFormatter stringFromDate:date];

Invoke stringFromDate on dateFormatter object to get the the date value of NSDate object in specified date format. This method consumes previously created date object and assign a NSString represent of the date on date object to dateString.

Method signature for stringFromDate

- (NSString *)stringFromDate:(NSDate *)date

Print the dateString using NSLog();

NSLog(@"The Date: %@", dateString);

Complete Code

    NSDate *date = [[NSDate alloc] init];

    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];

    [dateFormatter setDateFormat:@"dd-MM-YYYY HH:mm:ss"];

    NSString *dateString = [dateFormatter stringFromDate:date];

    NSLog(@"The Date: %@", dateString);

isDescendantOfView: To check whether UIView is a subview of another UIView in iOS

2014-04-07T06:28:00.000-07:00

Sometimes it's needed to check whether a UIView is a subview of another view. A typical use case is show and hide UIViews with button click events. it's better if the code check whether a UIView is a sub view of another view before the view is removed from the super view. UIView class have method "isDescendantOfView" [1] it returns "YES" if the specified UIView is a subview of another UIView if it's not it returns "NO".

In example code below, "I m in" is printed if the theSubView is a sub view of the UIView self.view otherwise it prints "I m out".

if ([theSubView isDescendantOfView:self.view]) {

        NSLog(@"I m in");

    }else{

        NSLog(@"I m out");

}

Reference:
[1]. http://developer.apple.com/library/ios/documentation/uikit/reference/uiview_class/uiview/uiview.html#//apple_ref/occ/instm/UIView/isDescendantOfView:

Converting NSString to NSData and NSData to NSString

2014-03-24T04:44:00.002-07:00

Description

Below code segment shows how to convert a NSString value to NSData and NSData value to NSString value. At the beginning, It create a NSString literal with the variable name "firstString" then value of "firstString" variable is taken as a way that can be assigned into NSData variable by using method [1]. The variable "firstString" is assigned with "nil" in order to show that the variable contains nothing. The value of the variable has printed to by using "NSLog". After the printing a NSString object is created with by using previously created NSData. Method [2] has been used. Then the "firstString" value is printed again with NSLog.

- (NSData *)dataUsingEncoding:(NSStringEncoding)encoding
- (instancetype)initWithData:(NSData *)data encoding:(NSStringEncoding)encoding

Code

    NSString *firstString = @"This is the string that is wanted to convert";

    NSData *dataFromString = [firstString dataUsingEncoding:NSUTF8StringEncoding];

    firstString = nil;

    NSLog(@"firstString (nil) = %@", firstString);

    firstString = [[NSString alloc] initWithData:dataFromString encoding:NSUTF8StringEncoding];

    NSLog(@"firstString (!nil) = %@", firstString);

Result

2014-03-24 17:10:11.367 TestProject[3048:60b] firstString (nil) = (null)

2014-03-24 17:10:11.369 TestProject[3048:60b] firstString (!nil) = This is the string that is wanted to convert