GUI Lock Down

2012-08-25T11:59:00.000-07:00

Dear folks, sorry for the delay in my this post (that is if any of you were waiting to read).
In this post I would mainly be stating a few points that should be in mind when you are are making a GUI program. This may be helpful to you and save a lot of head aches trying to debug the problem as it occurred to me.

As you all may know that when you click a button on a window, the window is in a frozen state until the execution of codes in the handler is completed. You would usually make the event handler of the button do some calculations and then reflect a few changes in the interface. Your program is a single thread, every thing happens on the UI thread, and you are out of trouble.

But what if your program is little more complex and other threads try to edit the UI components. What if there is shared memory and both the user interface thread and other threads acquire lock to these locations.

Here's the method of action GUI frameworks usually follow:

When ever a thread tries to access UI components, it acquires a lock on the user interface.
Any other call to the user interface will have to wait for the previously acquired lock to be released.
The button click event also acquires lock on the UI and releases it when its handlers execution is over.

Now here's the question, when does the program go into a Lock Down State (the program hangs execution due to some reasons) (I just call the state a lock down state).

Let me give you an example of such a situation. You will get the idea from it.
The program design is as follows:

Your program has two threads:
1. The User Interface thread. This holds all the UI objects.
2. Another thread, say Thread B, which samples data from a given steam at a particular rate (e.g. an Internet stream or a device connected to a serial port).
You have a function fun1() that is called by thread B and also by the button click event handler of the UI thread. This function has codes that acquires lock on object say LO1
Thread B and a button click on the UI, both try to change the text in a text box on the UI.

The program will go into Lock Down State when the following occurs:

Thread B has acquire lock on lock object LB1.
User has clicked the button and click event has acquired lock on the UI.
Thread B is waiting for the UI lock to be released so that it can edit the text in the text box.
The UI lock is not being released by the click event as it is waiting for Thread B to release the lock it has acquired.

And thus the Lock Down State is reached. It is really frustrating to be stuck in this situation and not know about it.

There are two ways I would suggest in which you can avoid this situation:

Never try to access UI components when there is a Lock acquired. Always free every lock object before doing so.
Put the execution part of the button event handler into another thread and free the UI (Clicking the button in this case will simply just start the thread execution).

These two ways will surely help you out. I personally prefer creating new threads and making them execute.Your UI will never freeze in this situation also.

Whose head to burn?

2012-06-27T22:58:00.000-07:00

When I am in the hostel, there is always some one to share my frustration with, if I have any while doing some computer science assignment (I am only a minor in it). Well presently, all of it is with me.
On the brighter side, I've learnt a lot (Though I had to eat my head off). So I thought of sharing my wisdom out here. People might find this stuff pretty useful.

Here are the guidelines for reading the later posts.

This blog is dedicated to share experiences in programming.
This is not a leisure time spending blog. (Except if you are a nut case and do stuff like these in your leisure time.)
If you ever try to say that I have made spelling mistakes and I'm writing dumb things, I WILL KILL YOU. Believe me.

I am working pretty extensively in Python these days so maybe I'll talk about it first (Not right now obviously, in my later posts). Till then, Burn your head.!

Burn your Head

GUI Lock Down

Whose head to burn?