Question

I’m trying to create a Kinect + WPF Application. The Kinect part works fine; now I want to create a method to check if the page is idle (no user is interacting with the program for a specific amount of time). If it is idle for 5 seconds, the screen will be locked. This is the algorithm:

1. Start Thread (I need to use Dispatcher as the thread needs to modify WPF elements)
2. Check if user is using. If not, then increase Count by 1.
3. If Count == 9 (which means that already 5 seconds have passed), lock the screen and set Count to 0 again.
4. Sleep for 500 milliseconds.
5. Repeat Step 2-4

This is my code. The method “startLockHandler” is invoked when the application starts.

 public void startLockHandler()
 {
     Application.Current.Dispatcher.BeginInvoke(new ThreadStart(() => lockHandler()), null);
 }

 public void lockHandler()
    {
        while (true)
        {
            if (myState.isSkeletonTracked == false) //if skeleton is no longer tracked 
            {
                if (myState.ActionAllowed == true) //if the page is not in transition
                {
                    lockCount++;

                    if (lockCount >= 10)
                    {
                        lockCount = 0;

                        myState.ActionAllowed = false;

                        //LOCKING MECHANISM INSERTED HERE. NEED TO MODIFY SOME WPF ELEMENTS
                        myState.ActionAllowed = true;
                    }
                }
            }

            else
            {
                lockCount = 0;
            }
            Console.WriteLine("lockHandler: THREAD SLEEP CALLED");
            Thread.Sleep ( 500 );

        }//end while
    }//end method lockHandler 

When I run the application, the application hangs right after it is started. What I think happened is that Thread.Sleep (500) instructs the main thread to sleep (Please correct me if I’m wrong). My question is how can I specify which thread to be put to sleep?
BTW, I’m not an expert in C# and a newbie in Threading 🙁

Thanks for your attention and hopefully you can help me 🙂

Answer 1

1. Dispatcher.BeginInvoke() does not create new thread, it just executes lockHandler() on main GUI thread. The fact that you pass ThreadStart delegate means nothing special here.

2. Thread.Sleep() puts to sleep the thread which called it, that is main GUI thread in your case. To control execution of one thead from another you should use synchronization primitives (ManualResetEvent, AutoResetEvent, Mutex, Semaphore, etc).

3. You should not access/modify UI directly from another thread, you must call Invoke or BeginInvoke Dispatcher methods from secondary thread (not the GUI thread) to ensure that all UI related code is executed from the thread which created UI elements.

4. You don’t really need a secondary thread with a dedicated Dispatcher instance for your case.

Minimum modification, which will at least use a separate thread:

var thread = new Thread(lockHandler);
thread.Start();

Remember that you also need to implement a way to stop your secondary thread or it may prevent your application from closing and unloading from memory. A quck and (very) dirty solution will be to mark this thread as a background thread: thread.IsBackground = true.

Update

Example of modifying UI from secondary thread:

Application.Current.Dispatcher.BeginInvoke(new Action(
    () =>
    {
         // access UI elements here, for example
         myState.ActionAllowed = false;
    }));