WPF - How to make main UI thread wait until the render thread is done doing its stuff

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Hi,

I have a scenario where I am starting a child thread which sets the 'busyIndicator.IsBusy' as true before doing some time taking stuff in main UI thread.
and when the execution in main UI thread is done I am again setting IsBusy as false.
But the problem is that the code in the delegate of the child thread doesn't get executed until the main thread is done doing its stuff making the use of the BusyIndicator pointless.
Is there any way to make the main thread wait until the child thread is done doing its task.
Here is my code sample:

C#

Thread thread = new System.Threading.Thread(delegate()
            {
                
                testBusyIndicator.Dispatcher.Invoke(DispatcherPriority.Send,
                   (ThreadStart) delegate
                    {
                        testBusyIndicator.IsBusy = true;
                    });
                
            });
            thread.Start();
            
            //DoTimeTakingStuff()
            testBusyIndicator.IsBusy = false; // It can also be done in a separate thread but not needed I think.
            thread.Abort();

Thanks in advance :)

Posted 21-Sep-11 1:30am

ashishks1987@gmail.com

Updated 4-Jul-22 23:01pm

v2

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Sergey Alexandrovich Kryukov 21-Sep-11 10:22am

Why on Earth doing so?! What's the idea?
--SA

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Sergey Alexandrovich Kryukov · Answer 1 · 2011-09-21T04:21:00

First, you should never make main thread (which is your UI thread I assume) to wait for anything. Not only this is bad, this is also never needed.

You code sample makes no sense at all, because the method of your thread really does nothing. Think of what are you doing. You're trying to create a race condition between you UI thread and thread competing over the value of testBusyIndicator; the assignment will be done in some random order. I prefer more descriptive term about race condition — "incorrect dependency on the time of execution". See http://en.wikipedia.org/wiki/Race_condition[^].

Now, I'm not answering what to do, because you don't explaining what you want to achieve. The mistake of you question is trying to ask about something which does not have to have any purpose (and apparently does not have in this case); you should ask a question based on your ultimate purpose, not on your opinion on how it should be achieved, especially when you don't explain what you want to achieve. I'm also not asking about what are you trying to do because it is apparent to me that you don't understand it yourself.

Focus on your ultimate goal. Picture what useful work should your thread do, not changing the status of any indicator. Perhaps you should simply prevent UI from some action until your temporary thread's work is complete. But this is very simple task: you should invoke change of UI status in the very beginning and the very end of the thread's body, that is in the same thread, not too different threads. (By the way the call to Invoke makes sure that actual calls on UI controls are always done in the UI thread anyway.) Be aware of race condition and avoid it.

You will find detailed explanation of how invocation works and code samples in my past answers:
Control.Invoke() vs. Control.BeginInvoke()[^],
Problem with Treeview Scanner And MD5[^].

See also more references on threading:
How to get a keydown event to operate on a different thread in vb.net[^],
Control events not firing after enable disable + multithreading[^].

—SA

Mac's Power · Answer 2 · 2022-07-04T23:01:00

Yes, this is a very common scenario that the designers of WPF did a bad job providing for.

e.g.

C#

StatusbarText = "Saving ...";
await SaveOperation();
StatusbarText = "Save complete";

In the above code the user won't see "Saving ..." appear on screen.

C#

StatusbarText = "Saving ...";
// wait for rendering thread to update text on screen
await SaveOperation();
StatusbarText = "Save complete";

Unfortunately WPF does not provide access to the rendering thread
(or they've hidden it away somewhere)

So you're stuck with having to wait on the main UI thread instead.

I've seen various post with Dispatcher.Invoke or Dispatcher.BeginInvoke but they didn't work for
me, or worked intermittently.

Others suggest re-writing everything to use background threads
(for something that should not need a background thread)

What worked for me was using a DispatcherTimer and Dispatcher.Yield()
If you put a breakpoint on the await SaveOperation() line you can see that the text has been updated

C#

StatusbarText = "Saving ...";
await UIFinishRender();
await SaveOperation();
StatusbarText = "Save complete";


public async Task UIFinishRender()
{
    // want to wait for render thread finish drawing
    RenderCount = 0;    // private int

    DispatcherTimer timer = new DispatcherTimer();
    timer.Interval = TimeSpan.FromMilliseconds(1);
    timer.Tick += timer_Tick;
    timer.Start();

    while (RenderCount < 30)
    {
        await Dispatcher.Yield();
    }
}

void timer_Tick(object sender, EventArgs e)
{
    RenderCount++;
}

WPF - How to make main UI thread wait until the render thread is done doing its stuff

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