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How to DoEvents in WPF?

From VB 5 (even 4) most advanced developers know little nice void method DoEvents. What is it? This is the great way to perform non-blocking wait. The method releases Windows messages pump, other words, performs execution loop. Why this good? Let’s see. Following code (C# 1.1) just hangs until the loop will reach it’s final value.

for(int i=0;i<1000;i++)
{
label1.Text = i.ToString();
}

How to force it to show values?

for(int i=0;i<1000;i++)
{
label1.Text = i.ToString();
DoEvents();
}

If you want to test this code and see something, put Thread.Sleep(1000); after label1.text… :) Just in case

But in WPF we have no DoEvents() method in application class? What to do? Well, we know, what Dispatcher is. We also know, that it use DispatcherFrame to pump messages, so, why not create our own DoEvents?

void DoEvents(){
DispatcherFrame f = new DispatcherFrame();
Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background, 
(SendOrPostCallback)delegate(object arg) {
    DispatcherFrame fr =  arg as DispatcherFrame;
    fr.Continue=False;
}, f);
Dispatcher.PushFrame(frame);
}

Now, by using this method, we’ll release message pump and make our long asynchronous methods not block dispatcher thread, but still wait for the end of execution. Here the example how to do it.

DispatcherOperation op = Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background,
(DispatcherOperationCallback)delegate {
    //DoSomethingReallyLong
    int res = 1;
    int pre = -1;
    for(int i=0;i<1000;++i) {
     int sum = res + pre;
    pre = res;
    res = sum;
     }
    return res;
},null);

while(op != DispatcherOperationStatus.Completed) {
   DoEvents();
}

Console.WriteLine(op.Result);

That’s all, folks.

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9 Responses to “How to DoEvents in WPF?”

  1. Philipp Says:

    @Peter:

    http://www.hardcodet.net/…/wpf-progress-dialog

  2. NMarian Says:

    Another solution of this problem is described on nmarian.blogspot.com/…/doevents-in-wpf.html

  3. Peter Says:

    I am looking for guidance for a progress dialog…without ‘doevents’ solution.

    I am looking to popup a progress dialog window which periodically updates the message and progress amount.  I have found no solid examples of this for WPF as well as not being an expert on threading.  This article helped but I was wondering if there was a proper implementation around for it.

    I have a UI which starts a class running several syncronous operations to hardware.  Between each operation the class fires an event with a progress message back to the UI.  I have the UI pop up a dialog at the start, but every time the event fires with a new progress message, it will not write it to the pop up window.  The ‘DoEvents’ here accomplishes it, but I would like to learn a better way.

  4. Tamir Khason Says:

    Now, I completely agree with you :)

    Fixed in article, thank you for your awarness

  5. Nir Says:

    A thread can do only one thing at a time, if code runs on your thread (even if that code is invoked in a async way) your thread is blocked.

    The only way to run true async operations is to run them on another thread or completely outside the CPU (async IO for example).

    Your example code in the post doesn’t run anything in another thread, you explicitly use CurrentDispacher in the same context as the DoEvent loop, so you are running the long operation on the same thread and you are blocking this thread, even if you use BeginInvoke (and there is no reason to use the debugger  threads window, you only use a single thread).

    Dispacher.BeginInvoke is async because it doesn’t block right now – when the invoked code runs it will block the running thread (otherwise it’s not running), you can use it without blocking your thread only if you are using another thread’s dispatcher (and then you are blocking that thread instead)

    And there are 3 bugs in your DoEvent code, as written in the post it never returns (unless you close the window that called it, then the dispatcher will force your frame to end)

    1. You use an undefined variable "frame", I assume you meant "f"

    2. fr.Continue=True should be fr.Continue=false;

    3. You should use a dispatcher priority of ApplicationIdle (or maybe ContextIdle)and not Background

    After those 3 changes this method does the same thing as the WinForms DoEvents (process all pending messages and the return).

  6. Tamir Khason Says:

    Nir, thank you for your comment

    A couple of things

    1) You are completely right, Dispatcher.BeginInvoke executes delegate asynchronously on it’s thread. It not opens new thread and that what I wrote in this article.

    2) After you begun asynchronous method, you might have to wait, but still let your thread not to block, thus the only thing you can do is to pump message loop (or other word push empty frame to message pump) instead of setting thread into sleep state. This why we doing it

    3) Try to attach to right thread to be able to debug it – debugging in multithreaded environment can be perfomed by using thread window in Visual Sudio

    4) BackgroundWorker is only handy class that open new thread – you can do it easely by yourself. This article is not about multi-threading, but about asynchronous non blocking operations

    Thank you again

  7. Nir Says:

    This doesn’t really work, just try it, put a breakpoint inside the long calculation and look at the call stack, the ling calculation is running inside your DoEvents and DoEvents doesn’t return until the long calculation is over, you are blocking the thread just like you would if you had just done the long calculation, all you did was add a lot of dispatching overhead.

    You can’t run a loop on your thread without blocking the thread (and dispatching with background priority doesn’t run the code in a background thread, it runs the code on the dispatcher’s thread (in this case your own thread).

    You have 3 basic mistakes:

    1. Calling Dispacher.BeginInvoke with background priority does not run the code in the background, it runs the code in the dispatcher’s thread (in this case your own thread) with low priority.

    2. To use the DoEvents technique you have to call DoEvents inside your long running loop, that way every DoEvents call the dispatcher will pump all pending events and let you continue the loop.

    3. Your DoEvents implementation is wrong, it only returns when you close the application.

    If you need to run a long operation you can use the BackgroundWorker class, the thread pool, the Thread class or the technique described here – http://msdn2.microsoft.com/en-us/library/ms771729.aspx

  8. Ross Says:

    I agree with Nir. This is a clusterfuck. Use another thread/BackgroundWorker if you don’t want to block the UI thread.

  9. JB Says:

    Here you find another solution of a progress dialog for WPF:
    http://jbaurle.wordpress.com/2011/04/22/how-to-implement-a-modern-progress-dialog-for-wpf-applications/

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