Synchronous Wait Without Blocking the UI-Thread

Question

Is there a synchronous wait function that won't tie up the UI-thread in .NET WPF? Something like:

Sub OnClick(sender As Object, e As MouseEventArgs) Handles button1.Click
     Wait(2000) 
     'Ui still processes other events here
     MessageBox.Show("Is has been 2 seconds since you clicked the button!")
End Sub

in plain Win32 you can use MsgWaitForMultipleObjects to achieve this. — David Heffernan, May 24 '11 at 23:10

H.B. · Accepted Answer · 2016-05-17T14:50:31.447

18

You can use a DispatcherTimer for that sort of thing.

Edit: This might do as well...

private void Wait(double seconds)
{
    var frame = new DispatcherFrame();
    new Thread((ThreadStart)(() =>
        {
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            frame.Continue = false;
        })).Start();
    Dispatcher.PushFrame(frame);
}

(Dispatcher.PushFrame documentation.)

Starting with .NET 4.5 you can use async event handlers and Task.Delay to get the same behaviour. To simply let the UI update during such a handler return Dispatcher.Yield.

edited May 17 '16 at 14:50

answered May 24 '11 at 21:46

H.B.

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1

In WPF you defiantly want to use the DispatcherTimer as it's ticks will execute on the UI's thread enabling you to modify UI controls. If you use a standard timer the ticks execute on its own thread and you will get some nasty exceptions if you try to touch UI controls. +1 – Jay May 24 '11 at 21:50
Is there anyway to use the timer to do the waits inline. Seems cumbersome and hard to read when you break up a method every time you need a wait. – PeterM May 24 '11 at 21:58
1

@PeterM: As i'm not all too well versed in threading issues you might want to take a look at the [Threading Model](http://msdn.microsoft.com/en-us/library/ms741870.aspx) reference and the [`Dispatcher.PushFrame`](http://msdn.microsoft.com/en-us/library/system.windows.threading.dispatcher.pushframe.aspx) method, which might help you simplify this. – H.B. May 24 '11 at 22:27
@PeterM: Actually i think i just got a solution using this method, see my updated answer. (I don't speak VB though...) – H.B. May 24 '11 at 23:03
As an example [A non blocking wait for event in C#](https://medium.com/home-wireless/a-non-blocking-wait-for-event-in-c-6662ba47373) – Eli Apr 15 '20 at 09:17
This solution works but should used with care. See [Which blocking operations cause an STA thread to pump COM messages?](https://stackoverflow.com/a/21573637/8051589) for example. Quoted from that answer: – Andre Kampling Jul 08 '20 at 10:55

score 1 · Answer 2 · answered Jul 13 '18 at 17:56

Here is a solution with Task.Delay. I'm using it in unit-tests for ViewModels that use DispatcherTimer.

var frame = new DispatcherFrame();

var t = Task.Run(
    async () => {
        await Task.Delay(TimeSpan.FromSeconds(1.5));
        frame.Continue = false;
    });

Dispatcher.PushFrame(frame);

t.Wait();

Synchronous Wait Without Blocking the UI-Thread

2 Answers2

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