WPF update UI from async method

Question

In public MainWindow() I call TestTask3(10000);. A TextBlock Text is bound to Answer in the XAML. The get is called and the initial value is displayed. I see the set call NotifyPropertyChanged. But the get is not called a second time to get the new value. I also have a button that changes Answer and that does change the TextBlock.

How to fix this or another approach? I want to keep the UI reactive and update some UI elements after a delay.

private int answser = -2;
public int Answer
{
    get { return answser; }
    set
    {
        if (answser != value)
        {
            answser = value;
            NotifyPropertyChanged("Answer");
        }
    }
}
public async void TestTask3(int delay)
{
    Debug.WriteLine($"TestTask3");
    int answer = -1;
    int i = await Task.Run(() =>
    {
        // … do compute-bound work here  
        Task.Delay(delay);
        answer = -1;
        return answer;
    });
    Debug.WriteLine($"TestTask3   {i}");
    Answer = answer;
    //return answer;
}

You shouldn’t use the same names for local variables and class fields, it’s very confusing. Have you tried putting a breakpoint in the setter and stepping through? — Dave M, May 11 '18 at 01:37
Yeah this is confusing, i cant see how on earth the property Answer would get updated twice anyway (if i'm understanding you correctly) — TheGeneral, May 11 '18 at 01:37
@DaveM As stated I see NotifyPropertyChanged called. That is how is see that. — paparazzo, May 11 '18 at 01:41
@TheGeneral It gets updated if I change Answer in a button click event. — paparazzo, May 11 '18 at 01:42

score 3 · Accepted Answer · edited May 11 '18 at 05:47

3

Task.Delay(delay) is awaitable method.

Following solution works for me. I took a long loop calculation and it returns me result on UI after 10 seconds. Tested and working.

public async void TestTask3(int delay)
{
    int answer = -1;
    int i = await Task.Run(async () =>
    {
        // … do compute-bound work here  
        for (int j = 0; j < 100000000; j++)
        {
            answer += j;
        }

        await Task.Delay(delay);

        //answer = -1;
        return answer;
    });
    Answer = answer;
    //return answer;
}

Usage:

private async void Button_Click(object sender, RoutedEventArgs e)
{
    await TestTask3(10000);
}

edited May 11 '18 at 05:47

Clemens

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answered May 11 '18 at 01:42

Gaurang Dave

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So it was missing an async. Not sure how that broke it but I am just glad it is fixed. – paparazzo May 11 '18 at 01:53
I am glad to help. Cheers :) – Gaurang Dave May 11 '18 at 01:54
2

@paparazzo Since you should usually avoid `async void` methods (except event handlers), you may declare `public async Task TestTask3(int delay)` and make it `return answer`. Then call it like `Answer = await TestTask3(10000);` – Clemens May 11 '18 at 05:49
@Clemens That syntax throws a design time error for me. ` int ans = TestTask3(1000, token).Result;` is valid syntax but is hangs on that statement. – paparazzo May 11 '18 at 14:18
@Clemens This is how I wanted to do it as it returns a value. https://stackoverflow.com/questions/50281183/delay-in-a-task Problem is even with the answer it blocks on Delay. – paparazzo May 11 '18 at 14:22
It must be `int ans = await TestTask3(...)`. Don't use `Result`. See here: https://stackoverflow.com/q/27464287/1136211 – Clemens May 11 '18 at 15:25

WPF update UI from async method

1 Answers1