异步以延迟在KeyDown事件中处理

Question

Let's assume we have this:

private void Input_KeyDown(object sender, KeyEventArgs e)
{
    e.Handled = true;
}

And then i add async-await like this:

private async void Input_KeyDown(object sender, KeyEventArgs e)
{
    await Task.Delay(1000);
    e.Handled = true;
}

Shouldn't that do the same, but only delay it for a second? So why doesn't it?

NOTE:

I'm not seeking for debugging help, i'm just wondering why can't i use async-await to delay handling the KeyDown

Answer 1

i'm just wondering why can't i use async-await to delay handling the KeyDown

Event handlers are synchronous by definition. So, unless your code has a way to inform the event handler that it is acting asynchronously (eg, a WinStore-style Deferral), then it must do all "communication" (eg, KeyEventArgs.Handled ) before its first await .

If you need to delay input in general, I recommend converting the code to use Reactive Extensions .

Answer 2

The problem is that you're trying to use await in an event handler. The "synchronous part" of event handler does nothing.

void OnKeyDown()
{
    Keys keyData = ...;
    KeyEventArgs args = new KeyEventArgs(keyData);
    // Thread-safe event raiser, 
    // equalient to KeyDown(this, args);
    RaiseKeyDownEvent(args);
    // Returns immediatelly when you reach
    // the first await method call
    bool isHandled = args.Handled;
    // In the meantime event handler
    // is awaiting for Task.Delay(),
    // but we have already saved Handled value
}

// This code does nothing useful
private async void Input_KeyDown(object sender, KeyEventArgs e)
{
    // Will return there
    await Task.Delay(1000);
    // The execution will be continued from there
    e.Handled = true;
}

// But this code will handle input, but without any delay
private async void Input_KeyDown(object sender, KeyEventArgs e)
{
    e.Handled = true;
    await Task.Delay(1000);
}