The problem

I came across this problem while writing an Android app for myself using Xamarin. Here's a shortened version of my problem.

• DisplayAlert is the simplest way to prompt the user with a question (ex: "Would you like to continue?").
• DisplayAlert is an async call.
• Unfortunately Button Clicked event handlers (which is often where these calls are made) must have a non-async function signature; meaning await cannot be performed on DisplayAlert

This means you cannot write code like the following:

myButton.Clicked += Button_Clicked;  <<<  ERROR!  Wrong return type
...

private async Task Button_Clicked(object sender, EventArgs e)
{
            
    var result = await DisplayAlert("", "Continue?", "Ok", "Cancel");
    if (result)
    {
        // Do something
    }
}

The Fix: ContinueWith

The solution was to use a call to ContinueWith on the Task returned by DisplayAlert. ContinueWith takes an Action>, and can be coded rather simply, as follows:

myButton.Clicked += Button_Clicked;  
...

private void Button_Clicked(object sender, EventArgs e)
{
            
    DisplayAlert("", "Continue?", "Ok", "Cancel").ContinueWith(task =>
    {
        if (task.Result == true)
        {
            // Do something
        }
    });
}

UI Updates on the Main UI Thread!

We're not quite done yet. Often times the code within the ContinueWith does something that updates the UI. If you've done any multi-threaded coding on a UI you'll recall that all UI changes must be done on the main thread.

Thankfully Xamarin has a simple fix for that: BeginInvokeOnMainThread. The solution now looks like:

myButton.Clicked += Button_Clicked;  
...

private void Button_Clicked(object sender, EventArgs e)
{
            
    DisplayAlert("", "Continue?", "Ok", "Cancel").ContinueWith(task =>
    {
        if (task.Result == true)
        {
            Xamarin.Forms.Device.BeginInvokeOnMainThread(() =>
            {
                // Do some UI updates
            });
        }
    });
}