Xamarin.Forms - BeginInvokeOnMainThread for an async Action

Question

I am familiar with the rules about updating UI elements on the UI thread using the Device.BeginInvokeOnMainThread, however I have an operation that needs to be run on the UI

Answer 1

Just make sure you handle exceptions in your Action and you should be fine. The problem you described occurs when you don't handle the exceptions. Below is a very simple example of running an async method from the main thread.

private void Test()
{
    Device.BeginInvokeOnMainThread(SomeMethod);
}

private async void SomeMethod()
{
    try
    {
        await SomeAsyncMethod();
    }
    catch (Exception e) // handle whatever exceptions you expect
    {
        //Handle exceptions
    }
}

private async Task SomeAsyncMethod()
{
    await Navigation.PushModalAsync(new ContentPage());
}

Answer 2

Since Xamarin.Forms 4.2 there is now a helper method to run tasks on the main thread and await them.

await Device.InvokeOnMainThreadAsync(SomeAsyncMethod);

Several overloads exist that should cover most scenarios:

System.Threading.Tasks.Task InvokeOnMainThreadAsync (System.Action action);
System.Threading.Tasks.Task InvokeOnMainThreadAsync (System.Func<System.Threading.Tasks.Task> funcTask);
System.Threading.Tasks.Task<T> InvokeOnMainThreadAsync<T> (System.Func<System.Threading.Tasks.Task<T>> funcTask);
System.Threading.Tasks.Task<T> InvokeOnMainThreadAsync<T> (System.Func<T> func);

Related PR: https://github.com/xamarin/Xamarin.Forms/pull/5028

NOTE: the PR had some bug that was fixed in v4.2, so don't use this in v4.1.

Answer 3

Dropping this here in case someone wants to await the end of an action which has to be executed on the main thread

public static class DeviceHelper
{
    public static Task RunOnMainThreadAsync(Action action)
    {
        var tcs = new TaskCompletionSource<object>();
        Device.BeginInvokeOnMainThread(
            () =>
            {
                try
                {
                    action();
                    tcs.SetResult(null);
                }
                catch (Exception e)
                {
                    tcs.SetException(e);
                }
            });

        return tcs.Task;
    }

    public static Task RunOnMainThreadAsync(Task action)
    {
        var tcs = new TaskCompletionSource<object>();
        Device.BeginInvokeOnMainThread(
            async () =>
            {
                try
                {
                    await action;
                    tcs.SetResult(null);
                }
                catch (Exception e)
                {
                    tcs.SetException(e);
                }
            });

        return tcs.Task;
    }
}