A reusable pattern to convert event into task

Question

I\'d like to have a generic reusable piece of code for wrapping EAP pattern as task, something similar to what Task.Factory.FromAsync does for BeginXXX/EndXXX APM pattern.

Answer 1

It is possible with a helper class and a fluent-like syntax:

public static class TaskExt
{
    public static EAPTask> FromEvent()
    {
        var tcs = new TaskCompletionSource();
        var handler = new EventHandler((s, e) => tcs.TrySetResult(e));
        return new EAPTask>(tcs, handler);
    }
}


public sealed class EAPTask
    where TEventHandler : class
{
    private readonly TaskCompletionSource _completionSource;
    private readonly TEventHandler _eventHandler;

    public EAPTask(
        TaskCompletionSource completionSource,
        TEventHandler eventHandler)
    {
        _completionSource = completionSource;
        _eventHandler = eventHandler;
    }

    public EAPTask WithHandlerConversion(
        Converter converter)
        where TOtherEventHandler : class
    {
        return new EAPTask(
            _completionSource, converter(_eventHandler));
    }

    public async Task Start(
        Action subscribe,
        Action action,
        Action unsubscribe,
        CancellationToken cancellationToken)
    {
        subscribe(_eventHandler);
        try
        {
            using(cancellationToken.Register(() => _completionSource.SetCanceled()))
            {
                action();
                return await _completionSource.Task;
            }
        }
        finally
        {
            unsubscribe(_eventHandler);
        }
    }
}

Now you have a WithHandlerConversion helper method, which can infer type parameter from converter argument, which means you need to write WebBrowserDocumentCompletedEventHandler only one time. Usage:

await TaskExt
    .FromEvent()
    .WithHandlerConversion(handler => new WebBrowserDocumentCompletedEventHandler(handler))
    .Start(
        handler => this.webBrowser.DocumentCompleted += handler,
        () => this.webBrowser.Navigate(@"about:blank"),
        handler => this.webBrowser.DocumentCompleted -= handler,
        CancellationToken.None);