Microsoft.Async.Transformations

A set of higher-order functions, which transform asynchronous functions of the form Func<CancellationToken, Task> into asynchronous functions with additional semantics.

For example, one of the transformations, Switch, creates an asynchronous function where each time it is called, it cancels the previous call.

Usage

Install the Nuget packages using your favorite NuGet tools:

• Microsoft.Async.Transformations
• Microsoft.Async.Transformations.Windows

As a matter of convenience, we've created both static methods and extension methods for most of the transformations where it made sense to do so. To use the extension methods, be sure to include the Microsoft.Async.Transformations namespace.The extension methods are available in Microsoft.Async.Transformations.AsyncTransformExtensions and the non-extension static methods are in Microsoft.Async.Transformations.AsyncTransform. In the example app and the code snippets below, we leverage the "Using Static" feature from C# 6.0 (see more here), i.e.:

using static Microsoft.Async.Transformations.AsyncTransform;

Example

The example included in Playground.App is a simple stopwatch application intended to demonstrate some of the interesting transformations.

The stopwatch has 3 basic functions:

1. A toggle switch to start and pause the stopwatch.
2. A reset button to reset the stopwatch.
3. A restart button to restart the stopwatch.

All of the functionality is implemented from a dead simple asynchronous function that waits one second, then calls a "tick" function, until the loop is cancelled by the input cancellation token. Here's the code:

public async Task RunAsync(CancellationToken token)
{
    while (!token.IsCancellationRequested)
    {
        try
        {
            await Task.Delay(s_interval, token);
            await _tickAsync(token);
        }
        catch (OperationCanceledException ex)
        {
            if (ex.CancellationToken != token)
            {
                throw;
            }
        }
    }
}

To implement the toggle switch, we simply used a transform on this RunAsync function a la:

var startStopCoreAsync = Identity(Context.RunAsync).Toggle();

Notice here we use the Identity function as syntactic sugar, albeit at the price of a wasted method call. The optimal way to write this code is simply:

var startStopCoreAsync = new Func<CancellationToken, Task>(Context.RunAsync).Toggle();

Or even:

Func<CancellationToken, Task> asyncFunc = Context.RunAsync;
var startStopCoreAsync = asyncFunc.Toggle();

But nonetheless, this is only for demo purposes and we'll use the Identity function throughout.

Now, the functionality of _tickAsync is to simply increment the current value of the Elapsed property on the context. The Elapsed property is hooked up via the System.ComponentModel.INotifyPropertyChanged and the binding mechanism in XAML, so we need to make sure we upodate the Elapsed property on a dispatcher thread. This is where the transformation from Microsoft.Async.Transformations comes in handy. We use the AsAsync transformation to treat the Tick method as an asynchronous one, then use the OnDispatcher transformation to ensure function is called from a dispatcher thread (assuming the caller to OnDispatcher is itself a dispatcher thread):

public StopwatchContext()
{
    _tickAsync = AsAsync(Tick).OnDispatcher();
    ...
}

...

private void Tick()
{
    Elapsed += s_interval;
}

We implement the Restart functionality using the same RunAsync method that the toggle button works with. Using the Switch transformation, the behavior of restart is to simply cancel the current call to RunAsync and start it over again (with the added behavior of "resetting" the Elapsed property prior to calling RunAsync).

Obviously, there are more efficient ways to implement restart (like allowing the RunAsync method to continue running while simply resetting the Elapsed property), but then we couldn't demo the Switch transformation.

The point is, there are many cases in XAML applications where you want to implement asynchronous functionality for toggle switches, or create buttons that simply restart any active asynchronous process, or create buttons that run only the first time the user presses them, or cancels other activities in the application before starting the expected activity. That is what the Microsoft.Async.Transformation library is designed for. The ultimate goal is to allow developers to write functional asynchronous behavior that is easier to reason about and more concise to develop.

Dependencies

• Rx.NET

Contributions

Contributions are awesome! If you want to help out, fix a bug, or get a feature fixed - check out the contributing guide.