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Lesson 2.2: Using ReceiveActor for Smarter Message Handling

In the first unit, you learned how to use the UntypedActor (docs) to build your first actors and handle some simple message types.

In this lesson we're going to show you how to use the ReceiveActor (docs) to easily handle more sophisticated types of pattern matching and message handling in Akka.NET.

Key Concepts / Background

Pattern matching

Actors in Akka.NET depend heavily on the concept of pattern matching - being able to selectively handle messages based on their .NET Type and/or values.

In the first module, you learned how to use the UntypedActor to handle and receive messages using blocks of code that looked a lot like this:

protected override void OnReceive(object message){
	if(message is Foo) {
		var foo = message as Foo;
		// do something with foo
	}
	else if(message is Bar) {
		var bar = message as Bar;
		// do something with bar
	}
	//.... other matches
	else {
		// couldn't match this message
		Unhandled(message);
	}
}

This method of pattern matching in Akka.NET works great for simple matches, but what if your matching needs were more complex?

Consider how you would handle these use cases with the UntypedActor we've seen so far:

  1. Match message if it's a string and begins with "AkkaDotNet" or
  2. Match message if it's of type Foo and Foo.Count is less than 4 and Foo.Max is greater than 10?

Hmm... if we tried to do all of that inside an UntypedActor we'd end up with something like this:

protected override void OnReceive(object message) {
	if(message is string
		&& message.AsInstanceOf<string>()
			.BeginsWith("AkkaDotNet")){
		var str = message as string;
		// do some work with str...
	}
	else if(message is Foo
			&& message.AsInstanceOf<Foo>().Count < 4
			&& message.AsInstanceOf<Foo>().Max > 10){
		var foo = message as Foo;
		// do something with foo
	}
	// ... other matches ...
	else {
		// couldn't match this message
		Unhandled(message);
	}
}

Yuck! There has to be a better way of doing this, right?

Yes, there is! Enter the ReceiveActor.

Introducing the ReceiveActor

The ReceiveActor is built on top of the UntypedActor and makes it easy to do sophisticated pattern matching and message handling.

Here's what that ugly code sample from a few moments ago would look like, rewritten with a ReceiveActor:

public class FooActor : ReceiveActor
{
    public FooActor()
    {
        Receive<string>(s => s.StartsWith("AkkaDotNet"), s =>
        {
            // handle string
        });

        Receive<Foo>(foo => foo.Count < 4 && foo.Max > 10, foo =>
        {
            // handle foo
        });
    }
}

Much better.

So, what's the secret sauce that helped us simplify and clean up all of that pattern matching code from earlier?

The secret sauce of ReceiveActor

The secret sauce that cleans up all that pattern matching code is the Receive<T> handler.

// this is what makes the ReceiveActor powerful!
Receive<T>(Predicate<T>, Action<T>);

A ReceiveActor lets you easily add a layer of strongly typed, compile-time pattern matching to your actors.

You can match messages easily based on type, and then use typed predicates to perform additional checks or validations when deciding whether or not your actor can handle a specific message.

Are there different kinds of Receive<T> handlers?

Yes, there are. Here are the different ways to use a Receive<T> handler:

1) Receive<T>(Action<T> handler)

This executes the message handler only if the message is of type T.

2) Receive<T>(Predicate<T> pred, Action<T> handler)

This executes the message handler only if the message is of type T AND the predicate function returns true for this instance of T.

3) Receive<T>(Action<T> handler, Predicate<T> pred)

Same as the previous.

4) Receive(Type type, Action<object> handler)

This is a concrete version of the typed + predicate message handlers from before (no longer generic).

5) Receive(Type type, Action<object> handler, Predicate<object> pred)

Same as the previous.

6) ReceiveAny()

This is a catch-all handler which accepts all object instances. This is usually used to handle any messages that aren't handled by a previous, more specific Receive() handler.

The order in which you declare Receive<T> handlers matters

What happens if we need to handle overlapping types of messages?

Consider the below messages: they start with the same substring, but assume they need to be handled differently.

  1. string messages that begin with AkkaDotNetSuccess, AND
  2. string messages that begin with AkkaDotNet?

What would happen if our ReceiveActor was written like this?

public class StringActor : ReceiveActor
{
    public StringActor()
    {
        Receive<string>(s => s.StartsWith("AkkaDotNet"), s =>
        {
            // handle string
        });

        Receive<string>(s => s.StartsWith("AkkaDotNetSuccess"), s =>
        {
            // handle string
        });
    }
}

What happens in this case is that the second handler (for s.StartsWith("AkkaDotNetSuccess")) is never invoked. Why not?

The order of the Receive<T> handlers matters!

This is because ReceiveActor will handle a message using the first matching handler, not the best matching handler and it evaluates its handlers for each message in the order in which they were declared.

So, how do we solve the above problem, where our handler for strings starting with "AkkaDotNetSuccess" is never triggered?

Simple: we fix this problem by making sure that the more specific handlers come first.

public class StringActor : ReceiveActor
{
    public StringActor()
    {
		// Now works as expected
        Receive<string>(s => s.StartsWith("AkkaDotNetSuccess"), s =>
        {
            // handle string
        });

        Receive<string>(s => s.StartsWith("AkkaDotNet"), s =>
        {
            // handle string
        });
    }
}

Where do I define message handlers in a ReceiveActor?

ReceiveActors do not have an OnReceive() method.

Instead, you must hook up Receive message handlers directly in the ReceiveActor constructor, or in a method called to by that constructor.

Knowing this, we can put ReceiveActor to work for us.

Exercise

In this exercise we're going to add the ability to add multiple data series to our chart, and we're going to modify the ChartingActor to handle commands to do this.

Step 1 - Add an "Add Series" Button to the UI

First thing we're going to do is add a new button called "Add Series" to our form. Go to the [Design] view of Main.cs and drag a Button onto the UI from the Toolbox. Here's where we put the button:

Adding a 'Add Series' button in Design view in Visual Studio

Double click on the button in the [Design] view and Visual Studio will automatically add a click handler for you inside Main.cs. That generated handler should look like this:

// automatically added inside Main.cs if you double click on button in designer
private void button1_Click(object sender, EventArgs e)
{

}

Leave this blank for now. We'll wire this button to our ChartingActor shortly.

Step 2 - Add an AddSeries Message Type to the ChartingActor

Let's define a new message class for putting additional Series on the Chart managed by the ChartingActor. We'll create the AddSeries message type to signify that we want to add a new Series.

Add the following code to ChartingActor.cs inside the Messages region:

// Actors/ChartingActor.cs, inside the #Messages region

/// <summary>
/// Add a new <see cref="Series"/> to the chart
/// </summary>
public class AddSeries
{
    public AddSeries(Series series)
    {
        Series = series;
    }

    public Series Series { get; private set; }
}

Step 3 - Have ChartingActor Inherit from ReceiveActor

Now for the meaty part - changing the ChartingActor from an UntypedActor to a ReceiveActor.

Now, let's change the declaration for ChartingActor. Change this:

// Actors/ChartingActor.cs
public class ChartingActor : UntypedActor

to this:

// Actors/ChartingActor.cs
public class ChartingActor : ReceiveActor

Remove the OnReceive method from ChartingActor

Don't forget to delete the current OnReceive method for the ChartingActor. Now that ChartingActor is a ReceiveActor, it doesn't need an OnReceive() method.

Step 4 - Define Receive<T> Handlers for ChartingActor

Right now our ChartingActor can't handle any messages that are sent to it - so let's fix that by defining some Receive<T> handlers for the types of messages we want to accept.

First things first, add the following method to the Individual Message Type Handlers region of the ChartingActor:

// Actors/ChartingActor.cs in the ChartingActor class
// (Individual Message Type Handlers region)

private void HandleAddSeries(AddSeries series)
{
    if(!string.IsNullOrEmpty(series.Series.Name) &&
    !_seriesIndex.ContainsKey(series.Series.Name))
    {
        _seriesIndex.Add(series.Series.Name, series.Series);
        _chart.Series.Add(series.Series);
    }
}

And now let's modify the constructor of the ChartingActor to set a Receive<T> hook for InitializeChart and AddSeries.

// Actors/ChartingActor.cs in the ChartingActor constructor

public ChartingActor(Chart chart, Dictionary<string, Series> seriesIndex)
{
    _chart = chart;
    _seriesIndex = seriesIndex;

    Receive<InitializeChart>(ic => HandleInitialize(ic));
    Receive<AddSeries>(addSeries => HandleAddSeries(addSeries));
}

NOTE: The other constructor for ChartingActor, ChartingActor(Chart chart) doesn't need to be modified, as it calls ChartingActor(Chart chart, Dictionary<string, Series> seriesIndex) anyway.

And with that, our ChartingActor should now be able to receive and process both types of messages easily.

Step 5 - Have the Button Clicked Handler for "Add Series" Button Send ChartingActor an AddSeries Message

Let's go back to the click handler we added for the button in Step 1.

In Main.cs, add this code to the body of the click handler:

// Main.cs - class Main
private void button1_Click(object sender, EventArgs e)
{
    var series = ChartDataHelper.RandomSeries("FakeSeries" +
        _seriesCounter.GetAndIncrement());
    _chartActor.Tell(new ChartingActor.AddSeries(series));
}

And that should do it!

Once you're done

Build and run SystemCharting.sln and you should see the following:

Successful Lesson 2 Output

NOTE: If you're following along using the eBook / .ePub, you won't see the animation. Click here to see it.

Compare your code to the code in the /Completed/ folder to compare your final output to what the instructors produced.

Great job!

Nice work, again. After having completed this lesson you should have a much better understanding of pattern matching in Akka.NET and an appreciation for how ReceiveActor is different than UntypedActor.

Let's move onto Lesson 3 - Using the Scheduler to Send Recurring Messages.

Any questions?

Come ask any questions you have, big or small, in this ongoing Bootcamp chat with the Petabridge & Akka.NET teams.

Problems with the code?

If there is a problem with the code running, or something else that needs to be fixed in this lesson, please create an issue and we'll get right on it. This will benefit everyone going through Bootcamp.