Event mapping in MonoTouch

When we were designing the MonoTouch APIs we had to map some of the constructs used by CocoaTouch and Objective-C to what C# developer expect.

Like other modern toolkits, the behavior of a control can be customized by the programmer by hooking up to events and the responses to those events.

This is a common pattern used in other toolkits. In Gtk+ objects emit signals, and the programmer can connect event handlers to individual signals, for example this is how you would connect

void pause_player ()
{
	// pause the player.
}

void configure_pause ()
{
	button = gtk_button_new_with_label ("pause");
	gtk_signal_connect (button, "clicked", pause_player, NULL);
}
	

In the ECMA CLI world developers hook up to events by connecting to events directly in the object, it is similar in spirit:

void configure_pause ()
{
	button = new Button ("Pause");
	button.Clicked += delegate {
		// Pause the player here.
	};
}

In both the Gtk+ and the ECMA CLI worlds there can be multiple subscribers to the event.

In CocoaTouch, instead of having an event per action they use a pattern where the components emits all of its messages to an instance of an object. If a button were to emit messages like "clicked", "clicked_violently" and "caption_changed" all of those messages will be sent to an object that implemented the required interface.

I am going to give myself a literary license and use some terms loosely and write a little bit of imaginary C#.

A button implementation would look like this:

interface ButtonDelegate {
	optional void clicked ();
	optional void clicked_violently ();
	optional void caption_changed ();
}

class Button {
	public ButtonDelegate Delegate { get; set; }

	public Button (string text) { ... }

	public string Text {
		get { return text; }
		set {
			text = value;
			DoSomeRepainting ();
			Delegate.caption_changed ();
		}
	}

	public EventHandler ()
	{
		...
		if (event_this_is_the_user_clicking){
			// Send the message to the Delegate
			Delegate.clicked ();
		}
		

The user of the Button class then needs to implement the interface ButtonDelegate to respond to messages. This in Objective-C is made simple by allowing this interface to have optional methods.

This would not work with C# interfaces as those require that all the methods on the interface are implemented.

So we came up with a design that would blend those worlds together.

Supporting the Objective-C approach

We can support the Objective-C model where a class can satisfy the contract by just registering methods to respond to the actions. A user could respond to some events like this:

class Foo : NSObject { 
	void ConfigureButton ()
	{
		button = new Button ("pause");
		button.WeakDelegate = this;
	}
	
	[Export ("clicked")]
	void pause_clicked ()
	{
		// Pause the button.
	}
}

In MonoTouch we expose the "WeakDelegate" property as the weakly-typed version of the delegate, that means that you can assign to it anything that derives from the NSObject class, and respond to messages by just annotating your method with the [Export ("name")] attribute.

But weakly typing could introduce errors. What if the parameters of the method or the return value do not match the actual signature of the selector. If the developer introduce a typo, the compiler would not be able to point this out, and you would get a nice crash or some corrupt memory.

So we introduced strongly typed delegates, these require the programmer to implement a class and override methods that have a very specific signature. The code would then look like this:

class Foo : ButtonDelegate {
	void ConfigureButton ()
	{
		button = new Button ("pause");
		button.Delegate = this;
	}

	// strongly typed: override ensures that this method exists in
	// the base class, or the contract is not satisfied. 
	public override void pause_clicked ()
	{
		// Pause the button.
	}
}

The ButtonDelegate class is flagged with a special attribute, the [Model] attribute which informs the MonoTouch runtime that only methods that are overwritten from this class will be dispatched, any other methods will be considered optional and treated like optional methods.

The problem of course is that if you wanted to respond to multiple buttons, you would have to actually distinguish them somewhere in your pause_clicked with some sort of closure value, or use helper classes, one for each button that you want to respond to. But that is a minor detail.

So Objective-C developers that like the separation between model, view and controller can continue to use those patterns.

That being said, although there is a lot of literature and discussion about the clean separation of Models, Views and Controllers, most samples I run across are a hodgepodge of Controller code for all sorts of things. At least the production code and the samples I have seen make it obvious that the separation of concerns is mostly academic, and in practice an instance object becomes a general purpose responder to all events, very much like you see in Gtk+, Qt, or Winforms.

Our approach also has a limitation: the hodgepodge approach does not really work with the strongly-typed signatures as you can only derive from one class at a time. The solution is to use separate classes for each controller. Although that cleans things up and has some aesthetics associated with it in terms of clean separation of concerns, in my opinion, it is not very practical.

Supporting the C# style

With MonoTouch we wanted to preserve the C# style of programming where you could attach code to these notifications one-by-one.

With C# you can use anonymous methods or lambda functions. These are incredibly powerful as they are able to capture the environment in which they were created. This means that you can trivially pass information and share information between multiple handlers.

Additionally, you more than one method can "subscribe" to the same event, you are not limited to a single method being the receiving of the events.

This means that in MonoTouch you configure buttons like this:

void configure_button ()
{
	button = new UIButton (dims) { Text = "Pause" };
	button.Clicked += delegate {
		// Pause the video
	};
}

What MonoTouch does internally is that the first time that you attach to a C#-like event, it create an instance of the ButtonDelegate that merely raises the event in the C# style.

This means that you only pay for the memory usage if you use the feature. But when you do, you can subscribe to individual events and you can have more than one listener attached to the event.

The Pattern

Although we brought the pattern to most places in MonoTouch where a delegate was used, it is notably missing from the UITableView as we felt that the number of methods that had to be overwritten were too big for the model to make sense.

In a few instances like UITableView, we suggest that developers just use the strongly typed version of the delegate classes and override the methods accordingly.

One of the things that I would like to see is a UITableView derived class that can present the data and style the data entirely based on properties discovered at runtime with System.Reflection.

Another thing that I want to see is support for System.Data data binding style versions of the UITableView and other UI controls in a single page.

More details on the low-level implementation are available on the API design document on the web site.

Posted on 15 Oct 2009 by Miguel de Icaza
This is a personal web page. Things said here do not represent the position of my employer.