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July 18, 2005
[Flash Platform] Delegates are not just for Events!
Some of you may already be familiar with the usefulness of mx.util.Delegate for events. However, you may not have realized their full potential!
Delegates are great, they resolve the scoping issue related to how functions are scoped to the invoking object. However, most examples I’ve seen using Delegates use them in events:
someComponent.addEventListener(this,Delegate.create(this, this.someMethod));
However, they can be most useful for solving other technical design issues as well:
Callbacks:
When you don’t need an event to broadcast itself to N number of listeners you can use a simple callback which will only respond to 1 object. Most callback implementations I’ve seen (and have written myself) look something like this:
class SomeClass
{
private var m_callbackObject:Object;
private var m_callbackMethod:String;
function setCallback(obj:Object, method:String):Void
{
this.m_callbackObject = obj;
this.m_callbackMethod = method;
}
function doCallback():Void
{
this.m_callbackObject[this.m_callbackMethod]();
}
}
// code on timeline
function sayHi(){
trace(“Hi!”);
}
var myClass = new SomeClass();
myClass.setCallback(this,”sayHi”);
myClass.doCallback();
// output
Hi!
While this works it is not ideal. Using a Delegate here could greatly reduce the amount of code (by 50%) which is especially useful when dealing with multiple callbacks:
import mx.utils.Delegate;
class SomeClass
{
private var m_callback:Function;
function setCallback(obj:Object, method:Function):Void
{
this.m_callback = Delegate.create(obj, method);
}
function doCallback():Void
{
this.m_callback();
}
}
NOTE: Delegate takes a reference to a function for the second parameter
Now you might find yourself going back to the first example and asking why you couldn’t use a single reference and eliminate the Delegate:
this.m_callback = obj[method];
The above code will invoke the function but that function will be scoped to SomeClass! The Flash scoping rule comes into play here because SomeClass executed the callback, that function gets scoped to the instance of SomeClass which is what Delegate resolves.
Event Handlers
Another common problem Delegate resolves is when dealing with Event Handlers such as MovieClip events, Buttons events, etc:
function sayHi(){
trace(“Hi!”);
}
function someFunction(){
this.sayHi();
}
myMovieClip.onPress = this.someFunction;
// User
myMovieClip Pressed
// output
Nothing, the “this” keyword is scoped to myMovieClip.
The above implementation shows bad design because someFunction is getting scoped to myMovieClip. So when someFunction tries to invoke sayHi it is looking for the sayHi method on myMovieClip which does not exisit. Also note that the compiler doesn’t apply these types of scoping rules so no error is ever thrown.
Another implantation I have seen (and used myself) is:
function sayHi(){
trace(“Hi!”);
}
myMovieClip.onPress = function ()
{
this._parent.sayHi();
}
This example IMO is better then the first but I am manually resolving the scope which assumes that the required function will always be on _parent. The compiler doesn't check the scope here either so it essentially ignores any reference to _parent. Using a Delegate however is a much more flexible (and compile safe) option which I find to be more readable and consistent to how other scope related issues are dealt with (consistency == good):
myMovieClip.onPress = Delegate.create(this,this.someFunction);
Also, in any of the above examples you could do some neat optimization tricks as well:
var myDelegate = Delegate.create(this,this.someFunction);
myMovieClip.onPress = this.myDelegate;
myMovieClip2.onPress = this.myDelegate;
myMovieClip3.onPress = this.myDelegate;
Some food for thought =)
-erik
Posted by erikbianchi at July 18, 2005 08:04 AM
Comments
You can also use Delegate with setInterval, instead of:
setInterval(this, "method", 1000);
You can use:
setInterval(Delegate.create(this, method), 1000);
The class form of setInterval has always acted flaky for me and the second parameter is not checked by the compiler, and using Delegate solves both issues. I have a tutorial on Delegate at actionscript.org that covers basics and advanced techniques:
http://www.actionscript.org/tutorials/beginner/the_delegate_class/index.shtml
As far as callbacks go, you're probably making this a bit overly complicated. First of all if you use EventDispatcher you have a backdoor that is always available as (type + "Handler"). Meaning:
myBtn.addEventListener('click', Delegate.create(this, doSomething));
Can also be written as:
myBtn.clickHandler = Delegate.create(this, doSomething);
There's another backdoor in EventDispatcher also that I can't remember the name of but just look at the code in mx.events.EventDispatcher.
Also if you want to implement straight callbacks, in the dispatcher class, use for example:
public var onMyEvent:Function;
function whenIllBeDispatchingMyEvent()
{
this.onMyEvent();
}
And in the listener:
inst.onMyEvent = Delegate.create(this, whatnot);
Just like you would with MovieClip.onEnterFrame on XML.onLoad.
Posted by: Patrick Mineault at July 18, 2005 10:16 AM
Hey Patrick,
In regards to callbacks I was referring to instances where you don't want to deal with EventDispatcher. For me, using events are for 1 - many relationships. Where as callbacks are for 1 to 1 relationships. I think for just callbacks using the EventDisptatcher is a little over kill. Plus, when using a callback it clearly communicates to other developers that this is a 1 - 1 relationship.
With your second example for me it's just a matter of preference. In my own classes I like to use methods just in case I need to do or check something when the user sets the callback.
=)
-erik
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