Flex: Help me understand data Binding on getters and setters

Question

Help me understand data Binding
When I create a variable in a class:

[Bindable] private var _name:String;

and then generate getters and setters, I get:

            private var _name:String;

            [Bindable]
            public function get name():String
            {
                return _name;
            }

            public function set name(value:String):void
            {
                _name = value;
            }

Why does it generate the tag '[Bindable]' only on the get function?
To me, it would seem that it should be on the set function, as I would want to know when the value changes, not when the value is just read.

Answer 1

What might help to understand what is going on here is the code that the MXML compiler will generate for you when you make something [Bindable]. The MXML compiler wraps your [Bindable] property in it's own getter/setter. It does this so that the wrapper setter method can dispatch a "propertyChange" event when a new value is set. This event notifies the parties binding to the property that the value has changed.

Getters/setters in Actionscript are considered to be properties of the object (they are not methods of the object). So it doesn't matter whether your annotate the getter or the setter as [Bindable], the generated code does the right thing.

It's worth noting that you can avoid the generated code and optimize the situation by dispatching your own event when your property changes. To do this, your [Bindable] metadata tag needs to include the event name that will be dispatched when the property changes:

private var _name:String;

[Bindable("nameChanged")]
public function get name():String
{
    return _name;
} 

public function set name(value:String)
{
    if (_name == value)
        return;
    _name = value;
    dispatchEvent(new Event("nameChanged"));
}

Because the bindable metadata contains an event string, no extra code is generated. Note, the compiler won't warn you if you forget to dispatch the event from the setter. In fact, you can dispatch your custom binding event from anywhere in your class (this can be useful with functions that are bindable).