Can somebody please explain this common binding pitfall to me? (using the wrong bindable event name)

Question

I refer to this site link text

Using the wrong event name in the [Bindable] tag can cause your application to not bind your property, and you will not even know why. When you use the [Bindable] tag with a custom name, the example below looks like a good idea:

public static const EVENT_CHANGED_CONST:String = "eventChangedConst"; 
private var _number:Number = 0; 
[Bindable(event=EVENT_CHANGED_CONST)] 
public function get number():Number 
{ 
  return _number; 
} 
public function set number(value:Number) : void 
{ 
  _number = value; 
  dispatchEvent(new Event(EVENT_CHANGED_CONST)); 
}

The code above assigns a static property to the event name, and then uses the same assignment to dispatch the event. However, when the value changes, the binding does not appear to work. The reason is that the event name will be EVENT_CHANGED_CONST and not the value of the variable.

The code should have been written as follows:

public static const EVENT_CHANGED_CONST:String = "eventChangedConst"; 
private var _number:Number = 0; 
[Bindable(event="eventChangedConst")] 
public function get number():Number 
{ 
  return _number; 
} 
public function set number(value:Number) : void 
{ 
  _number = value; 
  dispatchEvent(new Event(EVENT_CHANGED_CONST)); 
}

I agree, the wrong example does look like a good idea and I would do it that way because I think it's the right way and avoids the possibility of a typing error. Why is the name of the constant used instead of it's value? Surely this can't be right?

I appreciate your insights

Answer 1

Because the standard Flex compiler isn't that clever at times... and I feel your pain! I've complained about this exact problem more than a few times.

If I remember correctly, it's because the compiler does multiple passes. One of the early passes changes the Metadata into AS code. At this point in the compiler it hasn't parsed the rest of the AS code, so its not capable of parsing Constants or references to static variables in other files.

The only thing I can suggest is sign up to the Adobe JIRA, vote for the bug, and hope that the compiler fixes in 4.5 bring some relief.