Debugging event handlers in MXML gotchas

Everybody knows how difficult to debug event handlers in MXML attributes. So if we try to debug creationComplete event of our simple application with Flex Builder’s debugger we’ll fail:

XML:

<?xml version="1.0" encoding="utf-8"?> 
  <mx:Application  
    creationComplete="helloLabel.text = 'Hello, ' + event.currentTarget" 
    xmlns:mx="http://www.adobe.com/2006/mxml"  
    layout="vertical"> 
    <mx:Label 
      id="helloLabel" /> 
  </mx:Application>

Lets set breakpoint at line 3 and… we can debug Application’s addEventListener (namely Container’s) for subscribing to creationComplete event. It’s not that we need :no:

To debug inline MXML handlers we can use the other way of declarative expressiveness. Lets transform attribute to node:

XML:

<?xml version="1.0" encoding="utf-8"?> 
  <mx:Application  
    xmlns:mx="http://www.adobe.com/2006/mxml"  
    layout="vertical"> 
    <mx:creationComplete> 
      <![CDATA[ 
        helloLabel.text = 'Hello, ' + event.currentTarget; 
      ]]> 
    </mx:creationComplete> 
    <mx:Label 
      id="helloLabel" /> 
  </mx:Application>

Just set breakpoint at line 7 and voilà. We can examine event’s object properties in debugger’s Variable View.

The only problem is intellisense for event property, event object type (and properties) etc. in Flex Builder (tested in FB 3 beta 3). As the final release of Flex Builder will be very soon I’m looking forward for this behavior will be fixed.

Hope it helps!

Special thanks to Eldar ‘Pirrest’ Prilutskiy for pointing me this.

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This entry was posted on February 22nd, 2008 at 06:42:09 pm and is filed under Flex, Flex Builder, Tips and tricks.

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3 comments

Comment from: Tom Chiverton [Visitor]

· http://rachaelandtom.info/blogs/falken

Well, if you used
creationComplete="completeHandler(event)"
and a suitable private function in a Script block, that would work too, and you could correctly type the argument to get insight on the event too.

02/22/08 @ 19:35

Comment from: Konstantin Kovalev [Member]

You're right, Tom. But it takes a lot of exertions to write handler in Script block with right event object type in handler signature and then pointing this handler in creationComplete attribute. It much more easier in RAD to put some handling code right in mxml...

	`<?xml version="1.0" encoding="utf-8"?>`
	`<mx:Application`
	`creationComplete="helloLabel.text = 'Hello, ' + event.currentTarget"`
	`xmlns:mx="http://www.adobe.com/2006/mxml"`
	`layout="vertical">`
	`<mx:Label`
	`id="helloLabel" />`
	`</mx:Application>`

	`<?xml version="1.0" encoding="utf-8"?>`
	`<mx:Application`
	`xmlns:mx="http://www.adobe.com/2006/mxml"`
	`layout="vertical">`
	`<mx:creationComplete>`
	`<![CDATA[`
	`helloLabel.text = 'Hello, ' + event.currentTarget;`
	`]]>`
	`</mx:creationComplete>`
	`<mx:Label`
	`id="helloLabel" />`
	`</mx:Application>`

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