Well, not literally, of course, but:
I’m new to Flex and I’m trying to figure out where to put the code that I want to run when my app starts. In my example, I have a tree control defined in the markup, and I have AS code that can populate it from a local file or a webservice. How do I bind the one to the other such that when the control is created, the code is run?
The examples I have found in the documentation all use event handlers, i. e. they put ‘creationComplete=’myInitFunction(evt)” in the control’s opening tag. That would mean I write a little glue function that initializes things.
Now, as I have understood it, the Markup is really converted into AS classes internally. So can I not just define a class that is instantiated by the engine and put my startup code in the constructor? Maybe even like, extending the Tree class?
EDIT:
Months later, I’m a lot wiser. After trying to abuse the constructors of extended component classes, I had to learn that Flex doesn’t expect the constructors to do much – certainly not loading data off the network.
The component life cycle explanation in ‘Programming Flex 3’ was really helpful. Now, my constructors are all empty, and I’m overriding the life cycle methods, such as createChildren for a lot of things, and use events for anything that involves latency or depends on externalities, especially network stuff.
For example, I use creationComplete to initiate an asynchronous network request (using HTTPService), when that returns there are ResultEvent and FaultEvent handlers, the ResultEvent updates a data model to which I data-bound my controls. This is seemingly very Flex-y and works great.
The creationComplete event (on the Application object) is dispatched when the Application component is finished being created. The applicationComplete event is dispatched once the entire application is complete (layout, etc.) and is visible to the user (loading screen is finished).
For the situation you mention — where you’re populating a control at startup — I’d suggest using the creationComplete event of the component containing the control you want to populate (the Tree in your case), which sounds like it’d be your Application component. This is generally a good practice as often this code will depend on sibling components of the one you’re populating (e.g., an HTTPService component).
You’re right that all MXML is transformed in AS3 which is then compiled. There’s a compiler flag that will preserve the AS3 that’s generated (it’ll be read-only, though). You don’t want to mess with that. And due to the (somewhat) complex nature of the UIComponent instantiation sequence (constructor, styling, layout, etc.) it is definitely best practice to populate controls in a creationComplete event (or similar).