moock>> lecture slide

/5/ code structure: using movieclip subclasses

classes in GUI programming often want to display content on screen
visual objects such as a dialog box or pull-down menu are often best represented as MovieClip subclasses
Flash MX components are movie clip symbols that typically:
- allow properties to be set at authoring time via the component parameters panel
- subclass MovieClip
- contain MovieClip subclass code on frame 1 of the symbol, inside an #initclip block
when creating applications that are entirely generated at runtime:
- setting properties via the component parameters panel is not a concern
- separation of class code from Library symbol is often desirable
here's how to create and use a MovieClip subclass (not a component) for runtime use only:
- write the class, store it in ClassName.as (change ClassName to the class's actual name)
- in ClassName.as, remember to extend MovieClip, like this:
```
ClassName.prototype = new MovieClip();
```
- on frame 1 of the corresponding Library symbol, register the symbol with the class, in an #initclip block:
```
#initclip
Object.registerClass("ClassName", ReferenceToClass);
#endinitclip
```
- In the Library, select Linkage for the corresponding Library symbol
- under Linkage..., choose "Export for ActionScript", with a linkage identifier of "ClassName"
- under Linkage, make sure "Export in first frame" is not selected
- then, on the main timeline of the .fla that contains the symbol, first load the class (either using #include or loadMovie())
- on a later frame, place a dummy instance of the symbol on Stage (this loads the symbol and registers it)
- on a later frame, attach instances of the symbol to the Stage with attachMovie()
what about passing arguments to the MovieClip subclass constructor?
attachMovie() does not support passing args directly
an attachMovie() "initObj" can be used to transfer properties to the new clip instance
where possible, i prefer not to use initObj because:
- it clutters the clip with properties that may not be needed after construction
- it shifts the responsibility of property initialization to the instance creation mechanism, whereas properties should be initialized by the constructor
- it means there's no list of parameters in the class constructor. the users of the class are left to guess what args the constructor expects
for runtime-only classes, i use an init() method instead