MACROMEDIA FLEX-CREATING ADVANCED COMPONENTS, Manual

12

Creating Advanced Components

Implementing the layoutChildren() method

The 

layoutChildren()

 method positions subobjects within the confines set by the 

UIObject.layoutWidth

 and 

UIObject.layoutHeight

 properties of your component. Each 

component should implement this method. Use the 

layoutChildren()

 method rather than the 

size()

 method, which is used primarily by Macromedia Flash designers to perform the same 

function.

Use the 

UIObject.width

 and 

UIObject.height

 properties of the child controls when changing 

the size of the children. These properties are scaled for use inside the component. Use the 

layoutWidth

 and 

layoutHeight

 properties when changing the size of the component itself. 

These properties are not scaled, because the component is at the top level.

The 

layoutChildren()

 method does not update the screen unless you call an invalidation 

method. Flex only calls the 

layoutChildren()

 method if the 

invalidateLayout()

 method was 

previously called. For more information, see 

“About invalidation” on page 17

.

The following example checks for the value of the 

labelPlacement

 property and lays out the 

mode_mc object accordingly:

function layoutChildren():Void {

text_mc.setSize(layoutWidth - mode_mc.width, layoutHeight);

if (labelPlacement == "left")
{

mode_mc.move(layoutWidth - mode_mc.width, 0);
text_mc.move(0, 0);

}
else {

mode_mc.move(0, 0);
text_mc.move(mode_mc.width, 0);

}

}

Implementing the draw() method

The 

draw()

 method displays objects on the screen. Whenever the component needs to draw an 

interface element, it calls a 

draw()

 method. You use the 

draw()

 method to create or modify 

elements that are subject to change.

Everything is made visible in the 

draw()

 method. A border does not actually call the drawing API 

until its 

draw()

 method is called. Any graphical assets that you bring in for the purposes of 

measuring are invisible until the 

draw()

 method is called.

Do not call the 

draw()

 method directly. Instead, call one of the invalidation methods, which then 

calls the 

draw()

 method. Flex also calls the 

draw()

 method from the 

redraw()

 method. 

However, Flex calls the 

redraw()

 method only if the object is invalidated, so you should actually 

call an invalidation method if you want Flex to invoke the 

draw()

 or 

redraw()

 method. If you 

do not call an invalidation method, the component remains invisible unless you set its visibility 
property to 

true

 in MXML. For more information, see 

“About invalidation” on page 17

.

Flex also calls the 

draw()

 method after the 

layoutChildren()

 method.