as3 different between cacheAsBitmap vs CachedSprite - actionscript-3

Question

I have cacheAsBitmap = true
and the following class
package utility{
import flash.display.Bitmap;
import flash.display.BitmapData;
import flash.display.Sprite;
import flash.geom.Matrix;
import flash.geom.Rectangle;
import flash.utils.getQualifiedClassName;
public class CachedSprite extends Sprite {
//Declare a static data cache
protected static var cachedData:Object = { };
public var clip:Bitmap;
public function CachedSprite(asset:Class, centered:Boolean = false, scale:int = 2) {
//Check the cache to see if we've already cached this asset
var data:BitmapData = cachedData[getQualifiedClassName(asset)];
if (!data) {
// Not yet cached. Let's do it now
// This should make "Class", "Sprite", and "Bitmap" data types all work.
var instance:Sprite = new Sprite();
instance.addChild(new asset());
// Get the bounds of the object in case top-left isn't 0,0
var bounds:Rectangle = instance.getBounds(this);
// Optionally, use a matrix to up-scale the vector asset,
// this way you can increase scale later and it still looks good.
var m:Matrix = new Matrix();
m.translate(-bounds.x, -bounds.y);
m.scale(scale, scale);
// This shoves the data to our cache. For mobiles in GPU-rendering mode,
// also uploads automatically to the GPU as a texture at this point.
data = new BitmapData(instance.width * scale, instance.height * scale, true, 0x0);
data.draw(instance, m, null, null, null, true); // final true enables smoothing
cachedData[getQualifiedClassName(asset)] = data;
}
// This uses the data already in the GPU texture bank, saving a draw/memory/push call:
clip = new Bitmap(data, "auto", true);
// Use the bitmap class to inversely scale, so the asset still
// appear to be it's normal size
clip.scaleX = clip.scaleY = 1 / scale;
addChild(clip);
if (centered) {
// If we want the clip to be centered instead of top-left oriented:
clip.x = clip.width / -2;
clip.y = clip.height / -2;
}
// Optimize mouse children
mouseChildren = false;
}
public function kill():void {
// Just in case you want to clean up things the manual way
removeChild(clip);
clip = null;
}
}
}
Is there anyone can explain to me the different? Why do i need to implement this class rather than just use cacheAsBitmap = true? Thanks

Answer 1

To avoid redrawing the DisplayObject if moved, you can set the cacheAsBitmap property. If set to true, Flash runtimes cache an internal bitmap representation of the display object.
The cacheAsBitmap property is automatically set to true whenever you apply a filter to a display object. Best used with display objects that have mostly static content and that do not scale, rotate, or change alpha frequently, bitmap data must recalculated for all operations beyond horizontal or vertical movement.
Caching the bitmap yourself empowers control of the rendering lifecycle.
In your CachedSprite class, what's actually added to the display list is a Bitmap, versus adding your original display object. Any interaction with input devices must be applied to the cached sprite instance.

Answer 2

The main difference seems to be on this line:
var data:BitmapData = cachedData[getQualifiedClassName(asset)];
This class keeps a static reference to any previously cached bitmaps. If you have two instances of CachedSprite that are showing the same bitmap data (like a particle, for example) this class will only use one instance of BitmapData, saving memory.