Question

Does the actionscript preloader have to extend MovieClip?

...
public class Preloader extends MovieClip 
{

    public function Preloader() 
    {
        if (stage) {
            stage.scaleMode = StageScaleMode.NO_SCALE;
            stage.align = StageAlign.TOP_LEFT;
        }
        addEventListener(Event.ENTER_FRAME, checkFrame);
        loaderInfo.addEventListener(ProgressEvent.PROGRESS, progress);
        loaderInfo.addEventListener(IOErrorEvent.IO_ERROR, ioError);

        // TODO show loader
    }
...

Or can it also extend Sprite?

Answer 1

A preloader can only extend Sprite if it’s actually loading a separate SWF. If you make an SWF with a built-in preloader, you need two frames, because Flash player loads frames sequentially, so it’s the only way you can load and display a part of your SWF, which is required for a preloader to work. And for those frames you need a MovieClip, Sprites don’t have frames.