在自定义视图中，只有alpha动画不起作用

Question

我制作自定义视图。我也做了我的自定义视图（制作圆圈）。但在我的自定义视图中，Alpha动画只是不起作用....

我的自定义视图扩展了linearlayout

public class mylayout extends FrameLayout implements View.OnClickListener {
......
    public mylayout(Context context){
        super(context);
        init(context);
    }
    void init(Context context){
        this.setLayoutParams(new 
            LinearLayout.LayoutParams(LayoutParams.FILL_PARENT,
            LayoutParams.FILL_PARENT));
        mButton = new Button(getContext());
        mButton.setText("Delete all Created Circle.");
        mButton.setId(10);
        mButton.setOnClickListener(this);
        bitmap = BitmapFactory.decodeResource(context.getResources(),  
            R.drawable.change_sea01);

        this.addView(mButton,new 
            LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, 
            LayoutParams.WRAP_CONTENT));        
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        // TODO Auto-generated method stub
        if(event.getAction() == MotionEvent.ACTION_DOWN) {
            this.addView(new MyCircleView(getContext(), (int)event.getX(),
                (int)event.getY()));    
            return true;
        } else if(event.getAction() == MotionEvent.ACTION_MOVE) {
            this.addView(new MyCircleView(getContext(),(int)event.getX(), 
                (int)event.getY()));
            return true;
        }
        return super.onTouchEvent(event);
    }

这是myCircle Class，它在触摸点（x，y）

上形成圆圈

class MyCircleView extends ImageView{
    .............
    public MyCircleView(Context context, int xPos, int yPos) {
        super(context);
        pnt = new Paint();
        pnt.setAntiAlias(true);

        bDelete = false;
        bmpCircle = BitmapFactory.decodeResource(context.getResources(), 
            R.drawable.img_click);
        this.xPos = xPos;
        this.yPos = yPos;
        xWidth = bmpCircle.getWidth()/2;
        yWidht = bmpCircle.getHeight()/2;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if(setAnimation == null){
            createAnimation(canvas);            
        }
        canvas.drawBitmap(bmpCircle, xPos - xWidth, yPos - yWidht, pnt);
    }

    public void createAnimation(final Canvas canvas) {
        AlphaAnimation alpha;
        ScaleAnimation scale;
        alpha = new AlphaAnimation(1.0f, 0.0f);
        scale = new ScaleAnimation(1, 2, 1, 2, Animation.ABSOLUTE, xPos- xWidth,
            Animation.ABSOLUTE, yPos - yWidht);
        setAnimation = new AnimationSet(false);
        setAnimation.addAnimation(alpha);
        setAnimation.addAnimation(scale);
        setAnimation.setDuration(3000);
        startAnimation(setAnimation);        
    }
}