invalidate() on custom View does not cause onDraw(Canvas c) to be called

Question

For my activity i use 3 custom views stacked. the lower one is a SurfaceView fullscreen, the middle one is derived from GridView and overrides onDraw to add custom graphic.

The top one is derived directly from View, sits in a corner of the screen and act as a knob to control the others two views (this is the problematic view).

to add a custom animation to this view, i used this pattern:

public class StubKnobView extends View{

    private Drawable knob;
    private Point knobPos;
    private float rotation;
    private boolean needsToMove;

    public void moveKnob(){
            /* ...various calculations... */
            this.needsToMove=true;
            invalidate();   //to notify the intention to start animation
    }

    private void updateAnimation(){
        /* ........... */
        this.needsToMove= !animationComplete;
        invalidate();        
    }

    @Override
    protected void onDraw(Canvas canvas) {
        int saveCount=canvas.getSaveCount();
        canvas.save();
        canvas.rotate(this.rotation, this.knobPos.x, this.knobPos.y );
        this.knob.draw(canvas);
        canvas.restoreToCount(saveCount);
        if(this.needsToMove){
            updateAnimation();
        }
    }
}

ideally, if there is an animation pending, after each drawing cycle the view should auto invalidate.

right now this doesn't work, to force the animation i have to touch the screen to cause a onDraw cycle. Using "show screen updates" of Dev tools I see that no screen invalidate/update cycle happen , apart when i click the screen. specifying the dirty rect also ha no effect.

So, any idea where to look to know why this invalidate/draw cycle does not work the way is intended?

Answer 1

I encontered this situation, and also doubted that invalidate() doesn't make onDraw() called again.

After simplified the business codes, it's turn out that there is a dead loop - exactly too much iterations, which blocks the UI thread.

So, my advice is that make sure the UI thread going smoothly first.