Android : Canvas not update even I called invalidate()?

I want to create a custom view that animates some lines. so I created a class that extends View class. I called onDraw() & drawn on the canvas. Here are some codes that I've approached so far.

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    p += 10;
    canvas.drawLine(5, 5, p, 5, mPaint);
    invalidate();
}

note that p & mPaint are instantiated at the constructor.

but, even I called invalidate() method, it doesn't update the canvas, i.e. doesn't animate the line.

so, how to solve this?