覆盖onDraw时Android FrameLayout出现问题

Question

protected void onDraw(Canvas canvas) {
            float width = canvas.getWidth();
            float height = canvas.getHeight();
            Path path = new Path();
            path.addArc(new RectF(0, 0, width, height), 90, 180);
            canvas.clipPath(path);
            Paint paint = new Paint();
            paint.setAntiAlias(true);
            paint.setColor(Color.RED);
            canvas.drawRect(new RectF(0, 0, width, height), paint);
}

我使用此代码覆盖其他视图;没关系（显示半圆形）; 但是当我用它来覆盖FrameLayout时，给我看一个矩形？！

为什么呢？ 我该如何修改它？