why my invoke of Canvas.drawText() just doesn't work

Question

Hi all: I'm writing a class that inherit from TextView, and override its onDraw() method, but in the method, my invoke of canvas.drawText() doesn't seems to work, the code just like below:

protected void onDraw(Canvas canvas) {
    // super.onDraw(canvas);
    Paint paint = new Paint();
    paint.setColor(android.graphics.Color.WHITE);
    paint.setTextSize(20);

    String text = "hello";
    canvas.drawText(text, 0, 0, paint);
}