title: why my invoke of Canvas.drawText() just doesn't work
body: <p>Hi all:
     I'm writing a class that inherit from TextView, and override its <code>onDraw()</code> method, but in the method, my invoke of <code>canvas.drawText()</code> doesn't seems to work, the code just like below:</p>
  
  <pre><code>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);
  }
  </code></pre>