新手，怎么重绘RelativeLayout

新手，如何重绘RelativeLayout？
我自己写了个类继承RelativeLayout
public class ClientLayout extends RelativeLayout
{
public ClientLayout(Context context)
{
super(context);
}

public ClientLayout(Context context, AttributeSet attrs)
{
super(context, attrs);
}

@Override
protected void onDraw(Canvas canvas)
{
super.onDraw(canvas);
Paint paint=new Paint();
paint.setStyle(Style.FILL);
paint.setColor(Color.RED);
canvas.drawRect(0, 0, 100, 100, paint);
}
}
然后再Activity中创建。
  ClientLayout layout=new ClientLayout(this);
  layout.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT,LayoutParams.FILL_PARENT));
  setContentView(layout);
可是无法画出红色的矩形来啊，从View继承就可以画出来，这是为什么啊？

[解决办法]
@Override
protected void onDraw(Canvas canvas)
{
super.onDraw(canvas);
Paint paint=new Paint();
paint.setStyle(Style.FILL);
paint.setColor(Color.RED);
canvas.drawRect(0, 0, 100, 100, paint);
}

尝试把这段放在dispatchDraw里面。