onDraw方法中创建RectF对象，影响性能，建议优化。

[BiJim]

申明为成员变量，然后调用set方法，动态修改RectF属性。
public void set(float left, float top, float right, float bottom) {
this.left = left;
this.top = top;
this.right = right;
this.bottom = bottom;
}