关于属性动画的注意事项：
This method is accessed from an ObjectAnimator so it should be annotated with
@Keep to ensure that it is not discarded or renamed in release builds (The method
referenced here (setSweep) has not been annotated with @Keep which means it
could be discarded or renamed in release builds)
使用属性动画的时候，实际上使用反射的方式调用方法（比如： sweep 属性，是反射调用了setSweep方法，所以需要向这个方法增加@Keep，防止被混淆）
Paint#Shader 的方法的时候， 我在onLayout 中设置一次， 会使得stroke 变小。
所以每次onDraw方法都调用一次
