OnBecameVisible 和 OnBecameInvisible ，OnWillRenderObject 只有在所挂物体（不包括子物体）有render才有效
//可见
private void OnBecameVisible()
{
_isVisible = true;
}
//不可见
private void OnBecameInvisible()
{
_isVisible = false;
}
MonoBehaviour.OnBecameVisible 当可见
function OnBecameVisible () : void
Description描述
OnBecameVisible is called when the renderer became visible by any camera.
当renderer(渲染器)在任何相机上可见时调用OnBecameVisible。
This message is sent to all scripts attached to the renderer. OnBecameVisible and OnBecameInvisible is useful to avoid computations that are only necessary when the object is visible.
这个消息发送到所有附在渲染器的脚本上。OnBecameVisible 和 OnBecameInvisible可以用于只需要在物体可见时才进行的计算。