Monobehaviour的函数执行顺序图
yield后面可以加的表达式
- null - then coroutine executes the next time that it is eligible; 暂停协同程序，下一帧在继续执行
- WaitForEndOfFrame - the coroutine executes on the frame,after all of the rendering and GUI is complete
- WaitForFixedUpdate - causes this coroutine to execute at the next physics step, after all physics is calcuated
- WaitForSeconds - cause the corotine not to execute for a iven game time period
- WWW - waits for a web request to complete (resumes as if AwaiForSeconds or null)
- Another coroutine - in which case the new coroutine will run to completion before the yielder is resumed
值得注意的是WaitForSeconds()受Time.timeScale影响，当Time.timeScale = 0f时，yield return new WaitForSecond(x)将不会满足。
IEnumerator & Coroutine
协程其实就是一个迭代器，IEnumerator接口有两个方法Current和MoveNext(),只有当MoveNext()返回true时才会访问Current,否则回报错，迭代器犯法运行到yield return语句时，会返回一个expression表达式并保留当前代码中的位置。