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协程

协程是一个executed partially，遇到条件（ yield return 语句）会挂起，直到条件满足才会被唤醒继续执行后面代码的函数。

协程的作用: 控制代码在特定的时机执行 .

  1. 延时（等待）一段时间执行代码
  2. 等某个操作完成之后再执行后面的代码

协程不是线程，也不是异步执行的。协程和 MonoBehaviour的Update函数一样,也是在MainThread中执行的。使用协程你不用考虑同步和锁的问题。

Unity在每一帧（Frame）都会去处理对象上的协程, 主要是在Update后去处理协程（检查协程的条件是否满足），但也有写特例：

协程跟Update()其实一样的，都是Unity每帧对会去处理的函数（如果有的话）。如果MonoBehaviour 是处于激活（active）状态的而且yield的条件满足，就会协程方法的后面代码。

注: 图是从UnityGems.com上得来的，
上面显示如果yield return null, 协程会在同一帧被唤醒, 
D.S.Qiu用的是Unity 4.3.4f1 进行测试的。
经过测试验证，协程至少是每帧的LateUpdate()后去运行。

MonoBehaviour.enabled = false 协程会照常运行,但 gameObject.SetActive(false) 后协程却全部停止，即使在Inspector把 gameObject 激活还是没有继续执行.

也就说协程虽然是在MonoBehvaviour启动的（StartCoroutine）但是协程函数的地位完全是跟MonoBehaviour是一个层次的，不受MonoBehaviour的状态影响，但跟MonoBehaviour脚本一样受gameObject 控制，也应该是和MonoBehaviour脚本一样每帧“轮询” yield 的条件是否满足。

yield 后面可以有的表达式：

表达式 说明
null the coroutine executes the next time that it is eligible
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 calculated
WaitForSeconds causes the coroutine not to execute for a given game time period
WWW waits for a web request to complete (resumes as if WaitForSeconds or null)
Another coroutine in which case the new coroutine will run to completion before the yielder is resumed
number the total frames to wait

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IEnumerator（迭代器），IEnumerator 接口有两个方法 Current 和 MoveNext().

我们可以不使用 StartCoroutine 而是手动调用，或切换协程的执行。

下面是一个例子，DoHijack 是一个自动执行的协程, 它负责切换两个协程的执行

...
StartCoroutine(DoHijack());
...

IEnumerator DoHijack()
{
    while(true)
    {
        //Check if we have a current coroutine and MoveNext on it if we do
        if(_current != null && _current.MoveNext())
        {
            //Return whatever the coroutine yielded, so we will yield the
            //same thing
            yield return _current.Current;
        }
        else
            //Otherwise wait for the next frame
            yield return null;
    }
}

StartCoroutine

  1. public Coroutine StartCoroutine(IEnumerator routine);

The yield return value specifies when the coroutine is resumed.

StartCoroutine function always returns immediately:

    print("Starting " + Time.time);   // 0
    StartCoroutine(WaitAndPrint(2.0F));
    print("Before WaitAndPrint Finishes " + Time.time // 0 );

however you can yield the result. This will wait until the coroutine has finished execution (完全执行完，而不是 yield return ) .

    IEnumerator Start() {
        print("Starting " + Time.time);  // 0
        yield return StartCoroutine(WaitAndPrint(2.0F));
        print("Done " + Time.time);  // 2
    }
    IEnumerator WaitAndPrint(float waitTime) {
        yield return new WaitForSeconds(waitTime);
        print("WaitAndPrint " + Time.time);
    }
  1. public Coroutine StartCoroutine(string methodName, object value = null);

StartCoroutine using a string method name allows you to use StopCoroutine with a specific method name.

The downside is that the string version has a higher runtime overhead to start the coroutine and you can pass only one parameter.