yield return的作用

测试1：

　　

using UnityEngine;
using System.Collections;

public class test1 : MonoBehaviour {

    // Use this for initialization
    void Start () 
    {
        Debug.Log("in start " + Time.time.ToString());
        StartCoroutine(callYieldFunction()); //必须要用StartCoroutine
        Debug.Log("in start 2 " + Time.time.ToString());
    }
    
    // Update is called once per frame
    void Update () {
    
    }

    IEnumerator callYieldFunction()        
    {
        Debug.Log("callYieldFunction " + Time.time.ToString());
        //yield return StartCoroutine(callYieldFunction1());
        StartCoroutine(callYieldFunction1());
        yield return new WaitForSeconds(2);
        Debug.Log("callYieldFunction print after 2 seconds " + Time.time.ToString());
    }

    IEnumerator callYieldFunction1()        
    {
        Debug.Log("callYieldFunction1 " + Time.time.ToString());
        yield return new WaitForSeconds(2);
        Debug.Log("callYieldFunction print after 4 seconds " + Time.time.ToString());
    }
}

　　运行结果为

　　in start 0

　　callYieldFunction 0

　　callYieldFunction1 0

　　in start 2 0

　　callYieldFunction print after 4 seconds 2.004971

　　callYieldFunction print after 2 seconds 2.004971

　　

测试2：

　　

using UnityEngine;
using System.Collections;

public class test1 : MonoBehaviour {

    // Use this for initialization
    void Start () 
    {
        Debug.Log("in start " + Time.time.ToString());
        StartCoroutine(callYieldFunction()); //必须要用StartCoroutine
        Debug.Log("in start 2 " + Time.time.ToString());
    }
    
    // Update is called once per frame
    void Update () {
    
    }

    IEnumerator callYieldFunction()        
    {
        Debug.Log("callYieldFunction " + Time.time.ToString());
        yield return StartCoroutine(callYieldFunction1());
        //StartCoroutine(callYieldFunction1());
        yield return new WaitForSeconds(2);
        Debug.Log("callYieldFunction print after 2 seconds " + Time.time.ToString());
    }

    IEnumerator callYieldFunction1()        
    {
        Debug.Log("callYieldFunction1 " + Time.time.ToString());
        yield return new WaitForSeconds(2);
        Debug.Log("callYieldFunction print after 4 seconds " + Time.time.ToString());
    }
}

　　运行结果为：

　　in start 0

　　callYieldFunction 0

　　callYieldFunction1 0

　　in start 2 0

　　callYieldFunction print after 4 seconds 2.002947

　　callYieldFunction print after 2 seconds 4.007511

结果：

　　需要注意的是StartCoroutine(callYieldFunction());这句必须要加StartCoroutine，不然callYieldFunction()里面根本不会运行。

 1：StartCoroutine里面的函数是个闭包，主线程运行这个闭包，直到yield return，闭包中剩下的代码不运行，跳出闭包，运行主线程别的代码；
 2：主线程在下一帧运行yield return后面的函数（估计是把所有闭包放在一个list里面，每帧的一个特点时间执行list里面的闭包）。
 3：整个闭包应该保存了堆栈信息