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    Unity3D协程Coroutine解析

    2019/1/9      点击:

    本文只是从Unity的角度去分析理解协程的内部运行原理,而不是从C#底层的语法实现来介绍(后续有需要再进行介绍),一共分为三部分:

    1. 线程(Thread)和协程(Coroutine)


    使用协程的作用一共有两点: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。总结起来就是一句话:控制代码在特定的时机执行。 很多初学者,都会下意识地觉得协程是异步执行的,都会觉得协程是C# 线程的替代品,是Unity不使用线程的解决方案。 所以首先,请你牢记: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。

    2. Unity中协程的执行原理

    UnityGems.com给出了协程的定义: A coroutine is a function that is executed partially and, presuming suitable conditions are met, will be resumed at some point in the future until its work is done. 即协程是一个分部执行,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的代码。 Unity在每一帧(Frame)都会去处理对象上的协程。Unity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: 从上图的剖析就明白,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。还可以发现:如果在一个对象的前期调用协程,协程会立即运行到第一个 yield return 语句处,如果是 yield return null ,就会在同一帧再次被唤醒。如果没有考虑这个细节就会出现一些奇怪的问题『1』。 『1』注 图和结论都是从UnityGems.com 上得来的,经过下面的验证发现与实际不符,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。

    下面使用 yield return new WaitForSeconds(1f); 在Start,Update 和 LateUpdate 中分别进行测试:

    using UnityEngine;
    using System.Collections;
    
    public class TestCoroutine : MonoBehaviour {
    
        private bool isStartCall = false;  //Makesure Update() and LateUpdate() Log only once
        private bool isUpdateCall = false;
        private bool isLateUpdateCall = false;
        // Use this for initialization
        void Start () {
            if (!isStartCall)
            {
                Debug.Log("Start Call Begin");
                StartCoroutine(StartCoutine());
                Debug.Log("Start Call End");
                isStartCall = true;
            }
    
        }
        IEnumerator StartCoutine()
        {
    
            Debug.Log("This is Start Coroutine Call Before");
            yield return null;
            Debug.Log("This is Start Coroutine Call After");
    
        }
        // Update is called once per frame
        void Update () {
            if (!isUpdateCall)
            {
                Debug.Log("Update Call Begin");
                StartCoroutine(UpdateCoutine());
                Debug.Log("Update Call End");
                isUpdateCall = true;
            }
        }
        IEnumerator UpdateCoutine()
        {
            Debug.Log("This is Update Coroutine Call Before");
            yield return null;
            Debug.Log("This is Update Coroutine Call After");
        }
        void LateUpdate()
        {
            if (!isLateUpdateCall)
            {
                Debug.Log("LateUpdate Call Begin");
                StartCoroutine(LateCoutine());
                Debug.Log("LateUpdate Call End");
                isLateUpdateCall = true;
            }
        }
        IEnumerator LateCoutine()
        {
            Debug.Log("This is Late Coroutine Call Before");
            yield return null;
            Debug.Log("This is Late Coroutine Call After");
        }
    }
    得到日志输入结果如下:



    然后将yield return new WaitForSeconds(1f);改为 yield return null; 发现日志输入结果和上面是一样的,没有出现上面说的情况.

    MonoBehaviour 没有针对特定的协程提供Stop方法,其实不然,可以通过MonoBehaviour enabled = false 或者 gameObject.active = false 就可以停止协程的执行『2』。

    经过验证,『2』的结论也是错误的,正确的结论是,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:

    using UnityEngine;
    using System.Collections;
    
    public class TestCoroutine : MonoBehaviour {
    
      private bool isStartCall = false;  //Makesure Update() and LateUpdate() Log only once
      private bool isUpdateCall = false;
      private bool isLateUpdateCall = false;
      // Use this for initialization
      void Start () {
        if (!isStartCall)
        {
          Debug.Log("Start Call Begin");
          StartCoroutine(StartCoutine());
          Debug.Log("Start Call End");
          isStartCall = true;
        }
    
      }
      IEnumerator StartCoutine()
      {
    
        Debug.Log("This is Start Coroutine Call Before");
        yield return new WaitForSeconds(1f);
        Debug.Log("This is Start Coroutine Call After");
    
      }
      // Update is called once per frame
      void Update () {
        if (!isUpdateCall)
        {
          Debug.Log("Update Call Begin");
          StartCoroutine(UpdateCoutine());
          Debug.Log("Update Call End");
          isUpdateCall = true;
          this.enabled = false;
          //this.gameObject.SetActive(false);
        }
      }
      IEnumerator UpdateCoutine()
      {
        Debug.Log("This is Update Coroutine Call Before");
        yield return new WaitForSeconds(1f);
        Debug.Log("This is Update Coroutine Call After");
        yield return new WaitForSeconds(1f);
        Debug.Log("This is Update Coroutine Call Second");
      }
      void LateUpdate()
      {
        if (!isLateUpdateCall)
        {
          Debug.Log("LateUpdate Call Begin");
          StartCoroutine(LateCoutine());
          Debug.Log("LateUpdate Call End");
          isLateUpdateCall = true;
    
        }
      }
      IEnumerator LateCoutine()
      {
        Debug.Log("This is Late Coroutine Call Before");
        yield return null;
        Debug.Log("This is Late Coroutine Call After");
      }
    }
    先在Update中调用 this.enabled = false; 得到的结果:



    然后把 this.enabled = false; 注释掉,换成 this.gameObject.SetActive(false); 得到的结果如下:

    整理得到 :通过设置MonoBehaviour脚本的enabled对协程是没有影响的,但如果 gameObject.SetActive(false) 则已经启动的协程则完全停止了,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:

    a) null - the coroutine executes the next time that it is eligible 

     b) WaitForEndOfFrame - the coroutine executes on the frame, after all of the rendering and GUI is complete 

     c) WaitForFixedUpdate - causes this coroutine to execute at the next physics step, after all physics is calculated 

     d) WaitForSeconds - causes the coroutine not to execute for a given game time period 

     e) WWW - waits for a web request to complete (resumes as if WaitForSeconds or null) 

     f) 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。

    3. 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() ,前面介绍的TaskManager就是利用者两个方法对协程进行了管理,这里在介绍一个协程的交叉调用类 Hijack:

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using UnityEngine;
    using System.Collections;
    using UnityEngine.UI;
    
    [RequireComponent(typeof(Text))]
    public class HiJack : MonoBehaviour {
    
        //This will hold the counting up coroutine
        IEnumerator _countUp;
        //This will hold the counting down coroutine
        IEnumerator _countDown;
        //This is the coroutine we are currently
        //hijacking
        IEnumerator _current;
    
        //A value that will be updated by the coroutine
        //that is currently running
        int value = 0;
    
        void Start()
        {
            //Create our count up coroutine
            _countUp = CountUp();
            //Create our count down coroutine
            _countDown = CountDown();
            //Start our own coroutine for the hijack
            StartCoroutine(DoHijack());
        }
    
        void Update()
        {
            //Show the current value on the screen
            GetComponent().text = value.ToString ();
        }
    
        void OnGUI()
        {
            //Switch between the different functions
            if(GUILayout.Button("Switch functions"))
            {
                if(_current == _countUp)
                    _current = _countDown;
                else
                    _current = _countUp;
            }
        }
    
        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;
            }
        }
    
        IEnumerator CountUp()
        {
            //We have a local increment so the routines
            //get independently faster depending on how
            //long they have been active
            float increment = 0;
            while(true)
            {
                //Exit if the Q button is pressed
                if(Input.GetKey(KeyCode.Q))
                    break;
                increment+=Time.deltaTime;
                value += Mathf.RoundToInt(increment);
                yield return null;
            }
        }
    
        IEnumerator CountDown()
        {
            float increment = 0f;
            while(true)
            {
                if(Input.GetKey(KeyCode.Q))
                    break;
                increment+=Time.deltaTime;
                value -= Mathf.RoundToInt(increment);
                //This coroutine returns a yield instruction
                yield return new WaitForSeconds(0.1f);
            }
        }
    }
    上面的代码实现是两个协程交替调用,对有这种需求来说实在太精妙了。