Unity自定义协程——CustomYieldInstruction

Unity的Coroutine非常好用。在很多需要延迟执行的地方，都可以省掉大量功夫。

Coroutine的缺点

普通协程有以下这些缺点：

1. 嵌套协程依赖StartCoroutine，从而代码依赖MonoBehaviour
2. 执行异步逻辑的时候，需要写while等待。代码臃肿难看
3. 协程运行结果无法直接返回，需要另外处理，特别费力

这些缺点导致了下边这种写起来很难受的代码

 public IEnumerator Coroutine()
{
    //-----------------------------嵌套协程-----------------------------//
    yield return StartCoroutine(InternalCoroutine());

    //-----------------------------协程等待异步逻辑执行完毕-----------------------------//
    var running = false;
    DoSomething(() =>
    {
        running = true;
    });

    while (running == false)
    {
        yield return 0;
    }

    //-----------------------------协程返回值-----------------------------//
    running = false;
    var rtn = "bbb";
    DoSomthing2(s =>
    {
        running = true;
        rtn = s;
    });

    while (running == false)
    {
        yield return 0;
    }

}

CustomYieldInstruction

后来我看到Unity的一些类继承了CustomYieldInstruction这个接口，很好的解决了上边的问题。

public class WWW : CustomYieldInstruction, IDisposable {...}
public class WaitForSecondsRealtime : CustomYieldInstruction {...}
public sealed class WaitUntil : CustomYieldInstruction {...}
public sealed class WaitWhile : CustomYieldInstruction {...}

public abstract class CustomYieldInstruction : IEnumerator
{
    /// 

    ///   Indicates if coroutine should be kept suspended.
    /// 
    public abstract bool keepWaiting { get; }

    public object Current
    {
        get
        {
            return (object) null;
        }
    }

    public bool MoveNext()
    {
        return this.keepWaiting;
    }

    public void Reset()
    {
    }
}

比如下边这个下载文件的协程逻辑，我们把返回值Code放在类中。然后当下载完成后。再设置Code的值使协程被打断。

var download = new DownloadCoroutine(...);
yield return download;

Debug.Log(download.Code);

public class DownloadCoroutine : CustomYieldInstruction
{
    public int Code = -1;

    public DownloadCoroutine(string fromPath, string toPath, OnDownloadProgressDelegate onProgress)
    {
        Debug.Log("下载文件 ： " + fromPath + " -> " + toPath);

        var httpRequest = new HTTPRequest(new Uri(fromPath), HTTPMethods.Get, (request, response) =>
        {
            if (response == null)
            {
                Code = 500;
                return;
            }

            Debug.Log("下载完成 ： " + response.StatusCode);
            File.WriteAllBytes(toPath, response.Data);
            Code = response.StatusCode;
        });

        httpRequest.OnProgress = onProgress;
        httpRequest.Send();
    }

    public override bool keepWaiting
    {
        get { return Code == -1; }
    }
}