Unity自定義協程——CustomYieldInstruction

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

Coroutine的缺點

普通協程有以下這些缺點：

1. 嵌套協程依賴StartCoroutine，從而代碼依賴MonoBehaviour
2. 執行異步邏輯的時候，需要寫while等待。代碼臃腫難看
3. 協程運行結果無法直接返回，需要另外處理，特別費力

這些缺點導致了下邊這種寫起來很難受的代碼

 public IEnumerator Coroutine()
{
    //-----------------------------嵌套協程-----------------------------//
    yield return StartCoroutine(InternalCoroutine());

    //-----------------------------協程等待異步邏輯執行完畢-----------------------------//
    var running = false;
    DoSomething(() =>
    {
        running = true;
    });

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

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

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

}

CustomYieldInstruction

後來我看到Unity的一些類繼承了CustomYieldInstruction這個接口，很好的解決了上邊的問題。

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

public abstract class CustomYieldInstruction : IEnumerator
{
    /// <summary>
    ///   <para>Indicates if coroutine should be kept suspended.</para>
    /// </summary>
    public abstract bool keepWaiting { get; }

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

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

    public void Reset()
    {
    }
}

比如下邊這個下載文件的協程邏輯，我們把返回值Code放在類中。然後當下載完成後。再設置Code的值使協程被打斷。

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("下載文件 ： " + fromPath + " -> " + toPath);

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

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

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

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