.NET非同步編程模式(三)

EAP(Event-based Asynchronous Pattern) 是基於事件的非同步模式，在 .NET Framework 2.0 中引入。EAP 需要一個有 Async 尾碼方法和一個或多個事件。EAP不再推薦用於新開發。

EAP

一個符合 EAP 模式的示例聲明如下：

public class AsyncExample  
{  
    // Synchronous methods.  
    public int Method1(string param);  
    public void Method2(double param);  
  
    // Asynchronous methods.  
    public void Method1Async(string param);  
    public void Method1Async(string param, object userState);  
    public event Method1CompletedEventHandler Method1Completed;  
  
    public void Method2Async(double param);  
    public void Method2Async(double param, object userState);  
    public event Method2CompletedEventHandler Method2Completed;  
  
    public void CancelAsync(object userState);  
  
    public bool IsBusy { get; }  
  
    // Class implementation not shown.  
}

BackgroundWorker

SoundPlayer和PictureBox組件表示基於事件的非同步模式的簡單實現。WebClient和BackgroundWorker組件代表了基於事件的非同步模式的更複雜的實現。

 private void EAP_Btn_Click(object sender, RoutedEventArgs e)
{
    // 記錄時間
    Debug.WriteLine("1-" + DateTime.Now.TimeOfDay.ToString() +
                    ",ThreadID = " + Thread.CurrentThread.ManagedThreadId);

    BackgroundWorker worker = new BackgroundWorker();
    // 事件綁定
    worker.DoWork += Worker_DoWork;
    worker.RunWorkerCompleted += Worker_RunWorkerCompleted;
    // 非同步執行
    worker.RunWorkerAsync();

    // 記錄時間
    Debug.WriteLine("4-" + DateTime.Now.TimeOfDay.ToString() +
                    ",ThreadID = " + Thread.CurrentThread.ManagedThreadId);
}

private void Worker_RunWorkerCompleted(object? sender, RunWorkerCompletedEventArgs e)
{
    // 記錄時間
    Debug.WriteLine("3-" + DateTime.Now.TimeOfDay.ToString() +
                    ",ThreadID = " + Thread.CurrentThread.ManagedThreadId);
}

private void Worker_DoWork(object? sender, DoWorkEventArgs e)
{
    // 訪問外網網站網站
    var req = WebRequest.Create("https://docs.newrelic.com/docs/apm/agents/net-agent/getting-started/net-agent-compatibility-requirements-net-framework/");
    req.GetResponse();

    Debug.WriteLine("2-" + DateTime.Now.TimeOfDay.ToString() +
                    ",ThreadID = " + Thread.CurrentThread.ManagedThreadId);
}

程式運行效果：

日誌輸出：

1-17:00:10.3584040,ThreadID = 1
4-17:00:10.3609798,ThreadID = 1
2-17:00:11.3485887,ThreadID = 8
3-17:00:11.3514632,ThreadID = 1

從效果和日誌上看：

• EAP 不會阻塞調用線程
• 非同步操作真正執行是在另外一個線程
• RunWorkerCompleted 回調會回到調用線程(UI線程)

和APM比起來很像，好像只是把 委托綁定 放到了外面。
我們可以看一下 BackgroundWorker 的源碼：

• 在構造函數里實例化一個委托 threadStart

• 調用 RunWorkerAsync() 方法

看起來 EAP 的本質，還是使用了委托的非同步方式（BeginInvoke），實質上還是 APM 非同步模式。

多任務

如果有多個非同步任務，我們希望按照先後順序執行，並且需要在調用線程上得到所有返回值。

public Func<string, string> func1()
{
    return new Func<string, string>(t =>
    {
        Thread.Sleep(2000);
        return "name:" + t;
    });
}
public Func<string, string> func2()
{
    return new Func<string, string>(t =>
    {
        Thread.Sleep(2000);
        return "age:" + t;
    });
}
public Func<string, string> func3()
{
    return new Func<string, string>(t =>
    {
        Thread.Sleep(2000);
        return "sex:" + t;
    });
}

// 按照一定的順序去執行
public void Multi_APM_Btn_Click(object sender, RoutedEventArgs e)
{
    string str1 = string.Empty, str2 = string.Empty, str3 = string.Empty;
    IAsyncResult asyncResult1 = null, asyncResult2 = null, asyncResult3 = null;

    asyncResult1 = func1().BeginInvoke("張三", t =>
    {
        str1 = func1().EndInvoke(t);
        Debug.WriteLine("1-" + DateTime.Now.TimeOfDay.ToString() +",ThreadID = " + Thread.CurrentThread.ManagedThreadId);
        asyncResult2 = func2().BeginInvoke("26", a =>
        {
            str2 = func2().EndInvoke(a);
            Debug.WriteLine("2-" + DateTime.Now.TimeOfDay.ToString() +",ThreadID = " + Thread.CurrentThread.ManagedThreadId);
            asyncResult3 = func3().BeginInvoke("男", s =>
            {
                str3 = func3().EndInvoke(s);
                Debug.WriteLine("3-" + DateTime.Now.TimeOfDay.ToString() + ",ThreadID = " + Thread.CurrentThread.ManagedThreadId);
            }, null);
        }, null);
    }, null);

    asyncResult1.AsyncWaitHandle.WaitOne();
    asyncResult2.AsyncWaitHandle.WaitOne();
    asyncResult3.AsyncWaitHandle.WaitOne();
    Debug.WriteLine(str1 + str2 + str3);
} 

運行起來，發現有異常：

由此可見在完成第一個非同步操作之前沒有對asyncResult2進行賦值，asyncResult2執行非同步等待的時候報異常。也可以有其他方法來解決這個問題，但會比較複雜。