異步之後的消防回調任務方法

Question

當foreach循環搜索完List<>中的每個項目時，我需要觸發回調。

private async void startSearchBtn_Click(object sender, EventArgs e) 
{ 
    await Search(files, selectTxcDirectory.SelectedPath, status); 
} 

private static async Task Search(List<string> files, string path, Label statusText) 
{ 
    foreach (string file in files) 
    { 
     XmlDocument xmlDoc = new XmlDocument(); 
     xmlDoc.Load(file); 

     statusText.Text = "Started scanning..."; 
     using (XmlReader reader = XmlReader.Create(new StringReader(xmlDoc.InnerXml), new XmlReaderSettings() { Async = true })) 
     { 
      while (await reader.ReadAsync()) 
      { 
       if ((reader.NodeType == XmlNodeType.Element) && (reader.Name == "LineName")) 
       { 
        Console.WriteLine(reader.ReadInnerXml()); 
       } 
      } 
     } 
    } 
} 

這是可能的，如果是這樣怎麼辦？

Answer 1

這很簡單，只需在參數中傳遞一個方法作爲委託。然後在需要的地方調用它。

private async void startSearchBtn_Click(object sender, EventArgs e) 
{ 
    await Search(files, selectTxcDirectory.SelectedPath, status, SearchCompleted); // <-- pass the callback method here 
} 

private static async Task Search(List<string> files, string path, Label statusText, Action<string> callback) 
{ 
    foreach (string file in files) 
    { 
     XmlDocument xmlDoc = new XmlDocument(); 
     xmlDoc.Load(file); 

     statusText.Text = "Started scanning..."; 
     using (XmlReader reader = XmlReader.Create(new StringReader(xmlDoc.InnerXml), new XmlReaderSettings() { Async = true })) 
     { 
      while (await reader.ReadAsync()) 
      { 
       if ((reader.NodeType == XmlNodeType.Element) && (reader.Name == "LineName")) 
       { 
        Console.WriteLine(reader.ReadInnerXml()); 
       } 
      } 
     } 

     // Here you're done with the file so invoke the callback that's it. 
     callback(file); // pass which file is finished 
    } 
} 

private static void SearchCompleted(string file) 
{ 
    // This method will be called whenever a file is processed. 
} 

Answer 2

由於您使用await，您在startSearchBtn_Click代碼將不會繼續下去，直到Search完成。

所有你需要的是這樣的：

private async void startSearchBtn_Click(object sender, EventArgs e) 
{ 
    await Search(files, selectTxcDirectory.SelectedPath, status); 
    // run your callback here 
} 

Answer 3

我編寫它像下面。這樣，您仍然跟蹤待處理任務（_pendingSearch），而startSearchBtn_Click保持同步。

你應該跟蹤未決任務（並且能夠取消它們）。否則，用戶可以連續兩次點擊startSearchBtn併產生兩個搜索任務。在你的情況下，這仍然是一個有效的方案，但通常情況並非如此。

Task _pendingSearch = null; 
private void startSearchBtn_Click(object sender, EventArgs e) 
{ 
    // check if is _pendingSearch != null and still pending here 

    _pendingSearch = Search(files, 
     selectTxcDirectory.SelectedPath, status).ContinueWith((finishedTask) => 
    { 
     // Place your completion callback code here 
    }, TaskScheduler.FromCurrentSynchronizationContext); 
} 

private static async Task Search(List<string> files, string path, Label statusText) 
{ 
    // ... 
} 

將帖子使用await：

Task _pendingSearch = null; 
private async void startSearchBtn_Click(object sender, EventArgs e) 
{ 
    // check if is _pendingSearch != null and still pending here 

    _pendingSearch = Search(files, selectTxcDirectory.SelectedPath, status); 
    await _pendingSearch; 
    // Place your completion callback code here 
}