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1.原始的異步方法的調用

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我們來看個簡單的例子，在這里演示調用 WebClient.DownloadStringAsync 方法(這個方法不是 TAP)，然后由 WebClient.DownloadStringCompleted 事件通知 UI 更新，這是大多數人都會用的方法。

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private void DownloadString(string address)

{

????WebClient wc = new WebClient();

????wc.DownloadStringCompleted += (sender, e) =>

????{

????????if (e.Cancelled)

????????????this.textBox1.Text = "Cancel";

????????else if (e.Error != null)

????????????this.textBox1.Text = "Error";

????????else

????????????this.textBox1.Text = e.Result;

????};

????wc.DownloadStringAsync(new Uri(address));

}

客戶端調用

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private void button_DownloadString_Click(object sender, EventArgs e)

{

????DownloadString("https://www.google.com.tw/");

}

這是一個很簡單的例子，一旦若項目里有成千上萬的通知事件跟 UI 綁在一起，維護起來會相當的痛苦。

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2.將 EAP 轉換成 TAP步驟

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• 命名規則以 Async 為后綴
• 返回 Task 或是 Task<TResult>
• 調用 TaskCompletionSource 方法

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改變 Task 狀態可調用以下三個方法：SetCanceled、SetException、SetResult

private Task<string> DownloadStringAsync(string address)

{

????var tcs = new TaskCompletionSource<string>();

????WebClient wc = new WebClient();

????wc.DownloadStringCompleted += (sender, e) =>

????{

????????if (e.Cancelled)

????????????tcs.SetCanceled();

????????else if (e.Error != null)

????????????tcs.SetException(e.Error);

????????else

????????????tcs.SetResult(e.Result);

????};

????wc.DownloadStringAsync(new Uri(address));

????return tcs.Task;

}

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客戶端調用

private async void button_DownloadStringAsync_Click(object sender, EventArgs e)

{

????var task = DownloadStringAsync("https://www.google.com.tw/");

????await task;

????if (task.IsCanceled)

????{

????????this.textBox1.Text = "Cancel";

????}

????else if (task.IsFaulted)

????{

????????this.textBox1.Text = "Error";

????}

????else if (task.IsCompleted)

????{

????????this.textBox1.Text = task.Result;

????}

}

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轉自：http://www.it165.net/pro/html/201308/6710.html

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3.微軟的封裝

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public static Task<byte[]> DownloadDataTask(this WebClient webClient, Uri address)

{

????// Create the task to be returned

????var tcs = new TaskCompletionSource<byte[]>(address);

????// Setup the callback event handler

????DownloadDataCompletedEventHandler handler = null;

????handler = (sender, e) => EAPCommon.HandleCompletion(tcs, e, () => e.Result, () => webClient.DownloadDataCompleted -= handler);

????webClient.DownloadDataCompleted += handler;

????// Start the async work

????try

????{

????????webClient.DownloadDataAsync(address, tcs);

????}

????catch(Exception exc)

????{

????????// If something goes wrong kicking off the async work,

????????// unregister the callback and cancel the created task

????????webClient.DownloadDataCompleted -= handler;

????????tcs.TrySetException(exc);

????}

????// Return the task that represents the async operation

????return tcs.Task;

}

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internal class EAPCommon

{

????internal static void HandleCompletion<T>(

????????TaskCompletionSource<T> tcs, AsyncCompletedEventArgs e, Func<T> getResult, Action unregisterHandler)

????{

????????// Transfers the results from the AsyncCompletedEventArgs and getResult() to the

????????// TaskCompletionSource, but only AsyncCompletedEventArg's UserState matches the TCS

????????// (this check is important if the same WebClient is used for multiple, asynchronous

????????// operations concurrently). Also unregisters the handler to avoid a leak.

????????if (e.UserState == tcs)

????????{

????????????if (e.Cancelled) tcs.TrySetCanceled();

????????????else if (e.Error != null) tcs.TrySetException(e.Error);

????????????else tcs.TrySetResult(getResult());

????????????unregisterHandler();

????????}

????}

}

轉載于:https://www.cnblogs.com/pengzhen/p/4831339.html

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