C# Windows Phone 8 WP8 开发，将WebClient的DownloadStringCompleted事件改成非同步的awiat方法。

原文:C# Windows Phone 8 WP8 开发，将WebClient的DownloadStringCompleted事件改成非同步的awiat方法。

一般我们在撰写Windows Phone APP 的时候，很常使用WebClient来帮我们完成抓取网路资料的动作，并且利用DownloadStringCompleted的事件去帮我们得知载入的事件已经完成了，可以让我们进行下一步的动作，但是若我们想要将WebClient的DownloadStringCompleted事件改成非同步的awiat方法该如何做呢?!可以自订一延伸WebClient的方法来达成。?

本篇文章将引导您将WebClient的DownloadStringCompleted事件改成非同步的awiat方法。?

首先这是Windows Store App上开发时会使用的程式码，功能是从网路撷取网页原始资料:?

//网路动作

[code]
public async Task<string> Download_WebFrom(string url)

{

HttpClient http = new System.Net.Http.HttpClient();

HttpResponseMessage response = await http.GetAsync(url);

return await response.Content.ReadAsStringAsync();

}

[/code]
.csharpcode, .csharpcode pre{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt{
background-color: #f4f4f4;
width: 100%;
margin: 0em;}
.csharpcode .lnum { color: #606060; }]]>?

然後成功之後很开心地移植到Windows Phone 8上使用，结果悲剧发的奇怪不能用.....?

後来改回用原本的WebClient的DownloadStringCompleted事件来达成，功能一样是从网路撷取网页原始资料，可是就跟原本的写法不一样:?

private void DownloadFromWeb()

[code]{

?

WebClient wc = new WebClient();

wc.Encoding = Encoding.UTF8;

wc.DownloadStringAsync(new Uri(Albums_URL));

wc.DownloadStringCompleted += wc_DownloadStringCompleted;

}

?

void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)

{

//Debug.WriteLine("wc_DownloadStringCompleted" + e.Result);

try

{

string data = e.Result;

}

catch(Exception  ex)

{

//网路GG

MessageBox.Show(ex.Message);

}

}

[/code]
.csharpcode, .csharpcode pre{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt{
background-color: #f4f4f4;
width: 100%;
margin: 0em;}
.csharpcode .lnum { color: #606060; }]]>?

这样会造成移植上的困扰还要改一堆其他程式码实在不方便，

所以索性用延伸WebClient方法的方式来改造一下程式码。?

步骤一:

开一个新的类别档(class)，类别的型别是静态的，并命名为Extensions如下程式码所示:?

public static class Extensions

[code]{

public static Task<string> DownloadStringTask(this WebClient webClient, Uri uri)

{

var tcs = new TaskCompletionSource<string>();

?

webClient.DownloadStringCompleted += (s, e) =>

{

if (e.Error != null)

{

tcs.SetException(e.Error);

}

else

{

tcs.SetResult(e.Result);

}

};

?

webClient.DownloadStringAsync(uri);

?

return tcs.Task;

}

}

[/code]
.csharpcode, .csharpcode pre{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt{
background-color: #f4f4f4;
width: 100%;
margin: 0em;}
.csharpcode .lnum { color: #606060; }]]>?

上述程式码是将WebClient底下新增一个自定义的方法，那也是使用DownloadStringCompleted的方式来回传直。

而自订义的方法名称为DownloadStringTask。?

步骤二:

如此一来我们就可以在主程式中使用WebClient底下DownloadStringTask的方法罗!!?

public async Task<string> Download_WebFrom(string url)

[code]{

WebClient client = new WebClient();

string data = await client.DownloadStringTask(new Uri(url));

return data;

?

}

[/code]
.csharpcode, .csharpcode pre{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt{
background-color: #f4f4f4;
width: 100%;
margin: 0em;}
.csharpcode .lnum { color: #606060; }]]>?

这样我们就将将WebClient的DownloadStringCompleted事件改成非同步的awiat方法罗!!??

Refrence : Async await in Windows Phone web access APIs?

文章中的叙述如有观念不正确错误的部分，欢迎告知指正 谢谢

转载请注明出处，并且附上本篇文章网址 !? 感谢。

SUKI

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DotBlogs Tags:

C#

DownloadStringCompleted

DownloadStringTask

Visual Studio 2013

WebClient

Windows Phone 8

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