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C#(068):二、.NET 2.0基于事件的异步编程模式(EAP)

作者:互联网

一、引言

APM为我们实现异步编程提供了一定的支持,同时它也存在着一些明显的问题——不支持对异步操作的取消和没有提供对进度报告的功能,对于有界面的应用程序来说,进度报告和取消操作的支持也是必不可少的。

微软在.NET 2.0的时候就为我们提供了一个新的异步编程模型,也就是基于事件的异步编程模型——EAP(Event-based Asynchronous
Pattern )。

二、介绍

实现了基于事件的异步模式的类将具有一个或者多个以 Async 为后缀的方法和对应的 Completed
事件,并且这些类都支持异步方法的取消、进度报告和报告结果。

当我们调用实现基于事件的异步模式的类的
XxxAsync方法时,即代表开始了一个异步操作,该方法调用完之后会使一个线程池线程去执行耗时的操作,所以当UI线程调用该方法时,当然也就不会堵塞UI线程了。

并且基于事件的异步模式是建立了APM的基础之上的,而APM又是建立了在委托之上的。

    public static void GetInfomation()
    {
        WebClient client = new WebClient();
        Uri uri = new Uri("http://www.baidu.com");
        client.DownloadFileCompleted += new AsyncCompletedEventHandler(client_DownloadFileCompleted);
        client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressCallback);
        client.DownloadFileAsync(uri, "serverdata.txt");
    }
    
    static void client_DownloadProgressCallback(object sender, DownloadProgressChangedEventArgs e)
    {
        // Displays the operation identifier, and the transfer progress.
        Console.WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...", (string)e.UserState, e.BytesReceived, e.TotalBytesToReceive, e.ProgressPercentage);
    }
    
    static void client_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
    {
        string RetStr = (e.Error == null ? "成功" : e.Error.Message);
        Console.Write(RetStr);  //显示操作结果信息
    }

三、使用BackgroundWorker组件进行异步编程

BackgroundWorker类就是用EAP实现的。

BackgroundWorker类

公共属性

公共方法

公共事件

下面向大家演示一个使用BackgroundWorker组件实现异步下载文件的一个小程序,该程序支持异步下载(指的就是用线程池线程要执行下载操作),断点续传、下载取消和进度报告的功能。

image

事件绑定:

image

代码:

    public int DownloadSize = 0;
    public string downloadPath = null;
    private RequestState requestState = null;
    private long totalSize = 0;
    
    public FileDownloader()
    {
        InitializeComponent();
    
        string url = "http://download.microsoft.com/download/7/0/3/703455ee-a747-4cc8-bd3e-98a615c3aedb/dotNetFx35setup.exe";
        //string url = "http://download.microsoft.com/download/9/5/A/95A9616B-7A37-4AF6-BC36-D6EA96C8DAAE/dotNetFx40_Full_x86_x64.exe";
        txbUrl.Text = url;
        this.btnPause.Enabled = false;
        //this.status = DownloadStatus.Initialized;
        // Get download Path
    
        GetTotalSize();
        downloadPath = Environment.GetFolderPath(Environment.SpecialFolder.Desktop) + "\\" + Path.GetFileName(this.txbUrl.Text.Trim());
        if (File.Exists(downloadPath))
        {
            FileInfo fileInfo = new FileInfo(downloadPath);
            DownloadSize = (int)fileInfo.Length;
            progressBar1.Value = (int)((float)DownloadSize / (float)totalSize * 100);
        }
    
        // 启用支持 ReportProgress and Cancellation
        bgWorkerFileDownload.WorkerReportsProgress = true;
        bgWorkerFileDownload.WorkerSupportsCancellation = true;
    }
    
    //开始或恢复下载
    private void btnDownload_Click(object sender, EventArgs e)
    {
        if (bgWorkerFileDownload.IsBusy != true)
        {
            // 启动异步操作触发DoWork事件
            bgWorkerFileDownload.RunWorkerAsync();
    
            // 创建RequestState 实例
            requestState = new RequestState(downloadPath);
            requestState.filestream.Seek(DownloadSize, SeekOrigin.Begin);
            this.btnDownload.Enabled = false;
            this.btnPause.Enabled = true;
        }
        else
        {
            MessageBox.Show("正在执行操作,请稍后");
        }
    }
    
    // 暂停下载
    private void btnPause_Click(object sender, EventArgs e)
    {
        if (bgWorkerFileDownload.IsBusy && bgWorkerFileDownload.WorkerSupportsCancellation == true)
        {
            // 终止异步操作触发RunWorkerCompleted事件
            bgWorkerFileDownload.CancelAsync();
        }
    }
    
    #region BackGroundWorker Event
    
    // 1、当调用RunWorkerAsync方法时触发
    private void bgWorkerFileDownload_DoWork(object sender, DoWorkEventArgs e)
    {
        //获取事件源
        BackgroundWorker bgworker = sender as BackgroundWorker;
        try
        {
            // 执行下载操作
            // 初始化一个HttpWebRequest对象
            HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(txbUrl.Text.Trim());
    
            // 如果文件已下载了一部分,
            // 服务器应该从DownloadSize开始发送数据到HTTP实体中数据的末尾。
            if (DownloadSize != 0)
            {
                myHttpWebRequest.AddRange(DownloadSize);
            }
    
            // 指定一个 HttpWebRequest 实例到request字段.
            requestState.request = myHttpWebRequest;
            requestState.response = (HttpWebResponse)myHttpWebRequest.GetResponse();
            requestState.streamResponse = requestState.response.GetResponseStream();
            int readSize = 0;
            while (true)
            {
                if (bgworker.CancellationPending == true)
                {
                    e.Cancel = true;
                    break;
                }
    
                readSize = requestState.streamResponse.Read(requestState.BufferRead, 0, requestState.BufferRead.Length);
                if (readSize > 0)
                {
                    DownloadSize += readSize;
                    int percentComplete = (int)((float)DownloadSize / (float)totalSize * 100);
                    requestState.filestream.Write(requestState.BufferRead, 0, readSize);
    
                    // 报告进度,引发ProgressChanged事件的发生
                    bgworker.ReportProgress(percentComplete);
                }
                else
                {
                    break;
                }
            }
        }
        catch
        {
            throw;
        }
    }
    
    //  2、当调用ReportProgress方法时触发
    private void bgWorkerFileDownload_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        this.progressBar1.Value = e.ProgressPercentage;
    }
    
    // 3、当后台操作完成,取消或者有错误时触发
    private void bgWorkerFileDownload_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        if (e.Error != null)
        {
            MessageBox.Show(e.Error.Message);
            requestState.response.Close();
        }
        else if (e.Cancelled)
        {
            MessageBox.Show(String.Format("下载暂停,下载的文件地址为:{0}\n 已经下载的字节数为: {1}字节", downloadPath, DownloadSize));
            requestState.response.Close();
            requestState.filestream.Close();
    
            this.btnDownload.Enabled = true;
            this.btnPause.Enabled = false;
        }
        else
        {
            MessageBox.Show(String.Format("下载已完成,下载的文件地址为:{0},文件的总字节数为: {1}字节", downloadPath, totalSize));
    
            this.btnDownload.Enabled = false;
            this.btnPause.Enabled = false;
            requestState.response.Close();
            requestState.filestream.Close();
        }
    }
    
    #endregion BackGroundWorker Event
    
    // 获取文件总大小
    private void GetTotalSize()
    {
        HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(txbUrl.Text.Trim());
        HttpWebResponse response = (HttpWebResponse)myHttpWebRequest.GetResponse();
        totalSize = response.ContentLength;
        response.Close();
    }
    }
    
    // 此类存储请求的状态
    public class RequestState
    {
     public int BufferSize = 2048;
    
     public byte[] BufferRead;
     public HttpWebRequest request;
     public HttpWebResponse response;
     public Stream streamResponse;
    
     public FileStream filestream;
    
     public RequestState(string downloadPath)
     {
        BufferRead = new byte[BufferSize];
        request = null;
        streamResponse = null;
        filestream = new FileStream(downloadPath, FileMode.OpenOrCreate);
     }

运行程序点击"下载"按钮然后再点击"暂停"后的结果:

10001649_dc293a3d48a54471b32aeca5c72d373f[1]

当暂停下载后,我们还可以点 ”下载“按钮继续下载该文件,此时并不会从开开始下载,而会接着上次的下载继续下载。

这个实现主要是通过AddRange方法来实现的,该方法是指出向服务器请求文件的大小,上面代码中通过传入DownloadSize来告诉服务器,这次我需要的内容不是从开头开始的,而是从已经下载的文件字节数开始到该文件的总的字节结尾,这样就就实现了
断点续传 的功能了,使户暂停下载不至于之前下载的都白费了。程序的运行结果为:

10001620_04976ac9aedf44909ef1c09f4646c5ca[1]

标签:异步,DownloadSize,C#,068,requestState,bgWorkerFileDownload,EAP,public,下载
来源: https://www.cnblogs.com/springsnow/p/16272341.html