异步编程之EAP

一、概述

前面我们了解到了APM编程模式，但APM不支持对异步操作的取消和没有提供对进度报告的功能。

对于界面程序来说，进度报告和取消操作的支持也是必不可少的，为了支持这些功能，微软在.NET 2.0的时候提出了一个新的异步编程模型---基于事件的异步编程模型——EAP。

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。 

然而在.NET类库中并不是所有的类都支持EAP的，可能有朋友会误认为是不是支持APM的类都支持EAP的呢？在.NET 类库中只有部分的类支持EAP的(并且也只有部分类支持APM)，这些类有(共17个类)： 

System.Object的派生类型：

　　System.Activies.WorkflowInvoke　　

　　System.Deployment.Application.ApplicationDeployment

　　System.Deployment.Application.InPlaceHosingManager

　　System.Net.Mail.SmtpClient

　　System.Net.PeerToPeer.PeerNameResolver

　　System.Net.PeerToPeer.Collaboration.ContactManager

　　System.Net.PeerToPeer.Collaboration.Peer

　　System.Net.PeerToPeer.Collaboration.PeerContact

　　System.Net.PeerToPeer.Collaboration.PeerNearMe

　　System.ServiceModel.Activities.WorkflowControlClient

　　System.ServiceModel.Discovery.AnnoucementClient

　　System.ServiceModel.Discovery.DiscoveryClient

System.ComponentModel.Component的派生类型：

System.ComponentModel.BackgroundWorker

System.Media.SoundPlay

System.Net.WebClient

System.Net.NetworkInformation.Ping

System.Windows.Forms.PictureBox(继承于Control类，Control类派生于Component类)

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。

二、Demo

下面以BackgroundWorker类制作一个下载文件的demo。

BackgroundWorker类关键点

调用函数RunWorkerAsync时，会触发DoWork事件；

调用函数ReportProgress时，会触发ProgressChanged事件；

当后台操作已完成、被取消或引发异常时发生时，会触发RunWorkerCompleted事件。

using System;
using System.ComponentModel;
using System.IO;
using System.Net;
using System.Threading;
using System.Windows;

namespace Wpf_EAP
{
    public class RequestState
    {
        private HttpWebRequest request;
        public HttpWebRequest Request
        {
            get
            {
                return request;
            }
            set
            {
                request = value;
            }
        }
        private HttpWebResponse response;
        public HttpWebResponse Response
        {
            get
            {
                return response;
            }
            set
            {
                response = value;
            }
        }
        public Stream ResponseStream;
        public FileStream Filestream = null;

        public byte[] BufferRead = new byte[1024];
        public static int Index = 1;
        public RequestState(string fileSavePath)
        {
            //string fileName = "Pic" + (Index++).ToString();
            string fileName = "Pic";
            string saveFilePath = fileSavePath + fileName + ".jpg";//以下载jpg图片为例
            //if(File.Exists(saveFilePath))
            //{
            //    File.Delete(saveFilePath);
            //}
            Filestream = new FileStream(saveFilePath, FileMode.OpenOrCreate);
        }
    }
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private string downLoadUrl = @"https://ss1.bdstatic.com/70cFuXSh_Q1YnxGkpoWK1HF6hhy/it/u=2298824648,1812234339&fm=200&gp=0.jpg";
        public string DownLoadUrl
        {
            get { return downLoadUrl; }
            set { downLoadUrl = value; }
        }
        private string fileSavePath = @"D:360Downloads";
        public string FileSavePath
        {
            get { return fileSavePath; }
            set { fileSavePath = value; }
        }
        private int downLoadSize = 0;
        private BackgroundWorker bgWorker;
        private RequestState requestState;
        private long totalSize = 0;
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
            bgWorker = new BackgroundWorker();
            bgWorker.WorkerSupportsCancellation = true;
            bgWorker.WorkerReportsProgress = true;
            bgWorker.DoWork += bgWorkerFileDownload_DoWork;
            bgWorker.ProgressChanged += bgWorkerFileDownload_ProgressChanged;
            bgWorker.RunWorkerCompleted += bgWorkerFileDownload_RunWorkerCompleted;

        }
        private void GetTotalSize()
        {
            HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(DownLoadUrl);
            HttpWebResponse response = (HttpWebResponse)myHttpWebRequest.GetResponse();
            totalSize = response.ContentLength;
            response.Close();
        }
        private void bgWorkerFileDownload_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker bgworker = sender as BackgroundWorker;
            try
            {
                GetTotalSize();
                // Do the DownLoad operation
                // Initialize an HttpWebRequest object
                HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(DownLoadUrl);

                // If the part of the file have been downloaded, 
                // The server should start sending data from the DownloadSize to the end of the data in the HTTP entity.
                if (downLoadSize != 0)
                {
                    myHttpWebRequest.AddRange(downLoadSize);
                }

                // assign HttpWebRequest instance to its request field.
                requestState.Request = myHttpWebRequest;
                requestState.Response = (HttpWebResponse)myHttpWebRequest.GetResponse();
                requestState.ResponseStream = requestState.Response.GetResponseStream();
                int readSize = 0;
                while (true)
                {
                    if (bgworker.CancellationPending == true)
                    {
                        e.Cancel = true;
                        break;
                    }

                    readSize = requestState.ResponseStream.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);
                        Thread.Sleep(100);
                        // 报告进度，引发ProgressChanged事件的发生
                        bgworker.ReportProgress(percentComplete);
                    }
                    else
                    {
                        break;
                    }
                }
            }
            catch(Exception err)
            {
                MessageBox.Show(err.Message);
            }
        }
        private void bgWorkerFileDownload_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            //progressBar.Value = e.ProgressPercentage;
             Dispatcher.BeginInvoke(new Action( ()=> { progressBar.Value = e.ProgressPercentage; } ));
        }
        private void bgWorkerFileDownload_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if (e.Error != null)
            {
                MessageBox.Show(e.Error.Message);
                requestState.Response.Close();
            }
            else if (e.Cancelled)
            {
                int percentComplete = (int)((float)downLoadSize / (float)totalSize * 100);
                MessageBox.Show(String.Format("下载暂停，下载的文件地址为：{0}
 已经下载的字节数/总字节: {1}字节/{2}  百分比：{3} %", DownLoadUrl, downLoadSize, totalSize,percentComplete));
                requestState.Response.Close();
                requestState.Filestream.Close();

                this.btnDownLoad.IsEnabled = true;
                this.btnPause.IsEnabled = 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", DownLoadUrl, totalSize));
                downLoadSize = 0;
                Dispatcher.BeginInvoke(new Action(() => { progressBar.Value = 0; }));
                this.btnDownLoad.IsEnabled = true;
                this.btnPause.IsEnabled = false;
                requestState.Response.Close();
                requestState.Filestream.Close();
            }
       
        }

        private void btnPause_Click(object sender, RoutedEventArgs e)
        {
            if (bgWorker.IsBusy && bgWorker.WorkerSupportsCancellation == true)
            {
                // Pause the asynchronous operation
                // Fire RunWorkerCompleted event
                bgWorker.CancelAsync();
            }
        }

        private void btnDownLoad_Click(object sender, RoutedEventArgs e)
        {
            if (bgWorker.IsBusy != true)
            {
                bgWorker.RunWorkerAsync();//触发DoWork事件
                // Create an instance of the RequestState 
                requestState = new RequestState(FileSavePath);
                requestState.Filestream.Seek(downLoadSize, SeekOrigin.Begin);
                this.btnDownLoad.IsEnabled = false;
                this.btnPause.IsEnabled = true;
            }
            else
            {
                MessageBox.Show("下载进行中，请稍后...");
            }
        }
    }
}

<Window x:Class="Wpf_EAP.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Wpf_EAP"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50"/>
            <RowDefinition Height="50"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="100"/>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="100"/>
        </Grid.ColumnDefinitions>
        <Label Content="DownLoadUrl:" FontSize="14" VerticalAlignment="Center" HorizontalAlignment="Right"></Label>
        <Label Content="FileSavePath:" FontSize="14"  Grid.Row="1" VerticalAlignment="Center" HorizontalAlignment="Right"></Label>
        <TextBox FontSize="20" BorderBrush="Green" BorderThickness="1"  Grid.Column="1" Margin="5" Grid.Row="1" Text="{Binding FileSavePath}" Name="tbSavePath"/>
        <TextBox Text="{Binding DownLoadUrl}"  FontSize="20" BorderBrush="Green" BorderThickness="1" Name="lbUrl" Grid.Column="1" Margin="5"/>
        <Button Content="DownLoad" Grid.Column="2" Grid.Row="0" FontSize="20" Name="btnDownLoad" VerticalAlignment="Center" Click="btnDownLoad_Click"/>
        <Button Content="Pause" Grid.Column="2" Grid.Row="1" FontSize="20" Name="btnPause" VerticalAlignment="Center" Click="btnPause_Click"/>
        <ProgressBar Grid.Row="2" Grid.ColumnSpan="3" Height="30" Name="progressBar" Minimum="0" Maximum="100"></ProgressBar>
    </Grid>
</Window>

注：本文参考https://www.cnblogs.com/zhili/archive/2013/05/11/EAP.html