接着上一篇，我们继续来优化。我们的 SkyParallelWebClient 可否支持切换“同步下载模式”和“异步下载模式”呢，好处是大量的代码不用改，只需要调用 skyParallelWebClient.StartAsync() 就开始异步下载，而改为 skyParallelWebClient.StartSync(); 就同步下载。如图：

同步下载：

异步下载：

1. 同步下载模式

直接贴代码了：

  1.     public partial class Form1 : Form
  2.     {
  3.         public Form1()
  4.         {
  5.             InitializeComponent();
  6.         }
  7.  
  8.         private List<int> GetDownloadIds()
  9.         {
  10.             List<int> ids = new List<int>();
  11.             for (int i = ; i <= ; i++)
  12.             {
  13.                 ids.Add(i);
  14.             }
  15.             return ids;
  16.         }
  17.  
  18.         private void WhenAllDownloading()
  19.         {
  20.             this.listBoxLog.Items.Insert(, string.Format("当前时间：{0}，准备开始下载...", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss")));
  21.             //禁用按钮
  22.             EnableOrDisableButtons(false);
  23.         }
  24.  
  25.         private void EnableOrDisableButtons(bool enabled)
  26.         {
  27.             this.btnRunByHttpClient.Enabled = enabled;
  28.             this.btnRunByWebClient.Enabled = enabled;
  29.         }
  30.  
  31.         private void WhenSingleDownloading(WhenSingleDownloadEventArgs eventArg)
  32.         {
  33.             this.listBoxLog.Items.Insert(, string.Format("当前时间：{0}，剩余 {1} 个。编号 {2} 准备开始下载...", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"), eventArg.RemainingQueueCount, eventArg.CurrentDownloadEntry.Data));
  34.         }
  35.  
  36.         private void WhenSingleDownloaded(WhenSingleDownloadEventArgs eventArg)
  37.         {
  38.             this.listBoxLog.Items.Insert(, string.Format("当前时间：{0}，剩余 {1} 个。编号 {2} 下载完毕...", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"), eventArg.RemainingQueueCount, eventArg.CurrentDownloadEntry.Data));
  39.         }
  40.  
  41.         private void WhenAllDownloaded()
  42.         {
  43.             this.listBoxLog.Items.Insert(, string.Format("当前时间：{0}，下载完毕！", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss")));
  44.             //启用按钮
  45.             EnableOrDisableButtons(true);
  46.         }
  47.  
  48.         private async void btnRunByHttpClient_Click(object sender, EventArgs e)
  49.         {
  50.             //SkyHttpClient skyHttpClient = new SkyHttpClient();
  51.             //try
  52.             //{
  53.             //    WhenAllDownloading();
  54.             //    foreach (var id in GetDownloadIds())
  55.             //    {
  56.             //        bool singleDownloadSuccess = await TaskDemo101.RunByHttpClient(skyHttpClient, id);
  57.             //        WhenSingleDownloading(id);
  58.             //    }
  59.             //    WhenAllDownloaded();
  60.             //}
  61.             //catch (Exception ex)
  62.             //{
  63.             //    MessageBox.Show(ex.Message, "Download Error!");
  64.             //}
  65.         }
  66.  
  67.         private void btnRunByWebClient_Click(object sender, EventArgs e)
  68.         {
  69.             WhenAllDownloading();
  70.             var ids = GetDownloadIds();
  71.             List<DownloadEntry> downloadConfigs = new List<DownloadEntry>();
  72.             Random rd = new Random();
  73.             foreach (var id in ids)
  74.             {
  75.                 downloadConfigs.Add(new DownloadEntry(TaskDemo101.GetRandomUrl(rd), TaskDemo101.GetSavedFileFullName(), id));
  76.             }
  77.             // 方案4（同步下载，需要启动另外一个线程）
  78.             ThreadPool.QueueUserWorkItem(new WaitCallback(RunByWebClientCore), downloadConfigs);
  79.         }
  80.  
  81.         private void RunByWebClientCore(object state)
  82.         {
  83.             List<DownloadEntry> downloadConfigs = (List<DownloadEntry>)state;
  84.             SkyParallelWebClient skyParallelWebClient = new SkyParallelWebClient(downloadConfigs, );
  85.             skyParallelWebClient.SetMaximumForProgress += SkyParallelWebClient_SetMaximumForProgress;
  86.             skyParallelWebClient.SetRealTimeValueForProgress += SkyParallelWebClient_SetRealTimeValueForProgress;
  87.             skyParallelWebClient.WhenAllDownloaded += SkyWebClient_WhenAllDownloaded;
  88.             skyParallelWebClient.WhenSingleDownloading += SkyWebClient_WhenSingleDownloading;
  89.             skyParallelWebClient.WhenSingleDownloaded += SkyWebClient_WhenSingleDownloaded;
  90.             skyParallelWebClient.WhenDownloadingError += SkyWebClient_WhenDownloadingError;
  91.             skyParallelWebClient.StartSync();
  92.         }
  93.  
  94.         private void SkyParallelWebClient_SetRealTimeValueForProgress(int realTimeValue)
  95.         {
  96.             this.progressBar1.Value = realTimeValue;
  97.         }
  98.  
  99.         private void SkyParallelWebClient_SetMaximumForProgress(int maximum)
  100.         {
  101.             this.progressBar1.Maximum = maximum;
  102.         }
  103.  
  104.         private void SkyWebClient_WhenDownloadingError(Exception ex)
  105.         {
  106.             MessageBox.Show("下载时出现错误： " + ex.Message);
  107.         }
  108.  
  109.         private void SkyWebClient_WhenSingleDownloading(WhenSingleDownloadEventArgs eventArg)
  110.         {
  111.             WhenSingleDownloading(eventArg);
  112.         }
  113.  
  114.         private void SkyWebClient_WhenSingleDownloaded(WhenSingleDownloadEventArgs eventArg)
  115.         {
  116.             WhenSingleDownloaded(eventArg);
  117.         }
  118.  
  119.         private void SkyWebClient_WhenAllDownloaded()
  120.         {
  121.             btnRunByWebClient.Text = "用 WebClient 开始下载";
  122.             WhenAllDownloaded();
  123.         }
  124.  
  125.     }

运行截图：

2. 异步下载模式

直接贴代码了：

  1.     public partial class Form1 : Form
  2.     {
  3.         public Form1()
  4.         {
  5.             InitializeComponent();
  6.         }
  7.  
  8.         private List<int> GetDownloadIds()
  9.         {
  10.             List<int> ids = new List<int>();
  11.             for (int i = ; i <= ; i++)
  12.             {
  13.                 ids.Add(i);
  14.             }
  15.             return ids;
  16.         }
  17.  
  18.         private void WhenAllDownloading()
  19.         {
  20.             this.listBoxLog.Items.Insert(, string.Format("当前时间：{0}，准备开始下载...", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss")));
  21.             //禁用按钮
  22.             EnableOrDisableButtons(false);
  23.         }
  24.  
  25.         private void EnableOrDisableButtons(bool enabled)
  26.         {
  27.             this.btnRunByHttpClient.Enabled = enabled;
  28.             this.btnRunByWebClient.Enabled = enabled;
  29.         }
  30.  
  31.         private void WhenSingleDownloading(WhenSingleDownloadEventArgs eventArg)
  32.         {
  33.             this.listBoxLog.Items.Insert(, string.Format("当前时间：{0}，剩余 {1} 个。编号 {2} 准备开始下载...", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"), eventArg.RemainingQueueCount, eventArg.CurrentDownloadEntry.Data));
  34.         }
  35.  
  36.         private void WhenSingleDownloaded(WhenSingleDownloadEventArgs eventArg)
  37.         {
  38.             this.listBoxLog.Items.Insert(, string.Format("当前时间：{0}，剩余 {1} 个。编号 {2} 下载完毕...", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"), eventArg.RemainingQueueCount, eventArg.CurrentDownloadEntry.Data));
  39.         }
  40.  
  41.         private void WhenAllDownloaded()
  42.         {
  43.             this.listBoxLog.Items.Insert(, string.Format("当前时间：{0}，下载完毕！", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss")));
  44.             //启用按钮
  45.             EnableOrDisableButtons(true);
  46.         }
  47.  
  48.         private async void btnRunByHttpClient_Click(object sender, EventArgs e)
  49.         {
  50.             //SkyHttpClient skyHttpClient = new SkyHttpClient();
  51.             //try
  52.             //{
  53.             //    WhenAllDownloading();
  54.             //    foreach (var id in GetDownloadIds())
  55.             //    {
  56.             //        bool singleDownloadSuccess = await TaskDemo101.RunByHttpClient(skyHttpClient, id);
  57.             //        WhenSingleDownloading(id);
  58.             //    }
  59.             //    WhenAllDownloaded();
  60.             //}
  61.             //catch (Exception ex)
  62.             //{
  63.             //    MessageBox.Show(ex.Message, "Download Error!");
  64.             //}
  65.         }
  66.  
  67.         private void btnRunByWebClient_Click(object sender, EventArgs e)
  68.         {
  69.             WhenAllDownloading();
  70.             var ids = GetDownloadIds();
  71.             List<DownloadEntry> downloadConfigs = new List<DownloadEntry>();
  72.             Random rd = new Random();
  73.             foreach (var id in ids)
  74.             {
  75.                 downloadConfigs.Add(new DownloadEntry(TaskDemo101.GetRandomUrl(rd), TaskDemo101.GetSavedFileFullName(), id));
  76.             }
  77.             //搜索： Parallel WebClient
  78.             // 方案1
  79.             //SkyWebClient skyWebClient = new SkyWebClient(downloadConfigs, this.progressBar1);
  80.             //skyWebClient.WhenAllDownloaded += SkyWebClient_WhenAllDownloaded;
  81.             //skyWebClient.WhenSingleDownloading += SkyWebClient_WhenSingleDownloading;
  82.             //skyWebClient.WhenDownloadingError += SkyWebClient_WhenDownloadingError;
  83.             //skyWebClient.Start();
  84.             // 方案2（代码已经调整，无法恢复）
  85.             //ThreadPool.QueueUserWorkItem(new WaitCallback(AsyncRunSkyParallelWebClient), downloadConfigs);
  86.             // 方案3（异步下载，完美）
  87.             SkyParallelWebClient skyParallelWebClient = new SkyParallelWebClient(downloadConfigs, );
  88.             skyParallelWebClient.SetMaximumForProgress += SkyParallelWebClient_SetMaximumForProgress;
  89.             skyParallelWebClient.SetRealTimeValueForProgress += SkyParallelWebClient_SetRealTimeValueForProgress;
  90.             skyParallelWebClient.WhenAllDownloaded += SkyWebClient_WhenAllDownloaded;
  91.             skyParallelWebClient.WhenSingleDownloading += SkyWebClient_WhenSingleDownloading;
  92.             skyParallelWebClient.WhenSingleDownloaded += SkyWebClient_WhenSingleDownloaded;
  93.             skyParallelWebClient.WhenDownloadingError += SkyWebClient_WhenDownloadingError;
  94.             skyParallelWebClient.StartSync();
  95.             // 方案4（同步下载，需要启动另外一个线程）
  96.             //ThreadPool.QueueUserWorkItem(new WaitCallback(RunByWebClientCore), downloadConfigs);
  97.         }
  98.  
  99.         private void RunByWebClientCore(object state)
  100.         {
  101.             List<DownloadEntry> downloadConfigs = (List<DownloadEntry>)state;
  102.             SkyParallelWebClient skyParallelWebClient = new SkyParallelWebClient(downloadConfigs, );
  103.             skyParallelWebClient.SetMaximumForProgress += SkyParallelWebClient_SetMaximumForProgress;
  104.             skyParallelWebClient.SetRealTimeValueForProgress += SkyParallelWebClient_SetRealTimeValueForProgress;
  105.             skyParallelWebClient.WhenAllDownloaded += SkyWebClient_WhenAllDownloaded;
  106.             skyParallelWebClient.WhenSingleDownloading += SkyWebClient_WhenSingleDownloading;
  107.             skyParallelWebClient.WhenSingleDownloaded += SkyWebClient_WhenSingleDownloaded;
  108.             skyParallelWebClient.WhenDownloadingError += SkyWebClient_WhenDownloadingError;
  109.             skyParallelWebClient.StartSync();
  110.         }
  111.  
  112.         private void SkyParallelWebClient_SetRealTimeValueForProgress(int realTimeValue)
  113.         {
  114.             this.progressBar1.Value = realTimeValue;
  115.         }
  116.  
  117.         private void SkyParallelWebClient_SetMaximumForProgress(int maximum)
  118.         {
  119.             this.progressBar1.Maximum = maximum;
  120.         }
  121.  
  122.         private void SkyWebClient_WhenDownloadingError(Exception ex)
  123.         {
  124.             MessageBox.Show("下载时出现错误： " + ex.Message);
  125.         }
  126.  
  127.         private void SkyWebClient_WhenSingleDownloading(WhenSingleDownloadEventArgs eventArg)
  128.         {
  129.             WhenSingleDownloading(eventArg);
  130.         }
  131.  
  132.         private void SkyWebClient_WhenSingleDownloaded(WhenSingleDownloadEventArgs eventArg)
  133.         {
  134.             WhenSingleDownloaded(eventArg);
  135.         }
  136.  
  137.         private void SkyWebClient_WhenAllDownloaded()
  138.         {
  139.             btnRunByWebClient.Text = "用 WebClient 开始下载";
  140.             WhenAllDownloaded();
  141.         }
  142.  
  143.     }

运行截图：

如上图：当打印了“下载完毕”后，仍然打印了许多日志，这是因为异步下载的回调是不定时的。为了避免这种情况，建议 WhenSingleDownloading 事件和 WhenSingleDownloaded 事件二选一，因为实在没有必要下载开始前和下载后都打印日志。我们再次修改代码后，得到如下图的日志。

日志是不是清晰了很多？一般下载完成，可以注册完成事件，事件里面不用打印日志，而做一些比如“修改数据库表记录的状态字段”等等。

3. 总结

SkyParallelWebClient 完整的代码如下：

  1.     /// <summary>
  2.     /// 设置进度条的最大值的事件处理
  3.     /// </summary>
  4.     /// <param name="maximum"></param>
  5.     public delegate void SetMaximumForProgressEventHandler(int maximum);
  6.  
  7.     /// <summary>
  8.     /// 设置进度条的当前实时的值的事件处理
  9.     /// </summary>
  10.     /// <param name="maximum"></param>
  11.     public delegate void SetRealTimeValueForProgressEventHandler(int realTimeValue);
  12.  
  13.     /// <summary>
  14.     /// 并行的 WebClient
  15.     /// </summary>
  16.     public class SkyParallelWebClient : SkyWebClientBase
  17.     {
  18.         ConcurrentQueue<DownloadEntry> OptionDataList = new ConcurrentQueue<DownloadEntry>(); //比如说：有 500 个元素
  19.  
  20.         ConcurrentDictionary<WebClient, DownloadEntry> ProcessingTasks = new ConcurrentDictionary<WebClient, DownloadEntry>(); //当前运行中的
  21.  
  22.         public event SetMaximumForProgressEventHandler SetMaximumForProgress;
  23.         protected virtual void OnSetMaximumForProgress(int maximum)
  24.         {
  25.             if (SetMaximumForProgress != null)
  26.             {
  27.                 SetMaximumForProgress(maximum);
  28.             }
  29.         }
  30.  
  31.         public event SetRealTimeValueForProgressEventHandler SetRealTimeValueForProgress;
  32.         protected virtual void OnSetRealTimeValueForProgress(int realTimeValue)
  33.         {
  34.             if (SetRealTimeValueForProgress != null)
  35.             {
  36.                 SetRealTimeValueForProgress(realTimeValue);
  37.             }
  38.         }
  39.  
  40.         public int ParallelCount { get; }
  41.  
  42.         public bool IsCompleted { get; private set; }
  43.  
  44.         public int Maximum { get;}
  45.  
  46.         private static object lockObj = new object();
  47.  
  48.         /// <summary>
  49.         /// 构造函数
  50.         /// </summary>
  51.         /// <param name="downloadConfigs">要下载的全部集合，比如 N 多要下载的，N无限制</param>
  52.         public SkyParallelWebClient(IEnumerable<DownloadEntry> downloadConfigs)
  53.             :this(downloadConfigs, )
  54.         {
  55.  
  56.         }
  57.  
  58.         /// <summary>
  59.         /// 构造函数
  60.         /// </summary>
  61.         /// <param name="downloadConfigs">要下载的全部集合，比如 N 多要下载的，N无限制</param>
  62.         /// <param name="parallelCount">单位内，并行下载的个数。切忌：该数字不能过大，否则可能很多文件因为 WebClient 超时，而导致乱文件（即：文件不完整）一般推荐 20 个左右</param>
  63.         public SkyParallelWebClient(IEnumerable<DownloadEntry> downloadConfigs, int parallelCount)
  64.         {
  65.             if (downloadConfigs == null)
  66.             {
  67.                 throw new ArgumentNullException("downloadConfigs");
  68.             }
  69.             this.ParallelCount = parallelCount;
  70.             foreach (var item in downloadConfigs)
  71.             {
  72.                 OptionDataList.Enqueue(item);
  73.             }
  74.             this.Maximum = OptionDataList.Count;
  75.             System.Net.ServicePointManager.DefaultConnectionLimit = int.MaxValue;
  76.             OnSetMaximumForProgress(this.Maximum);
  77.         }
  78.  
  79.         /// <summary>
  80.         /// 异步启动（备注：由于内部采用异步下载，所以方法不用加 Try 和返回值）
  81.         /// </summary>
  82.         public void StartAsync()
  83.         {
  84.             StartAsyncCore();
  85.         }
  86.  
  87.         protected void StartAsyncCore()
  88.         {
  89.             if (OptionDataList.Count <= )
  90.             {
  91.                 SetIsCompletedTrue();
  92.                 return;
  93.             }
  94.             while (OptionDataList.Count >  && ProcessingTasks.Count <= ParallelCount)
  95.             {
  96.                 DownloadEntry downloadEntry;
  97.                 if (!OptionDataList.TryDequeue(out downloadEntry))
  98.                 {
  99.                     break;
  100.                 }
  101.                 DownloadFileAsync(downloadEntry);
  102.                 OnWhenSingleDownloading(new WhenSingleDownloadEventArgs
  103.                 {
  104.                     CurrentDownloadEntry = downloadEntry,
  105.                     RemainingQueueCount = OptionDataList.Count
  106.                 });
  107.             }
  108.         }
  109.  
  110.         /// <summary>
  111.         /// 同步启动
  112.         /// </summary>
  113.         public void StartSync()
  114.         {
  115.             StartSyncCore();
  116.         }
  117.  
  118.         /// <summary>
  119.         /// 同步启动
  120.         /// </summary>
  121.         public void StartSync(WebClient webClient)
  122.         {
  123.             StartSyncCore(webClient);
  124.         }
  125.  
  126.         protected void StartSyncCore()
  127.         {
  128.             using (WebClient webClient = new WebClient())
  129.             {
  130.                 StartSyncCore(webClient);
  131.             }
  132.         }
  133.  
  134.         protected void StartSyncCore(WebClient webClient)
  135.         {
  136.             while (OptionDataList.Count > )
  137.             {
  138.                 DownloadEntry downloadEntry;
  139.                 if (!OptionDataList.TryDequeue(out downloadEntry))
  140.                 {
  141.                     break;
  142.                 }
  143.                 OnWhenSingleDownloading(new WhenSingleDownloadEventArgs
  144.                 {
  145.                     CurrentDownloadEntry = downloadEntry,
  146.                     RemainingQueueCount = OptionDataList.Count
  147.                 });
  148.                 DownloadFile(downloadEntry, webClient);
  149.                 OnWhenSingleDownloaded(new WhenSingleDownloadEventArgs
  150.                 {
  151.                     CurrentDownloadEntry = downloadEntry,
  152.                     RemainingQueueCount = OptionDataList.Count
  153.                 });
  154.             }
  155.             SetIsCompletedTrue();
  156.         }
  157.  
  158.         protected void SetIsCompletedTrue()
  159.         {
  160.             if (!IsCompleted)
  161.             {
  162.                 lock (lockObj)
  163.                 {
  164.                     if (!IsCompleted)
  165.                     {
  166.                         OnSetRealTimeValueForProgress();//表示已经完成
  167.                         OnWhenAllDownloaded();
  168.                         IsCompleted = true;
  169.                     }
  170.                 }
  171.             }
  172.         }
  173.  
  174.         private void DownloadFileAsync(DownloadEntry downloadEntry)
  175.         {
  176.             using (WebClient webClient = new WebClient())
  177.             {
  178.                 webClient.Proxy = null;
  179.                 webClient.DownloadFileCompleted += WebClient_DownloadFileCompleted;
  180.                 webClient.DownloadFileAsync(new Uri(downloadEntry.Url), downloadEntry.Path);
  181.                 ProcessingTasks.TryAdd(webClient, downloadEntry);
  182.             }
  183.         }
  184.  
  185.         private void DownloadFile(DownloadEntry downloadEntry)
  186.         {
  187.             using (WebClient webClient = new WebClient())
  188.             {
  189.                 DownloadFile(downloadEntry, webClient);
  190.             }
  191.         }
  192.  
  193.         private void DownloadFile(DownloadEntry downloadEntry, WebClient webClient)
  194.         {
  195.             try
  196.             {
  197.                 webClient.Proxy = null;
  198.                 webClient.DownloadFile(new Uri(downloadEntry.Url), downloadEntry.Path);
  199.             }
  200.             catch (Exception ex)
  201.             {
  202.                 OnWhenDownloadingError(ex);
  203.             }
  204.         }
  205.  
  206.         private void WebClient_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
  207.         {
  208.             WebClient webClient = (WebClient)sender;
  209.             DownloadEntry downloadEntry;
  210.             if (ProcessingTasks.TryRemove(webClient, out downloadEntry))
  211.             {
  212.                 OnWhenSingleDownloaded(new WhenSingleDownloadEventArgs
  213.                 {
  214.                     CurrentDownloadEntry = downloadEntry,
  215.                     RemainingQueueCount = OptionDataList.Count
  216.                 });
  217.                 try
  218.                 {
  219.                     int realTimeValue = (this.Maximum - OptionDataList.Count) *  / this.Maximum;
  220.                     OnSetRealTimeValueForProgress(realTimeValue);   //表示单个已经完成
  221.                 }
  222.                 catch (Exception)
  223.                 {
  224.  
  225.                 }
  226.             }
  227.             StartAsyncCore();
  228.         }
  229.     }

谢谢浏览！

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