如何从并发运行的任务更新进度条

Question

我尝试将并行任务绑定到包含pprogressBars的listView。 我正在使用一个有限的调度程序，它只允许指定的最大并行度。 到目前为止它大部分时间都可以正常工作，但偶尔会有两个任务更新listView中的相同进度条。 以下是我的代码。

知道如何防止两个任务更新listView中的相同进度条吗？ 或者如何从同时运行的任务中更新进度条？

public class MyClass
{
  public ObservableCollection<StatusInfo> StatusItems { get; set; }
  private Object thisLock = new Object();

  public int Process() //handled
  {
    StatusItems = new ObservableCollection<StatusInfo>();
    for (int i = 0; i < 4; i++)  // initialize progress bar collection
    {
      StatusInfo sInfo = new StatusInfo();
      sInfo.ThreadID = i;
      sInfo.Table = "No Table";
      sInfo.Percentage = 0;
      sInfo.Status = AppInfo.AVAILABLE;
      sInfo.Minimum = 0;
      sInfo.Maximum = 100;
      sInfo.Visibility = Visibility.Visible;
      StatusItems.Add(sInfo);
    }
    Parent.StatusItems = StatusItems; // assign to viewmodel

    int numberOfBackGroundThread = 4;
    LimitedTaskScheduler scheduler = new LimitedTaskScheduler(numberOfBackGroundThread);
    TaskFactory factory = new TaskFactory(scheduler);
    var ui = LimitedTaskScheduler.FromCurrentSynchronizationContext();

    Task[] tasks = new Task[rows.Count];
    for (int i = 0; i < rows.Count; i++)
    {
      ......
      tasks[i] = factory.StartNew<string>(() =>
      {
        int barIndex = -1;
        int threadID = Thread.CurrentThread.ManagedThreadId;
        cnt++;
        if (cnt > numberOfBackGroundThread - 1)
        {
          while (true)
          {                 
            for (int j = 0; j < numberOfBackGroundThread; j++)
            {
              lock (thisLock)
              {
                if (StatusItems[j].Status == "AVAILABLE" || StatusItems[j].Status == "DONE") 
                {
                  //Console.WriteLine(String.Format("Current table = {0}, Cuurent table type = {1}, BarIndex = {2}, ThreadID = {3}, Prev TableName = {4}, Prev TableType = {5}, Prev Status = {6} PrevThreadID = {7}", tabName, tabType, j.ToString(), Thread.CurrentThread.ManagedThreadId.ToString(), StatusItems[j].Table, StatusItems[j].TabType, StatusItems[j].Status, StatusItems[j].ThreadID));
                  // 0. Assign the current task to a display slot StatusItems[j] that is currently free.
                  // 1. We lock this piece of code to prevent another concurrent task from picking the same slot.
                  // 2. And we update the slot as "Occupied" immediately to avoid to prevent another concurrent task from picking the same slot.
                  // 3. We also add one extra slot to avoid concurrent tasks competing on the same sidplay slot and reduce wait on the lock.
                  // All of the above cannot completely avoid two thread using the same display slot. We may need a new way to 
                  // 4. Since multiple tasks may run on the same thread we may see the same thread ID appear on different display slots.
                  StatusItems[j].Status = "Occupied";
                  barIndex = j;
                  break; // break for loop
                }
              }
            }
            if (barIndex >= 0) { break; }  // break while loop
          }
        }
        else { barIndex = cnt; }

        StatusItems[barIndex].TabType = tabType;
        StatusItems[barIndex].ThreadID = threadID;

        int nStatus = IndividualProcess(barIndex);
        if (nStatus < 0) { AppInfo.JobStatus = "01"; }
        return result;
      });
    }
    var done = factory.ContinueWhenAll(tasks, completedTasks => { AppInfo.Finished = true; });
    done.ContinueWith(completedTasks => { int nStatus = PostProcess(); }, ui);
    return returnStatus;
  }

  private int IndividualProcess(int barIndex)
  {
     for (int i=0; i< 100; i++)  
     {
       perform work...
       SetProgressbar (i, StatusItems, barIndex, "in progress")
     }
     SetProgressbar (100, StatusItems, barIndex, "DONE")
  } 

  public void SetProgressbar(int pPercent, ObservableCollection<StatusInfo> pInfo, int pInfoIndex, string pStatus)
  {
    try // Increment percentage for COPY or nested PURGE 
    {
      if (Application.Current.Dispatcher.Thread != System.Threading.Thread.CurrentThread)
      {
        Application.Current.Dispatcher.BeginInvoke(new Action(() =>
        {
          ((StatusInfo)pInfo[pInfoIndex]).Percentage = pPercent;
          ((StatusInfo)pInfo[pInfoIndex]).Status = pStatus; 
          ((StatusInfo)pInfo[pInfoIndex]).PCT = pPercent.ToString() + "%";
        }));
      }
      else // When the current thread is main UI thread. The label won't be updated until the EntityCopy() finishes.
      {
        ((StatusInfo)pInfo[pInfoIndex]).Percentage = pPercent;
        ((StatusInfo)pInfo[pInfoIndex]).Status = pStatus; 
        ((StatusInfo)pInfo[pInfoIndex]).PCT = pPercent.ToString() + "%";
      }
    }
    catch { throw; }
  }      
}

public class LimitedTaskScheduler : TaskScheduler
{
  // Fields
  // Whether the current thread is processing work items.
  [ThreadStatic]
  private static bool _currentThreadIsProcessingItems;
  // The list of tasks to be executed. 
  private readonly LinkedList<Task> _tasks = new LinkedList<Task>(); // protected by lock(_tasks) 
  /// <summary>The maximum concurrency level allowed by this scheduler.</summary> 
  private readonly int _maxDegreeOfParallelism;
  /// <summary>Whether the scheduler is currently processing work items.</summary> 
  private int _delegatesQueuedOrRunning = 0; // protected by lock(_tasks) 

  /// <summary> 
  /// Initializes an instance of the LimitedConcurrencyLevelTaskScheduler class with the 
  /// specified degree of parallelism. 
  /// </summary> 
  /// <param name="maxDegreeOfParallelism">The maximum degree of parallelism provided by this scheduler.</param>
  public LimitedTaskScheduler(int maxDegreeOfParallelism)
  {
      if (maxDegreeOfParallelism < 1) throw new ArgumentOutOfRangeException("maxDegreeOfParallelism");
      _maxDegreeOfParallelism = maxDegreeOfParallelism;
  }

  /// <summary>Queues a task to the scheduler.</summary> 
  /// <param name="task">The task to be queued.</param>
  protected sealed override void QueueTask(Task task)
  {
      // Add the task to the list of tasks to be processed.  If there aren't enough 
      // delegates currently queued or running to process tasks, schedule another. 
    lock (_tasks)
      {
          _tasks.AddLast(task);
          if (_delegatesQueuedOrRunning < _maxDegreeOfParallelism)
          {
              ++_delegatesQueuedOrRunning;
              NotifyThreadPoolOfPendingWork();
          }
      }
  }

  /// <summary> 
  /// Informs the ThreadPool that there's work to be executed for this scheduler. 
  /// </summary> 
  private void NotifyThreadPoolOfPendingWork()
  {
      ThreadPool.UnsafeQueueUserWorkItem(_ =>
      {
          // Note that the current thread is now processing work items. 
          // This is necessary to enable inlining of tasks into this thread.
          _currentThreadIsProcessingItems = true;
          try
          {
              // Process all available items in the queue. 
              while (true)
              {
                  Task item;
                  lock (_tasks)
                  {
                      // When there are no more items to be processed, 
                      // note that we're done processing, and get out. 
                      if (_tasks.Count == 0)
                      {
                          --_delegatesQueuedOrRunning;
                          break;
                      }

                      // Get the next item from the queue
                      item = _tasks.First.Value;
                      _tasks.RemoveFirst();
                  }

                  // Execute the task we pulled out of the queue 
                  base.TryExecuteTask(item);
              }
          }
          // We're done processing items on the current thread 
          finally { _currentThreadIsProcessingItems = false; }
      }, null);
  }

  /// <summary>Attempts to execute the specified task on the current thread.</summary> 
  /// <param name="task">The task to be executed.</param>
  /// <param name="taskWasPreviouslyQueued"></param>
  /// <returns>Whether the task could be executed on the current thread.</returns> 
  protected sealed override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
  {
      // If this thread isn't already processing a task, we don't support inlining 
      if (!_currentThreadIsProcessingItems) return false;

      // If the task was previously queued, remove it from the queue 
      if (taskWasPreviouslyQueued) TryDequeue(task);

      // Try to run the task. 
      return base.TryExecuteTask(task);
  }

  /// <summary>Attempts to remove a previously scheduled task from the scheduler.</summary> 
  /// <param name="task">The task to be removed.</param>
  /// <returns>Whether the task could be found and removed.</returns> 
  protected sealed override bool TryDequeue(Task task)
  {
      lock (_tasks) return _tasks.Remove(task);
  }

  /// <summary>Gets the maximum concurrency level supported by this scheduler.</summary> 
  public sealed override int MaximumConcurrencyLevel { get { return _maxDegreeOfParallelism; } }

  /// <summary>Gets an enumerable of the tasks currently scheduled on this scheduler.</summary> 
  /// <returns>An enumerable of the tasks currently scheduled.</returns> 
  protected sealed override IEnumerable<Task> GetScheduledTasks()
  {
      bool lockTaken = false;
      try
      {
          Monitor.TryEnter(_tasks, ref lockTaken);
          if (lockTaken) return _tasks.ToArray();
          else throw new NotSupportedException();
      }
      finally
      {
          if (lockTaken) Monitor.Exit(_tasks);
      }
  }
}

更新： 任务调度程序可能不相关。我把它放在这里以防万一有人能找到一些我从未想过要做或丢失的新东西。

Answer 1

  

知道如何防止两个任务更新相同的进度条   listView？或者如何从正在运行的任务更新进度条   同时？

如果您有多个并行运行的任务，并且您确实希望向用户显示每个个别任务的进度，那么您需要为每个任务显示单独的进度条。

如何操作取决于UI结构。例如，如果您有一个列表视图，其中每个项目都是一个任务，您可以为每个列表视图项目添加一个进度条作为子窗口：

这可能是一种矫枉过正，通常只有一个进度条来跟踪整体进度。即，如果您有100个任务，则当完成所有100个任务时，您的进度条将显示100％。

请注意，任务＃50（例如）可能在任务＃45之前完成，因此您无法使用任务的号码来更新进度。要正确显示进度，您需要计算已完成的任务，并将该计数器用作进度指示器。

更新了以解决评论：

  

我在listview中有4个进度条和500个任务。在任何给定的时间   由于受限制，只有4个任务同时运行   调度。我尝试在listview中分配一个免费的进度条   状态到新的传入任务，然后设置的状态   任务完成后进度条自由，以便进度条可以   被另一个新的传入任务重用。那有意义吗？或者我   去了一个deadend？

我不确定这是一个明智的用户界面设计决定，但如果你想这样做，你可以使用SemaphoreSlim将进度条分配为有限的资源。例如：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace TaskProgress
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private async void Form1_Load(object sender, EventArgs e)
        {
            await DoWorkAsync();
        }

        const int MAX_PARALLEL = 4;

        readonly object _lock = new Object();

        readonly SemaphoreSlim _semaphore = 
            new SemaphoreSlim(initialCount: MAX_PARALLEL);

        HashSet<Task> _pendingTasks;

        Queue<ProgressBar> _availableProgressBars;

        // do all Task
        async Task DoWorkAsync()
        {
            _availableProgressBars = new Queue<ProgressBar>();
            _pendingTasks = new HashSet<Task>();

            var progressBars = new ProgressBar[] { 
                this.progressBar1, 
                this.progressBar2, 
                this.progressBar3, 
                this.progressBar4 };

            foreach (var item in progressBars)
                _availableProgressBars.Enqueue(item);

            for (int i = 0; i < 50; i++) // start 50 tasks
                QueueTaskAsync(DoTaskAsync());

            await Task.WhenAll(WithLock(() => _pendingTasks.ToArray()));
        }

        // do a sigle Task
        readonly Random _random = new Random(Environment.TickCount);

        async Task DoTaskAsync()
        {
            await _semaphore.WaitAsync();
            try
            {
                var progressBar = WithLock(() => _availableProgressBars.Dequeue());
                try
                {
                    progressBar.Maximum = 100;
                    progressBar.Value = 0;
                    IProgress<int> progress = 
                        new Progress<int>(value => progressBar.Value = value);

                    await Task.Run(() =>
                    {
                        // our simulated work takes no more than 10s
                        var sleepMs = _random.Next(10000) / 100;
                        for (int i = 0; i < 100; i++)
                        {
                            Thread.Sleep(sleepMs); // simulate work item
                            progress.Report(i);
                        }
                    });
                }
                finally
                {
                    WithLock(() => _availableProgressBars.Enqueue(progressBar));
                }
            }
            finally
            {
                _semaphore.Release();
            }
        }

        // Add/remove a task to the list of pending tasks
        async void QueueTaskAsync(Task task)
        {
            WithLock(() => _pendingTasks.Add(task));
            try
            {
                await task;
            }
            catch
            {
                if (!task.IsCanceled && !task.IsFaulted)
                    throw;
                return;
            }
            WithLock(() => _pendingTasks.Remove(task));
        }

        // execute func inside a lock
        T WithLock<T>(Func<T> func)
        {
            lock (_lock)
                return func();
        }

        // execute action inside a lock
        void WithLock(Action action)
        {
            lock (_lock)
                action();
        }

    }
}

此代码不使用自定义任务调度程序，SemaphoreSlim足以限制并发性。另请注意，保护锁（WithLock）在这里是多余的，因为Task.Run lambda之外的所有内容都在UI线程上运行。但是，我决定保留锁，因为您的应用程序可能有不同的线程模型。在这种情况下，无论您在何处访问ProgressBar或其他用户界面，都应该使用BeginInvoke或类似的方式在UI线程上进行操作。

另外，请检查"Async in 4.5: Enabling Progress and Cancellation in Async APIs"以获取有关Progress<T>模式的详细信息。