IProgress<T> 同步

Question

我在 C# 中有以下内容

public static void Main()
{
    var result = Foo(new Progress<int>(i =>
        Console.WriteLine("Progress: " + i)));

    Console.WriteLine("Result: " + result);            
    Console.ReadLine();
}

static int Foo(IProgress<int> progress)
{
    for (int i = 0; i < 10; i++)
        progress.Report(i);

    return 1001;
}

Main 的一些输出是：

第一次运行：

Result: 1001
Progress: 4
Progress: 6
Progress: 7
Progress: 8
Progress: 9
Progress: 3
Progress: 0
Progress: 1
Progress: 5
Progress: 2

第二次运行：

Progress: 4
Progress: 5
Progress: 6
Progress: 7
Progress: 8
Progress: 9
Progress: 0
Progress: 1
Progress: 2
Result: 1001
Progress: 3

等等……

每次运行，输出都是不同的。如何同步这些方法，以便按照它们报告的顺序显示进度 0,1,...9，然后是结果 1001。我希望输出如下所示：

Progress: 0
.
.
.
Progress: 9
Result: 1001

Answer 1

Progress 类使用 SynchronizationContext.Current 属性来 Post() 进度更新。这样做是为了确保在程序的 UI 线程上触发 ProgressChanged 事件，以便更新 UI 是安全的。安全更新 ProgressBar.Value 属性所必需的。

控制台模式应用的问题在于它没有同步提供程序。不像 Winforms 或 WPF 应用程序。 Synchronization.Current 属性具有默认提供程序，其 Post() 方法在线程池线程上运行。在没有任何联锁的情况下，哪个 TP 线程首先报告其更新是完全不可预测的。也没有什么好的联锁方法。

只是不要在这里使用 Progress 类，没有意义。您在控制台模式应用程序中没有 UI 线程安全问题，控制台类已经是线程安全的。修复：

static int Foo()
{
    for (int i = 0; i < 10; i++)
        Console.WriteLine("Progress: {0}", i);

    return 1001;
}

Answer 2

如Hans' answer 中所述，Progress<T> 的 .NET 实现使用 SynchronizationContext.Post 发送其请求。你可以让它像Yves' answer一样直接报告，或者你可以使用SynchronizationContext.Send，这样请求将被阻塞，直到接收者处理它。

因为Reference Source is available 的实现就像复制源并将Post 更改为Send 并将SynchronizationContext.CurrentNoFlow 更改为SynchronizationContext.Current 一样简单，因为CurrentNoFlow 是一个内部属性。

/// <summary>
/// Provides an IProgress{T} that invokes callbacks for each reported progress value.
/// </summary>
/// <typeparam name="T">Specifies the type of the progress report value.</typeparam>
/// <remarks>
/// Any handler provided to the constructor or event handlers registered with
/// the <see cref="ProgressChanged"/> event are invoked through a 
/// <see cref="System.Threading.SynchronizationContext"/> instance captured
/// when the instance is constructed.  If there is no current SynchronizationContext
/// at the time of construction, the callbacks will be invoked on the ThreadPool.
/// </remarks>
public class SynchronousProgress<T> : IProgress<T>
{
    /// <summary>The synchronization context captured upon construction.  This will never be null.</summary>
    private readonly SynchronizationContext m_synchronizationContext;
    /// <summary>The handler specified to the constructor.  This may be null.</summary>
    private readonly Action<T> m_handler;
    /// <summary>A cached delegate used to post invocation to the synchronization context.</summary>
    private readonly SendOrPostCallback m_invokeHandlers;

    /// <summary>Initializes the <see cref="Progress{T}"/>.</summary>
    public SynchronousProgress()
    {
        // Capture the current synchronization context.  "current" is determined by Current.
        // If there is no current context, we use a default instance targeting the ThreadPool.
        m_synchronizationContext = SynchronizationContext.Current ?? ProgressStatics.DefaultContext;
        Contract.Assert(m_synchronizationContext != null);
        m_invokeHandlers = new SendOrPostCallback(InvokeHandlers);
    }

    /// <summary>Initializes the <see cref="Progress{T}"/> with the specified callback.</summary>
    /// <param name="handler">
    /// A handler to invoke for each reported progress value.  This handler will be invoked
    /// in addition to any delegates registered with the <see cref="ProgressChanged"/> event.
    /// Depending on the <see cref="System.Threading.SynchronizationContext"/> instance captured by 
    /// the <see cref="Progress"/> at construction, it's possible that this handler instance
    /// could be invoked concurrently with itself.
    /// </param>
    /// <exception cref="System.ArgumentNullException">The <paramref name="handler"/> is null (Nothing in Visual Basic).</exception>
    public SynchronousProgress(Action<T> handler) : this()
    {
        if (handler == null) throw new ArgumentNullException("handler");
        m_handler = handler;
    }

    /// <summary>Raised for each reported progress value.</summary>
    /// <remarks>
    /// Handlers registered with this event will be invoked on the 
    /// <see cref="System.Threading.SynchronizationContext"/> captured when the instance was constructed.
    /// </remarks>
    public event EventHandler<T> ProgressChanged;

    /// <summary>Reports a progress change.</summary>
    /// <param name="value">The value of the updated progress.</param>
    protected virtual void OnReport(T value)
    {
        // If there's no handler, don't bother going through the [....] context.
        // Inside the callback, we'll need to check again, in case 
        // an event handler is removed between now and then.
        Action<T> handler = m_handler;
        EventHandler<T> changedEvent = ProgressChanged;
        if (handler != null || changedEvent != null)
        {
            // Post the processing to the [....] context.
            // (If T is a value type, it will get boxed here.)
            m_synchronizationContext.Send(m_invokeHandlers, value);
        }
    }

    /// <summary>Reports a progress change.</summary>
    /// <param name="value">The value of the updated progress.</param>
    void IProgress<T>.Report(T value) { OnReport(value); }

    /// <summary>Invokes the action and event callbacks.</summary>
    /// <param name="state">The progress value.</param>
    private void InvokeHandlers(object state)
    {
        T value = (T)state;

        Action<T> handler = m_handler;
        EventHandler<T> changedEvent = ProgressChanged;

        if (handler != null) handler(value);
        if (changedEvent != null) changedEvent(this, value);
    }
}

/// <summary>Holds static values for <see cref="Progress{T}"/>.</summary>
/// <remarks>This avoids one static instance per type T.</remarks>
internal static class ProgressStatics
{
    /// <summary>A default synchronization context that targets the ThreadPool.</summary>
    internal static readonly SynchronizationContext DefaultContext = new SynchronizationContext();
}

Answer 3

正如其他答案之前多次指出的那样，这是由于Progress<T> 的实现方式。您可以为您的客户（库的用户）提供示例代码，或为控制台项目提供IProgress<T> 的实现。这是基本的，但应该这样做。

public class ConsoleProgress<T> : IProgress<T>
{
    private Action<T> _action;

    public ConsoleProgress(Action<T> action) {
        if(action == null) {
            throw new ArgumentNullException(nameof(action));
        }

        _action = action;
    }

    public void Report(T value) {
        _action(value);
    }
}

Answer 4

这是Progress<T> 编写方式的线程问题。您需要编写自己的 IProgress<T> 实现来获得所需的内容。

然而，这个场景已经告诉你一些重要的事情，虽然在这个例子中，你只是在做简单的Console.Writeline 语句，在实际场景中，由于需要更长或更短的时间，一些报告可能会以其他顺序报告，所以在我的认为无论如何你都不应该依赖订单。