C# Task WhenAny和WhenAll 以及TaskFactory 的ContinueWhenAny和ContinueWhenAll的实现
个人感觉Task 的WaitAny和WhenAny以及TaskFactory 的ContinueWhenAny有相似的地方，而WaitAll和WhenAll以及TaskFactory 的ContinueWhenAll也是相同，但是WaitAny和WhenAny的返回值有所不同。我们首先来看看Task WhenAny和WhenAll 的实现吧，
public class Task : IThreadPoolWorkItem, IAsyncResult, IDisposable { //Creates a task that will complete when any of the supplied tasks have completed. public static Task<Task> WhenAny(IEnumerable<Task> tasks) { if (tasks == null) throw new ArgumentNullException("tasks"); Contract.EndContractBlock(); List<Task> taskList = new List<Task>(); foreach (Task task in tasks) { if (task == null) throw new ArgumentException(Environment.GetResourceString("Task_MultiTaskContinuation_NullTask"), "tasks"); taskList.Add(task); } if (taskList.Count == 0) { throw new ArgumentException(Environment.GetResourceString("Task_MultiTaskContinuation_EmptyTaskList"), "tasks"); } // Previously implemented CommonCWAnyLogic() can handle the rest return TaskFactory.CommonCWAnyLogic(taskList); } //Creates a task that will complete when all of the supplied tasks have completed. public static Task<TResult[]> WhenAll<TResult>(params Task<TResult>[] tasks) { if (tasks == null) throw new ArgumentNullException("tasks"); Contract.EndContractBlock(); int taskCount = tasks.Length; if (taskCount == 0) return InternalWhenAll<TResult>(tasks); // small optimization in the case of an empty task array Task<TResult>[] tasksCopy = new Task<TResult>[taskCount]; for (int i = 0; i < taskCount; i++) { Task<TResult> task = tasks[i]; if (task == null) throw new ArgumentException(Environment.GetResourceString("Task_MultiTaskContinuation_NullTask"), "tasks"); tasksCopy[i] = task; } return InternalWhenAll<TResult>(tasksCopy); } private static Task<TResult[]> InternalWhenAll<TResult>(Task<TResult>[] tasks) { Contract.Requires(tasks != null, "Expected a non-null tasks array"); return (tasks.Length == 0) ? new Task<TResult[]>(false, new TResult[0], TaskCreationOptions.None, default(CancellationToken)) : new WhenAllPromise<TResult>(tasks); } private sealed class WhenAllPromise<T> : Task<T[]>, ITaskCompletionAction { private readonly Task<T>[] m_tasks; private int m_count; internal WhenAllPromise(Task<T>[] tasks) :base() { Contract.Requires(tasks != null, "Expected a non-null task array"); Contract.Requires(tasks.Length > 0, "Expected a non-zero length task array"); m_tasks = tasks; m_count = tasks.Length; if (AsyncCausalityTracer.LoggingOn) AsyncCausalityTracer.TraceOperationCreation(CausalityTraceLevel.Required, this.Id, "Task.WhenAll", 0); if (s_asyncDebuggingEnabled) { AddToActiveTasks(this); } foreach (var task in tasks) { if (task.IsCompleted) this.Invoke(task); // short-circuit the completion action, if possible else task.AddCompletionAction(this); // simple completion action } } public void Invoke(Task ignored) { if (AsyncCausalityTracer.LoggingOn) AsyncCausalityTracer.TraceOperationRelation(CausalityTraceLevel.Important, this.Id, CausalityRelation.Join); // Decrement the count, and only continue to complete the promise if we're the last one. if (Interlocked.Decrement(ref m_count) == 0) { T[] results = new T[m_tasks.Length]; List<ExceptionDispatchInfo> observedExceptions = null; Task canceledTask = null; for (int i = 0; i < m_tasks.Length; i++) { Task<T> task = m_tasks[i]; Contract.Assert(task != null, "Constituent task in WhenAll should never be null"); if (task.IsFaulted) { if (observedExceptions == null) observedExceptions = new List<ExceptionDispatchInfo>(); observedExceptions.AddRange(task.GetExceptionDispatchInfos()); } else if (task.IsCanceled) { if (canceledTask == null) canceledTask = task; // use the first task that's canceled } else { Contract.Assert(task.Status == TaskStatus.RanToCompletion); results[i] = task.GetResultCore(waitCompletionNotification: false); // avoid Result, which would triggering debug notification } if (task.IsWaitNotificationEnabled) this.SetNotificationForWaitCompletion(enabled: true); else m_tasks[i] = null; // avoid holding onto tasks unnecessarily } if (observedExceptions != null) { Contract.Assert(observedExceptions.Count > 0, "Expected at least one exception"); TrySetException(observedExceptions); } else if (canceledTask != null) { TrySetCanceled(canceledTask.CancellationToken, canceledTask.GetCancellationExceptionDispatchInfo()); } else { if (AsyncCausalityTracer.LoggingOn) AsyncCausalityTracer.TraceOperationCompletion(CausalityTraceLevel.Required, this.Id, AsyncCausalityStatus.Completed); if (Task.s_asyncDebuggingEnabled) { RemoveFromActiveTasks(this.Id); } TrySetResult(results); } } Contract.Assert(m_count >= 0, "Count should never go below 0"); } internal override bool ShouldNotifyDebuggerOfWaitCompletion { get { return base.ShouldNotifyDebuggerOfWaitCompletion && Task.AnyTaskRequiresNotifyDebuggerOfWaitCompletion(m_tasks); } } } }
首先我们来看看Task的WhenAny的实现，非常简单调用TaskFactory.CommonCWAnyLogic方法，直接返回Task，而WaitAny【也调用TaskFactory.CommonCWAnyLogic】则需要等待这个Task完成。接下来我们来看看WhenAll的实现，WhenAll核心方法是InternalWhenAll，在InternalWhenAll里面返回了一个WhenAllPromise的Task，WhenAllPromise里面有一个计数器m_count，每当Task完成一个，就调用WhenAllPromise的Invoke方法，实现计数器m_count减1，当计数器m_count为0时表示所有的Task有已经完成。【在Task的WaitAll里面用的是SetOnCountdownMres，和这里的WhenAllPromise相似，都有一个计数器】。
那么我们现在来看看TaskFactory的ContinueWhenAny和ContinueWhenAll的实现
public class TaskFactory { public Task ContinueWhenAny<TAntecedentResult>(Task<TAntecedentResult>[] tasks, Action<Task<TAntecedentResult>> continuationAction, CancellationToken cancellationToken, TaskContinuationOptions continuationOptions, TaskScheduler scheduler) { if (continuationAction == null) throw new ArgumentNullException("continuationAction"); Contract.EndContractBlock(); StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller; return TaskFactory<VoidTaskResult>.ContinueWhenAnyImpl<TAntecedentResult>(tasks, null, continuationAction, continuationOptions, cancellationToken, scheduler, ref stackMark); } // Creates a continuation Task<TResult> ,that will be started upon the completion of a set of provided Tasks. public Task<TResult> ContinueWhenAll<TAntecedentResult>(Task<TAntecedentResult>[] tasks, Func<Task<TAntecedentResult>[], TResult> continuationFunction, CancellationToken cancellationToken) { if (continuationFunction == null) throw new ArgumentNullException("continuationFunction"); Contract.EndContractBlock(); StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller; return ContinueWhenAllImpl<TAntecedentResult>(tasks, continuationFunction, null, m_defaultContinuationOptions, cancellationToken, DefaultScheduler, ref stackMark); } internal static Task<TResult> ContinueWhenAnyImpl<TAntecedentResult>(Task<TAntecedentResult>[] tasks, Func<Task<TAntecedentResult>, TResult> continuationFunction, Action<Task<TAntecedentResult>> continuationAction, TaskContinuationOptions continuationOptions, CancellationToken cancellationToken, TaskScheduler scheduler, ref StackCrawlMark stackMark) { TaskFactory.CheckMultiTaskContinuationOptions(continuationOptions); if (tasks == null) throw new ArgumentNullException("tasks"); if (tasks.Length == 0) throw new ArgumentException(Environment.GetResourceString("Task_MultiTaskContinuation_EmptyTaskList"), "tasks"); Contract.Requires((continuationFunction != null) != (continuationAction != null), "Expected exactly one of endFunction/endAction to be non-null"); if (scheduler == null) throw new ArgumentNullException("scheduler"); Contract.EndContractBlock(); var starter = TaskFactory.CommonCWAnyLogic(tasks); // Bail early if cancellation has been requested. if (cancellationToken.IsCancellationRequested && ((continuationOptions & TaskContinuationOptions.LazyCancellation) == 0) ) { return CreateCanceledTask(continuationOptions, cancellationToken); } // returned continuation task, off of starter if (continuationFunction != null) { return starter.ContinueWith<TResult>( GenericDelegateCache<TAntecedentResult, TResult>.CWAnyFuncDelegate, continuationFunction, scheduler, cancellationToken, continuationOptions, ref stackMark); } else { Contract.Assert(continuationAction != null); return starter.ContinueWith<TResult>( // Use a cached delegate GenericDelegateCache<TAntecedentResult,TResult>.CWAnyActionDelegate, continuationAction, scheduler, cancellationToken, continuationOptions, ref stackMark); } } internal static Task<TResult> ContinueWhenAllImpl<TAntecedentResult>(Task<TAntecedentResult>[] tasks, Func<Task<TAntecedentResult>[], TResult> continuationFunction, Action<Task<TAntecedentResult>[]> continuationAction, TaskContinuationOptions continuationOptions, CancellationToken cancellationToken, TaskScheduler scheduler, ref StackCrawlMark stackMark) { TaskFactory.CheckMultiTaskContinuationOptions(continuationOptions); if (tasks == null) throw new ArgumentNullException("tasks"); Contract.Requires((continuationFunction != null) != (continuationAction != null), "Expected exactly one of endFunction/endAction to be non-null"); if (scheduler == null) throw new ArgumentNullException("scheduler"); Contract.EndContractBlock(); Task<TAntecedentResult>[] tasksCopy = TaskFactory.CheckMultiContinuationTasksAndCopy<TAntecedentResult>(tasks); if (cancellationToken.IsCancellationRequested && ((continuationOptions & TaskContinuationOptions.LazyCancellation) == 0) ) { return CreateCanceledTask(continuationOptions, cancellationToken); } var starter = TaskFactory.CommonCWAllLogic(tasksCopy); if (continuationFunction != null) { return starter.ContinueWith<TResult>( // use a cached delegate GenericDelegateCache<TAntecedentResult, TResult>.CWAllFuncDelegate, continuationFunction, scheduler, cancellationToken, continuationOptions, ref stackMark); } else { Contract.Assert(continuationAction != null); return starter.ContinueWith<TResult>( // use a cached delegate GenericDelegateCache<TAntecedentResult, TResult>.CWAllActionDelegate, continuationAction, scheduler, cancellationToken, continuationOptions, ref stackMark); } } internal static Task<Task<T>[]> CommonCWAllLogic<T>(Task<T>[] tasksCopy) { Contract.Requires(tasksCopy != null); // Create a promise task to be returned to the user CompleteOnCountdownPromise<T> promise = new CompleteOnCountdownPromise<T>(tasksCopy); for (int i = 0; i < tasksCopy.Length; i++) { if (tasksCopy[i].IsCompleted) promise.Invoke(tasksCopy[i]); // Short-circuit the completion action, if possible else tasksCopy[i].AddCompletionAction(promise); // simple completion action } return promise; } private sealed class CompleteOnCountdownPromise<T> : Task<Task<T>[]>, ITaskCompletionAction { private readonly Task<T>[] _tasks; private int _count; internal CompleteOnCountdownPromise(Task<T>[] tasksCopy) : base() { Contract.Requires((tasksCopy != null) && (tasksCopy.Length > 0), "Expected non-null task array with at least one element in it"); _tasks = tasksCopy; _count = tasksCopy.Length; if (AsyncCausalityTracer.LoggingOn) AsyncCausalityTracer.TraceOperationCreation(CausalityTraceLevel.Required, this.Id, "TaskFactory.ContinueWhenAll<>", 0); if (Task.s_asyncDebuggingEnabled) { AddToActiveTasks(this); } } public void Invoke(Task completingTask) { if (AsyncCausalityTracer.LoggingOn) AsyncCausalityTracer.TraceOperationRelation(CausalityTraceLevel.Important, this.Id, CausalityRelation.Join); if (completingTask.IsWaitNotificationEnabled) this.SetNotificationForWaitCompletion(enabled: true); if (Interlocked.Decrement(ref _count) == 0) { if (AsyncCausalityTracer.LoggingOn) AsyncCausalityTracer.TraceOperationCompletion(CausalityTraceLevel.Required, this.Id, AsyncCausalityStatus.Completed); if (Task.s_asyncDebuggingEnabled) { RemoveFromActiveTasks(this.Id); } TrySetResult(_tasks); } Contract.Assert(_count >= 0, "Count should never go below 0"); } internal override bool ShouldNotifyDebuggerOfWaitCompletion { get { return base.ShouldNotifyDebuggerOfWaitCompletion && Task.AnyTaskRequiresNotifyDebuggerOfWaitCompletion(_tasks); } } } }
首先我们来看看TaskFactory的ContinueWhenAny方法，ContinueWhenAny方法主要调用的是ContinueWhenAnyImpl，在ContinueWhenAnyImpl里面主要调用的是TaskFactory.CommonCWAnyLogic(tasks)方法，这个方法方返回的是一个CompleteOnInvokePromise Task，在Task的WhenAny方法中直接返回这个CompleteOnInvokePromise task,而Task的WaitAny则需要等待这个CompleteOnInvokePromise task的完成；而TaskFactory的ContinueWhenAny则是返回这个CompleteOnInvokePromise task的ContinueWith方法。
接下来我们在看看TaskFactory的ContinueWhenAll方法，ContinueWhenAll方法主要调用的是ContinueWhenAllImpl方法，ContinueWhenAllImpl方法主要是调用TaskFactory.CommonCWAnyLogic(tasks)方法，TaskFactory.CommonCWAnyLogic(tasks)方法返回一个CompleteOnCountdownPromise<T> 的task，然后ContinueWhenAllImpl最后返回这个task的ContinueWith方法，这里的CompleteOnCountdownPromise和Task的WhenAllPromise相似，里面都有一个计数器来标记里面的task是否执行完毕。
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