69 lines
2.9 KiB
C#
69 lines
2.9 KiB
C#
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using System;
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using System.Collections.Generic;
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using System.Threading;
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using System.Threading.Tasks;
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namespace LeanCloud.Storage.Internal {
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/// <summary>
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/// A helper class for enqueuing tasks
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/// </summary>
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public class TaskQueue {
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/// <summary>
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/// We only need to keep the tail of the queue. Cancelled tasks will
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/// just complete normally/immediately when their turn arrives.
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/// </summary>
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private Task tail;
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private readonly object mutex = new object();
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/// <summary>
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/// Gets a cancellable task that can be safely awaited and is dependent
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/// on the current tail of the queue. This essentially gives us a proxy
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/// for the tail end of the queue whose awaiting can be cancelled.
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/// </summary>
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/// <param name="cancellationToken">A cancellation token that cancels
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/// the task even if the task is still in the queue. This allows the
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/// running task to return immediately without breaking the dependency
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/// chain. It also ensures that errors do not propagate.</param>
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/// <returns>A new task that should be awaited by enqueued tasks.</returns>
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private Task GetTaskToAwait(CancellationToken cancellationToken) {
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lock (mutex) {
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Task toAwait = tail ?? Task.FromResult(true);
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return toAwait.ContinueWith(task => { }, cancellationToken);
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}
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}
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/// <summary>
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/// Enqueues a task created by <paramref name="taskStart"/>. If the task is
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/// cancellable (or should be able to be cancelled while it is waiting in the
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/// queue), pass a cancellationToken.
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/// </summary>
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/// <typeparam name="T">The type of task.</typeparam>
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/// <param name="taskStart">A function given a task to await once state is
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/// snapshotted (e.g. after capturing session tokens at the time of the save call).
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/// Awaiting this task will wait for the created task's turn in the queue.</param>
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/// <param name="cancellationToken">A cancellation token that can be used to
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/// cancel waiting in the queue.</param>
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/// <returns>The task created by the taskStart function.</returns>
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public T Enqueue<T>(Func<Task, T> taskStart, CancellationToken cancellationToken)
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where T : Task {
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Task oldTail;
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T task;
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lock (mutex) {
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oldTail = this.tail ?? Task.FromResult(true);
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// The task created by taskStart is responsible for waiting the
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// task passed to it before doing its work (this gives it an opportunity
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// to do startup work or save state before waiting for its turn in the queue
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task = taskStart(GetTaskToAwait(cancellationToken));
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// The tail task should be dependent on the old tail as well as the newly-created
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// task. This prevents cancellation of the new task from causing the queue to run
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// out of order.
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this.tail = Task.WhenAll(oldTail, task);
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}
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return task;
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}
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public object Mutex { get { return mutex; } }
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}
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}
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