TaskScheduler.cs

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// 
// TaskScheduler.cs
//  
// Authors:
//       Jérémie "Garuma" Laval <jeremie.laval@gmail.com>
//       Marek Safar <marek.safar@gmail.com>
// 
// Copyright (c) 2009 Jérémie "Garuma" Laval
// Copyright 2012 Xamarin, Inc (http://www.xamarin.com)
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

#if NET_4_0 || UNITY_5_3_OR_NEWER

using System.Collections.Generic;
using System.Diagnostics;

namespace System.Threading.Tasks
{
    [DebuggerDisplay ("Id={Id}")]
    [DebuggerTypeProxy (typeof (TaskSchedulerDebuggerView))]
    public abstract class TaskScheduler
    {
        sealed class TaskSchedulerDebuggerView
        {
            readonly TaskScheduler scheduler;

            public TaskSchedulerDebuggerView (TaskScheduler scheduler)
            {
                this.scheduler = scheduler;
            }

            public IEnumerable<Task> ScheduledTasks {
                get {
                    return scheduler.GetScheduledTasks ();
                }
            }
        }

        static readonly TaskScheduler defaultScheduler = new TpScheduler ();
        
        [ThreadStatic]
        static TaskScheduler currentScheduler;
        
        int id;
        static int lastId = int.MinValue;
        
        public static event EventHandler<UnobservedTaskExceptionEventArgs> UnobservedTaskException;
        
        protected TaskScheduler ()
        {
            this.id = Interlocked.Increment (ref lastId);
        }
        
        public static TaskScheduler FromCurrentSynchronizationContext ()
        {
            var syncCtx = SynchronizationContext.Current;
            if (syncCtx == null)
                throw new InvalidOperationException ("The current SynchronizationContext is null and cannot be used as a TaskScheduler");

            return new SynchronizationContextScheduler (syncCtx);
        }
        
        public static TaskScheduler Default  {
            get {
                return defaultScheduler;
            }
        }
        
        public static TaskScheduler Current  {
            get {
                if (currentScheduler != null)
                    return currentScheduler;
                
                return defaultScheduler;
            }
            internal set {
                currentScheduler = value;
            }
        }
        
        public int Id {
            get {
                return id;
            }
        }
        
        public virtual int MaximumConcurrencyLevel {
            get {
                return int.MaxValue;
            }
        }

        protected abstract IEnumerable<Task> GetScheduledTasks ();
        protected internal abstract void QueueTask (Task task);

        protected internal virtual bool TryDequeue (Task task)
        {
            return false;
        }

        internal protected bool TryExecuteTask (Task task)
        {
            if (task.IsCompleted)
                return false;

            if (task.Status == TaskStatus.WaitingToRun) {
                task.Execute ();
                if (task.WaitOnChildren ())
                    task.Wait ();

                return true;
            }

            return false;
        }

        protected abstract bool TryExecuteTaskInline (Task task, bool taskWasPreviouslyQueued);

        internal bool RunInline (Task task, bool taskWasPreviouslyQueued)
        {
            if (!TryExecuteTaskInline (task, taskWasPreviouslyQueued))
                return false;

            if (!task.IsCompleted)
                throw new InvalidOperationException ("The TryExecuteTaskInline call to the underlying scheduler succeeded, but the task body was not invoked");

            return true;
        }

        internal static UnobservedTaskExceptionEventArgs FireUnobservedEvent (Task task, AggregateException e)
        {
            UnobservedTaskExceptionEventArgs args = new UnobservedTaskExceptionEventArgs (e);
            
            EventHandler<UnobservedTaskExceptionEventArgs> temp = UnobservedTaskException;
            if (temp == null)
                return args;
            
            temp (task, args);
            
            return args;
        }
    }
}
#endif