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Lazy Alternatives - LazyAndWeak and BackgroundLoader

, 1 Dec 2011
This article will present two alternatives to Lazy.
LazyAlternatives.zip
LazyAlternatives
LazyAlternatives.suo
LazyAlternativesSample
Properties
Settings.settings
Pfz
Caching
Collections
DataTypes
DynamicObjects
Internal
Extensions
Factoring
Pfz.csproj.user
Pfz.Phone.csproj.user
Pfz.ruleset
Pfz.Silverlight.csproj.user
Pfz.snk
Pfz.suo
PhoneSpecific
Properties
Remoting
Instructions
Internal
Serializers
Udp
Serialization
BinaryBuiltIn
Threading
Contexts
Disposers
Unsafe
using System;
using System.Collections.Generic;
using System.Threading;

namespace Pfz.Threading.Unsafe
{
	/// <summary>
	/// Inheriting this class you can program an iterator in the most "natural" way, doing what it need to do
	/// and then waiting to do the next action.
	/// This uses another thread to effectivelly do the iteration and this is why it is considered "unsafe", as
	/// forgetting to dispose keeps a thread alive. So, to manipulate it assign it to a ThreadedIterator
	/// variable.
	/// </summary>
	public abstract class UnsafeThreadedIterator:
		IDisposable
	{
		internal static ManagedSemaphore _parallelSemaphore = new ManagedSemaphore(Environment.ProcessorCount);
		private bool _parallelSemaphoreGot;

		private readonly object _finishLock = new object();
		private bool _finished;
		private bool _disposed;
		private ManagedAutoResetEvent _nextFrameEvent = new ManagedAutoResetEvent();
		private ManagedAutoResetEvent _finishFrameEvent = new ManagedAutoResetEvent();

		private bool _useParallelLimit = false;

		/// <summary>
		/// Gets or sets a valud indicating if this threaded iterator should be limited by the number of CPUs.
		/// If the limit is used, only one parallel iterator/enumerator is allowed per cpu.
		/// </summary>
		public bool UseParallelLimit
		{
			get
			{
				return _useParallelLimit;
			}
			set
			{
				_useParallelLimit = value;
			}
		}

		private bool _started;
		internal void _Start()
		{
			lock(_finishLock)
			{
				if (_started)
					return;

				_started = true;
			}

			try
			{
				UnlimitedThreadPool.Run(_Run);
			}
			catch
			{
				_DisposeEvents();
				throw;
			}
		}

		/// <summary>
		/// Asks the actual iterator to finish, or releases all resources if it is already finished.
		/// </summary>
		public void Dispose()
		{
			lock(_finishLock)
			{
				if (_finished)
				{
					_DisposeEvents();
					return;
				}

				_disposed = true;
			}

			_nextFrameEvent.Set();
		}
		private void _DisposeEvents()
		{
			Disposer.Dispose(ref _nextFrameEvent);
			Disposer.Dispose(ref _finishFrameEvent);
		}

		/// <summary>
		/// Must be implemented to run the iterator code.
		/// </summary>
		protected abstract void OnRun();

		private void _Run()
		{
			try
			{
				_nextFrameEvent.WaitOne();

				lock(_finishLock)
					if (_disposed)
						return;

				_ParallelWait();

				OnRun();
			}
			catch(IteratorDisposedException)
			{
			}
			finally
			{
				lock(_finishLock)
				{
					if (_disposed)
						_DisposeEvents();
					else
					{
						_finished = true;
						_finishFrameEvent.Set();
					}
				}

				_ParallelRelease();
			}
		}

		private void _ParallelWait()
		{
			if (UseParallelLimit)
			{
				_parallelSemaphore.Enter();
				_parallelSemaphoreGot = true;
			}
		}
		private void _ParallelRelease()
		{
			if (_parallelSemaphoreGot)
			{
				_parallelSemaphoreGot = false;
				_parallelSemaphore.Exit();
			}
		}

		/// <summary>
		/// Must be called when ending a frame.
		/// If it returns false, the iterator was disposed, so it is better to return immediatelly.
		/// </summary>
		protected bool YieldReturnNoException()
		{
			_ParallelRelease();

			lock(_finishLock)
				if (_disposed)
					return false;

			var thread = Thread.CurrentThread;
			_finishFrameEvent.Set();
			thread.IsBackground = true;
			_nextFrameEvent.WaitOne();
			thread.IsBackground = false;

			lock(_finishLock)
				if (_disposed)
					return false;

			_ParallelWait();

			return true;
		}

		/// <summary>
		/// Similar to YieldReturnNoException, but throws an exception of type IteratorDisposedException
		/// if the iterator was already disposed, this way you don't need to check for the bool result.
		/// </summary>
		protected void YieldReturn()
		{
			if (!YieldReturnNoException())
				throw new IteratorDisposedException(GetType().FullName);
		}

		internal bool _BeginUpdate()
		{
			lock(_finishLock)
				if (_finished || _disposed)
					return false;

			_nextFrameEvent.Set();
			return true;
		}

		internal bool _EndUpdate()
		{
			_finishFrameEvent.WaitOne();

			lock(_finishLock)
				return !(_finished || _disposed);
		}

		internal bool _Update()
		{
			if (!_BeginUpdate())
				return false;

			return _EndUpdate();
		}
	}
}

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License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Paulo Zemek
Architect
Canada Canada
I started to program computers when I was 11 years old, as a hobbist, programming in AMOS Basic and Blitz Basic for Amiga.
At 12 I had my first try with assembler, but it was too difficult at the time. Then, in the same year, I learned C and, after learning C, I was finally able to learn assembler (for Motorola 680x0).
Not sure, but probably between 12 and 13, I started to learn C++. I always programmed "in an object oriented way", but using function pointers instead of virtual methods.
 
At 15 I started to learn Pascal at school and to use Delphi. At 16 I started my first internship (using Delphi). At 18 I started to work professionally using C++ and since then I've developed my programming skills as a professional developer in C++ and C#, generally creating libraries that help other developers do they work easier, faster and with less errors.
 
Want more info or simply want to contact me?
Take a look at: http://paulozemek.azurewebsites.net/
Or e-mail me at: paulozemek@outlook.com
 
Codeproject MVP 2012
Microsoft MVP 2013

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