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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.Linq;
using System.Threading;

namespace Pfz.Threading
{
	/// <summary>
	/// Class similar to Lazy. But instead of only loading values when they are first accessed, it tries to load the
	/// value when the application is idle. Will load immediatelly if the value is requested.
	/// You must instantiate the generic version.
	/// </summary>
	public abstract class BackgroundLoader:
		ThreadSafeDisposable
	{
		private static readonly ManagedAutoResetEvent _event = new ManagedAutoResetEvent();
		internal static readonly HashSet<BackgroundLoader> _items = new HashSet<BackgroundLoader>();

		static BackgroundLoader()
		{
			Thread thread = new Thread(_Run);
			thread.Name = "Pfz.Threading.BackgroundLoader";
			thread.Start();
		}
		private static void _Run()
		{
			var currentThread = Thread.CurrentThread;
			while(true)
			{
				currentThread.IsBackground = true;

				#if !SILVERLIGHT
					currentThread.Priority = ThreadPriority.Lowest;
				#endif

				_event.WaitOne();

				while(true)
				{
					#if !SILVERLIGHT
						while(true)
						{
							// Keeps yielding while there are other threads wanting to run.
							if (!Thread.Yield())
								break;
						}

						currentThread.Priority = ThreadPriority.Normal;
					#endif

					currentThread.IsBackground = false;

					BackgroundLoader item;

					lock(_items)
					{
						item = _items.FirstOrDefault();

						if (item == null)
							break;

						_items.Remove(item);
					}

					try
					{
						lock(item.DisposeLock)
							if (!item.WasDisposed)
								item._CreateValue();
					}
					catch
					{
						// Ignore any exceptions here.
					}

					currentThread.IsBackground = true;

					#if !SILVERLIGHT
						currentThread.Priority = ThreadPriority.Lowest;
					#endif
				}
			}
		}

		internal BackgroundLoader()
		{
			lock(_items)
				_items.Add(this);

			_event.Set();
		}

		/// <summary>
		/// Clears the loaded value or removes this BackgroundLoader from the
		/// pending objects to load.
		/// </summary>
		protected override void Dispose(bool disposing)
		{
			if (disposing)
				_ClearValueOrRemoveFromLoader();

			base.Dispose(disposing);
		}

		internal abstract void _CreateValue();
		internal abstract void _ClearValueOrRemoveFromLoader();
	}

	/// <summary>
	/// Class similar to Lazy. But instead of only loading values when they are first accessed, it tries to load the
	/// value when the application is idle. Will load immediatelly if the value is requested.
	/// </summary>
	public sealed class BackgroundLoader<T>:
		BackgroundLoader,
		IValueLoader<T>
	where
		T: class
	{
		private Func<T> _createDelegate;

		/// <summary>
		/// Creates a new background loader that will create the value using the default constructor.
		/// </summary>
		public BackgroundLoader()
		{
		}

		/// <summary>
		/// Creates a new BackgroundLoaderByDelegate setting the delegate used
		/// to create the Value.
		/// </summary>
		public BackgroundLoader(Func<T> createDelegate)
		{
			_createDelegate = createDelegate;
		}

		internal override void _CreateValue()
		{
			if (_value != null)
				return;

			if (_createDelegate != null)
			{
				_value = _createDelegate();
				_createDelegate = null;
			}
			else
				_value = (T)Activator.CreateInstance(typeof(T));
		}

		private T _value;
		/// <summary>
		/// Gets the Value, creating it immediatelly if needed.
		/// </summary>
		public T Value
		{
			get
			{
				T result = _value;
				if (result != null)
					return result;

				lock(DisposeLock)
				{
					if (WasDisposed)
						return null;

					result = _value;
					if (result != null)
						return result;

					lock(_items)
						_items.Remove(this);

					_CreateValue();
					return _value;
				}
			}
		}

		/// <summary>
		/// Gets the value that is actually in memory.
		/// If the value was not created or if this object was disposed, returns null.
		/// </summary>
		public T InMemoryValue
		{
			get
			{
				return _value;
			}
		}

		internal override void _ClearValueOrRemoveFromLoader()
		{
			T value = _value;
			if (value == null)
			{
				_createDelegate = null;

				lock(_items)
					_items.Remove(this);

				return;
			}

			_value = null;

			var disposable = value as IDisposable;
			if (disposable != null)
				disposable.Dispose();
		}
	}
}

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

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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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