ProgramQueuer/ProgramQueuer/Helpers/ValueConverterGroup.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Globalization;
using System.Reflection;
using System.Text;
using System.Windows.Data;

namespace ProgramQueuer.Helpers
{
	/// <summary>
	/// A value converter which contains a list of IValueConverters and invokes their Convert or ConvertBack methods
	/// in the order that they exist in the list.  The output of one converter is piped into the next converter
	/// allowing for modular value converters to be chained together.  If the ConvertBack method is invoked, the
	/// value converters are executed in reverse order (highest to lowest index).  Do not leave an element in the
	/// Converters property collection null, every element must reference a valid IValueConverter instance. If a
	/// value converter's type is not decorated with the ValueConversionAttribute, an InvalidOperationException will be
	/// thrown when the converter is added to the Converters collection.
	/// </summary>
	[System.Windows.Markup.ContentProperty("Converters")]
	public class ValueConverterGroup : IValueConverter
	{
		#region Data

		private readonly ObservableCollection<IValueConverter> converters = new ObservableCollection<IValueConverter>();
		private readonly Dictionary<IValueConverter, ValueConversionAttribute> cachedAttributes = new Dictionary<IValueConverter, ValueConversionAttribute>();

		#endregion // Data

		#region Constructor

		public ValueConverterGroup()
		{
			this.converters.CollectionChanged += this.OnConvertersCollectionChanged;
		}

		#endregion // Constructor

		#region Converters

		/// <summary>
		/// Returns the list of IValueConverters contained in this converter.
		/// </summary>
		public ObservableCollection<IValueConverter> Converters
		{
			get { return this.converters; }
		}

		#endregion // Converters

		#region IValueConverter Members

		object IValueConverter.Convert(object value, Type targetType, object parameter, CultureInfo culture)
		{
			object output = value;

			for (int i = 0; i < this.Converters.Count; i++)
			{
				//Run the converter.
				output = this.Converters[i].Convert(output, this.GetTargetType(i, targetType, true), parameter, culture);
				if (output == Binding.DoNothing)
					break;
			}

			return output;
		}

		object IValueConverter.ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
		{
			object output = value;

			for (int i = this.Converters.Count - 1; i > -1; i--)
			{
				//Run the converter.
				output = this.Converters[i].Convert(output, this.GetTargetType(i, targetType, true), parameter, culture);
				if (output == Binding.DoNothing)
					break;
			}

			return output;
		}

		#endregion // IValueConverter Members

		#region Private Helpers

		#region GetTargetType

		/// <summary>
		/// Returns the target type for a conversion operation.
		/// </summary>
		/// <param name="converterIndex">The index of the current converter about to be executed.</param>
		/// <param name="finalTargetType">The 'targetType' argument passed into the conversion method.</param>
		/// <param name="convert">Pass true if calling from the Convert method, or false if calling from ConvertBack.</param>
		protected virtual Type GetTargetType(int converterIndex, Type finalTargetType, bool convert)
		{
			// If the current converter is not the last/first in the list, 
			// get a reference to the next/previous converter.
			IValueConverter nextConverter = null;
			int nextIndex = converterIndex;

			//Grab the index for the next converter.
			if (convert && nextIndex < this.Converters.Count - 1)
				nextIndex++;
			else if (!convert && nextIndex > 0)
				nextIndex--;

			//Check to see if we have a new index.
			if (nextIndex != converterIndex)
				//Make sure the converter at our new index is not null.
				if (this.Converters[nextIndex] == null)
					throw new InvalidOperationException("The Converters collection of the ValueConverterGroup contains a null reference at index: " + nextIndex);
				else
					//Get our next converter.
					nextConverter = this.Converters[nextIndex];

			//Check if we have to go through another converter. If this (current) is the last converter then this will be null.
			if (nextConverter != null)
			{
				var conversionAttribute = cachedAttributes[nextConverter];

				// If the Convert method is going to be called, we need to use the SourceType of the next 
				// converter in the list.  If ConvertBack is called, use the TargetType.
				return convert ? conversionAttribute.SourceType : conversionAttribute.TargetType;
			}

			// If the current converter is the last one to be executed return the target type passed into the conversion method.
			return finalTargetType;
		}

		#endregion // GetTargetType

		#region OnConvertersCollectionChanged

		void OnConvertersCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
		{
			// The 'Converters' collection has been modified, so validate that each value converter it now
			// contains is decorated with ValueConversionAttribute and then cache the attribute value.

			IList convertersToProcess = null;
			if (e.Action == NotifyCollectionChangedAction.Add ||
				e.Action == NotifyCollectionChangedAction.Replace)
			{
				convertersToProcess = e.NewItems;
			}
			else if (e.Action == NotifyCollectionChangedAction.Remove)
			{
				foreach (IValueConverter converter in e.OldItems)
					this.cachedAttributes.Remove(converter);
			}
			else if (e.Action == NotifyCollectionChangedAction.Reset)
			{
				this.cachedAttributes.Clear();
				convertersToProcess = this.converters;
			}

			if (convertersToProcess != null && convertersToProcess.Count > 0)
			{
				foreach (IValueConverter converter in convertersToProcess)
				{
					object[] attributes = converter.GetType().GetCustomAttributes(typeof(ValueConversionAttribute), false);

					if (attributes.Length != 1)
						throw new InvalidOperationException("All value converters added to a ValueConverterGroup must be decorated with the ValueConversionAttribute attribute exactly once.");

					this.cachedAttributes.Add(converter, attributes[0] as ValueConversionAttribute);
				}
			}
		}

		#endregion // OnConvertersCollectionChanged

		#endregion // Private Helpers
	}
}