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Catel - Part 4 of n: Unit testing with Catel

, 28 Jan 2011 CPOL
This article explains how to write unit tests for MVVM using Catel.
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// --------------------------------------------------------------------------------------------------------------------
// <copyright file="ValueConverterGroup.cs" company="Catel development team">
//   Copyright (c) 2008 - 2011 Catel development team. All rights reserved.
// </copyright>
// <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.
//   Known limitations: the supplied parameter is used for ALL! converters.
// </summary>
// --------------------------------------------------------------------------------------------------------------------

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Globalization;
using System.Windows.Data;
using System.Windows.Markup;

namespace Catel.Windows.Data.Converters
{
	/// <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.
	/// 
	/// Known limitations: the supplied parameter is used for ALL! converters.
	/// </summary>
	/// <remarks>
	/// Original source: http://www.codeproject.com/KB/WPF/PipingValueConverters_WPF.aspx
	/// </remarks>
	[ContentProperty("Converters")]
	public class ValueConverterGroup : IValueConverter
	{
		#region Variables
		private readonly ObservableCollection<IValueConverter> _converters = new ObservableCollection<IValueConverter>();
		private readonly Dictionary<IValueConverter, ValueConversionAttribute> _cachedAttributes = new Dictionary<IValueConverter, ValueConversionAttribute>();
		#endregion

		#region Constructor & destructor
		/// <summary>
		/// Initializes a new instance of the <see cref="ValueConverterGroup"/> class.
		/// </summary>
		public ValueConverterGroup()
		{
			_converters.CollectionChanged += OnConvertersCollectionChanged;
		}
		#endregion

		#region Properties
		/// <summary>
		/// Returns the list of IValueConverters contained in this converter.
		/// </summary>
		public ObservableCollection<IValueConverter> Converters
		{
			get { return _converters; }
		}
		#endregion

		#region Methods
		/// <summary>
		/// Converts a value.
		/// </summary>
		/// <param name="value">The value produced by the binding source.</param>
		/// <param name="targetType">The type of the binding target property.</param>
		/// <param name="parameter">The converter parameter to use.</param>
		/// <param name="culture">The culture to use in the converter.</param>
		/// <returns>
		/// A converted value. If the method returns null, the valid null value is used.
		/// </returns>
		object IValueConverter.Convert(object value, Type targetType, object parameter, CultureInfo culture)
		{
			object output = value;

			for (int i = 0; i < Converters.Count; ++i)
			{
				IValueConverter converter = Converters[i];
				Type currentTargetType = GetTargetType(i, targetType, true);
				output = converter.Convert(output, currentTargetType, parameter, culture);

				// If the converter returns 'DoNothing' then the binding operation should terminate.
				if (output == Binding.DoNothing)
				{
				    break;
				}
			}

			return output;
		}

		/// <summary>
		/// Converts a value.
		/// </summary>
		/// <param name="value">The value that is produced by the binding target.</param>
		/// <param name="targetType">The type to convert to.</param>
		/// <param name="parameter">The converter parameter to use.</param>
		/// <param name="culture">The culture to use in the converter.</param>
		/// <returns>
		/// A converted value. If the method returns null, the valid null value is used.
		/// </returns>
		object IValueConverter.ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
		{
			object output = value;

			for (int i = Converters.Count - 1; i > -1; --i)
			{
				IValueConverter converter = Converters[i];
				Type currentTargetType = GetTargetType(i, targetType, false);
				output = converter.ConvertBack(output, currentTargetType, parameter, culture);

				// When a converter returns 'DoNothing' the binding operation should terminate.
				if (output == Binding.DoNothing)
				{
				    break;
				}
			}

			return output;
		}

		/// <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;
			if (convert)
			{
				if (converterIndex < Converters.Count - 1)
				{
					nextConverter = Converters[converterIndex + 1];
					if (nextConverter == null)
					{
					    throw new InvalidOperationException("The Converters collection of the ValueConverterGroup contains a null reference at index: " + (converterIndex + 1));
					}
				}
			}
			else
			{
				if (converterIndex > 0)
				{
					nextConverter = Converters[converterIndex - 1];
					if (nextConverter == null)
					{
					    throw new InvalidOperationException("The Converters collection of the ValueConverterGroup contains a null reference at index: " + (converterIndex - 1));
					}
				}
			}

			if (nextConverter != null)
			{
				ValueConversionAttribute 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;
		}

		/// <summary>
		/// Called when [converters collection changed].
		/// </summary>
		/// <param name="sender">The sender.</param>
		/// <param name="e">The <see cref="System.Collections.Specialized.NotifyCollectionChangedEventArgs"/> instance containing the event data.</param>
		private 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)
				{
				    _cachedAttributes.Remove(converter);
				}
			}
			else if (e.Action == NotifyCollectionChangedAction.Reset)
			{
				_cachedAttributes.Clear();
				convertersToProcess = _converters;
			}

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

					// Maybe it is 'beter' (more robust) to use a default ValueConversion(typeof(object), typeof(bool)) attribute.
					if (attributes.Length != 1)
					{
					    throw new InvalidOperationException("All value converters added to a ValueConverterGroup must be decorated with the ValueConversionAttribute attribute exactly once.");
					}

					_cachedAttributes.Add(converter, attributes[0] as ValueConversionAttribute);
				}
			}
		}
		#endregion
	}
}

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About the Author

Geert van Horrik
Software Developer CatenaLogic
Netherlands Netherlands
I am Geert van Horrik, and I have studied Computer Science in the Netherlands.
 
I love to write software using .NET (especially the combination of WPF and C#). I am also the lead developer of Catel, an open-source application development framework for WPF, Silverlight, WP7 and WinRT with the focus on MVVM.
 
I have my own company since January 1st 2007, called CatenaLogic. This company develops commercial and non-commercial software.
 
To download (or buy) applications I have written, visit my website: http://www.catenalogic.com
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