/V4/Quickstarts/MVVM/MVVM/ViewModels/MultipleSelectionQuestionViewModel.cs

//===================================================================================

// Microsoft patterns & practices

// Composite Application Guidance for Windows Presentation Foundation and Silverlight

//===================================================================================

// Copyright (c) Microsoft Corporation.  All rights reserved.

// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY

// OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT

// LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY AND

// FITNESS FOR A PARTICULAR PURPOSE.

//===================================================================================

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// e-mail addresses, logos, people, places, and events depicted

// herein are fictitious.  No association with any real company,

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// places, or events is intended or should be inferred.

//===================================================================================

using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

using System.ComponentModel;

using System.Linq;

using MVVM.Model;



namespace MVVM.ViewModels

{

    /// <summary>

    /// View model for multiple selection questions.

    /// </summary>

    /// <remarks>

    /// The model for a multiple selection question expects a list of selections as its answer, but the desired 

    /// View for this question consists of a series of check boxes, one for each option. In order to support this

    /// UI, this view model is a hierarchical view model, exposing a collection of child view models representing 

    /// each option. However, rather than mapping the checked state on each child view model to presence of an option

    /// into the list of selections, the view model exposes the list of selections and expects the view to 

    /// populate it appropriately, which the view does by using the 

    /// <see cref="MVVM.Infrastructure.Behaviors.SynchronizeSelectedItems">SynchronizeSelectedItems</see> class.

    /// </remarks>

    public class MultipleSelectionQuestionViewModel : QuestionViewModel<MultipleSelectionQuestion>

    {

        private readonly ObservableCollection<string> selections;



        public MultipleSelectionQuestionViewModel(MultipleSelectionQuestion question)

            : base(PreAnswerQuestion(question))

        {

            this.selections = new ObservableCollection<string>();

            this.selections.CollectionChanged += this.OnSelectionsChanged;

        }



        /// <summary>

        /// Gets the list of selections.

        /// </summary>

        public IList<string> Selections

        {

            get { return this.selections; }

        }



        protected override void OnQuestionPropertyChanged(object sender, PropertyChangedEventArgs e)

        {

            if (e.PropertyName == "Response")

            {

                this.OnResponseChanged();

            }

        }



        private static MultipleSelectionQuestion PreAnswerQuestion(MultipleSelectionQuestion question)

        {

            if (question != null && question.Response == null)

            {

                question.Response = new string[0];

            }



            return question;

        }



        /// <summary>

        /// Overwrites the response value in the model, triggering validation and change updates.

        /// </summary>

        private void OnSelectionsChanged(object sender, EventArgs args)

        {

            this.Question.Response = this.selections.ToArray();

        }

    }

}