Nuclex.Windows.Forms/Source/ViewModels/MultiPageViewModel.cs

#region CPL License
/*
Nuclex Framework
Copyright (C) 2002-2019 Nuclex Development Labs

This library is free software; you can redistribute it and/or
modify it under the terms of the IBM Common Public License as
published by the IBM Corporation; either version 1.0 of the
License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
IBM Common Public License for more details.

You should have received a copy of the IBM Common Public
License along with this library
*/
#endregion

using System;
using System.Collections.Concurrent;

using Nuclex.Support;

namespace Nuclex.Windows.Forms.ViewModels {

  /// <summary>Base class for view models that have multiple child view models</summary>
  /// <typeparam name="TPageEnumeration">Enum type by which pages can be indicated</typeparam>
  public abstract class MultiPageViewModel<TPageEnumeration> :
    Observable, IMultiPageViewModel, IDisposable { 

    /// <summary>Initializes a new multi-page view model</summary>
    /// <param name="windowManager">
    ///   Window manager the view model uses to create child views
    /// </param>
    /// <param name="cachePageViewModels">
    ///   Whether child view models will be kept alive and reused
    /// </param>
    public MultiPageViewModel(IWindowManager windowManager, bool cachePageViewModels = false) {
      this.windowManager = windowManager;
      if(cachePageViewModels) {
        this.cachedViewModels = new ConcurrentDictionary<TPageEnumeration, object>();
      }
    }

    /// <summary>Immediately releases all resources owned by the instance</summary>
    public virtual void Dispose() {
      if(this.cachedViewModels != null) {
        foreach(object cacheViewModel in this.cachedViewModels.Values) {
          disposeIfSupported(cacheViewModel);
        }
        this.activePageViewModel = null;

        this.cachedViewModels.Clear();
        this.cachedViewModels = null;
      } else if(this.activePageViewModel != null) {
        disposeIfSupported(this.activePageViewModel);
        this.activePageViewModel = null;
      }
    }

    /// <summary>Child page that is currently being displayed by the view model</summary>
    public TPageEnumeration ActivePage {
      get { return this.activePage; }
      set {
        if(!this.activePage.Equals(value)) {
          this.activePage = value;
          if(this.activePageViewModel != null) {
            if(this.cachedViewModels == null) {
              disposeIfSupported(this.activePageViewModel);
            }
            this.activePageViewModel = null;
          }
          OnPropertyChanged(nameof(ActivePage));
        }
      }
    }

    /// <summary>Retrieves (and, if needed, creates) the view model for the active page</summary>
    /// <returns>A view model for the active page on the multi-page view model</returns>
    public object GetActivePageViewModel() {
      if(this.cachedViewModels == null) {
        if(this.activePageViewModel == null) {
          this.activePageViewModel = CreateViewModelForPage(this.activePage);
        }
      } else if(this.activePageViewModel == null) {
        this.activePageViewModel = this.cachedViewModels.GetOrAdd(
          this.activePage,
          delegate(TPageEnumeration activePage) {
            return CreateViewModelForPage(this.activePage);
          }
        );
      }

      return this.activePageViewModel;
    }

    /// <summary>Windowmanager that can create view models and display other views</summary>
    protected IWindowManager WindowManager {
      get { return this.windowManager; }
    }

    /// <summary>Creates a view model for the specified page</summary>
    /// <param name="page">Page for which a view model will be created</param>
    /// <returns>The view model for the specified page</returns>
    protected abstract object CreateViewModelForPage(TPageEnumeration page);

    /// <summary>Disposes the specified object if it is disposable</summary>
    /// <param name="potentiallyDisposable">Object that will be disposed if supported</param>
    private static void disposeIfSupported(object potentiallyDisposable) {
      var disposable = potentiallyDisposable as IDisposable;
      if(disposable != null) {
        disposable.Dispose();
      }
    }

    /// <summary>Page that is currently active in the multi-page view model</summary>
    private TPageEnumeration activePage;
    /// <summary>Window manager that can be used to display other views</summary>
    private IWindowManager windowManager;

    /// <summary>View model for the active page</summary>
    private object activePageViewModel;
    /// <summary>Cached page view models, if caching is enabled</summary>
    private ConcurrentDictionary<TPageEnumeration, object> cachedViewModels;

  }

} // namespace Nuclex.Windows.Forms.ViewModels