ViewModel Active Awareness in a Prism Based Application

Yesterday, while chatting with the highly talented Jeremiah Morrill and other WPF Disciples about some MVVM subtleties (for the full post see here), Jeremiah briefly touched on the topic of providing ViewModels with the awareness of being active or inactive within a Prism based application. I wanted to explore this further, and decided to integrate this functionality into Calcium. What I provide here isn't rocket science, and merely serves to illustrate one of indeed many design approaches that could be applied to accomplish the same thing.

For this, I had two goals. The first, to provide the capability without coupling the ViewModel to the View. That is, without requiring the ViewModel to have a reference to the View. The second, to not depend on a base class for the functionality; forever tying the developer to my base class implementation (ViewModelBase). So, indeed, I chose an interface based approach. To be Prism-esque, I have adapted Jeremiah's approach which was to implement Prism's IActiveAware interface on my base view class. I then feed an intermediary object to the ViewModel via an interface named IViewAware. The intermediary object is an instance of ActiveAwareUIElementAdapter. This class is used to provide a UIElement instance with Prisms IActiveAware interface. It does so by monitoring its Got and LostFocus events.

ActiveAwareUIElementAdapter

/// <summary>
/// Wraps a <see cref="UIElement"/> to provide an <see cref="IActiveAware"/>
/// implementation based on its focus state.
/// </summary>
public class ActiveAwareUIElementAdapter : IActiveAware
{
    bool active;

    public ActiveAwareUIElementAdapter(UIElement uiElement)
    {
        ArgumentValidator.AssertNotNull(uiElement, "uiElement");
        uiElement.GotFocus += OnGotFocus;
        uiElement.LostFocus += OnLostFocus;
    }

    void OnLostFocus(object sender, RoutedEventArgs e)
    {
        IsActive = false;
    }

    void OnGotFocus(object sender, RoutedEventArgs e)
    {
        IsActive = true;
    }

    public bool IsActive
    {
        get
        {
            return active;
        }
        set
        {
            if (active != value)
            {
                active = value;
            }
            OnIsActiveChanged(EventArgs.Empty);
        }
    }

    #region event IsActiveChanged

    event EventHandler isActiveChanged;

    public event EventHandler IsActiveChanged
    {
        add
        {
            isActiveChanged += value;
        }
        remove
        {
            isActiveChanged -= value;
        }
    }

    protected void OnIsActiveChanged(EventArgs e)
    {
        if (isActiveChanged != null)
        {
            isActiveChanged(this, e);
        }
    }

    #endregion
}

We then use this class within any IView UIElement implementation, but in particular, the base ViewControl class. We instantiate the ActiveAwareUIElement adapter within the view's constructor, and pass it the instance of the view itself. The ActiveAwareUIElement then simply subscribes to the GotFocus and LostFocus events of the view, which is of course a UIElement. 

ViewControl Implementation

/// <summary>
/// The base class for <see cref="IView"/>s.
/// </summary>
public class ViewControl : UserControl, IView, IActiveAware /* (not abstract for Blendability) */
{
    #region ViewModel Dependency Property

    public static DependencyProperty ViewModelProperty = DependencyProperty.Register(
        "ViewModel", typeof(IViewModel), 
        typeof(ViewControl), new PropertyMetadata(null, OnViewModelChanged));

    static void OnViewModelChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var viewControl = (ViewControl)d;
        viewControl.SetViewAwareAssociations((IViewModel)e.OldValue, (IViewModel)e.NewValue);
    }

    [Description("The view model for this view.")]
#if !SILVERLIGHT
    [Browsable(true)]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
#endif
    public IViewModel ViewModel
    {
        get
        {
            return (IViewModel)GetValue(ViewModelProperty);
        }
        set
        {
            SetValue(ViewModelProperty, value);
        }
    }

    #endregion

    /// <summary>
    /// Initializes a new instance of the <see cref="ViewControl"/> class.
    /// </summary>
    public ViewControl()
    {
        Loaded += OnLoaded;
        activeAwareUIElementAdapter = new ActiveAwareUIElementAdapter(this); 
    }

    bool alreadyLoaded;

    void OnLoaded(object sender, RoutedEventArgs e)
    {
        if (!alreadyLoaded)
        {
            alreadyLoaded = true;
            OnViewLoaded(e);
        }
    }

    #region ViewLoaded event

    event EventHandler<EventArgs> viewLoaded;

    /// <summary>
    /// Occurs when the view has been loaded.
    /// </summary>
    public event EventHandler<EventArgs> ViewLoaded
    {
        add
        {
            viewLoaded += value;
        }
        remove
        {
            viewLoaded -= value;
        }
    }

    /// <summary>
    /// Closes the view.
    /// </summary>
    /// <param name="force">if set to <c>true</c> the control will be forced
    /// to close even if e.g., there is unsaved data and the user chooses 
    /// to cancel the closure.</param>
    /// <returns></returns>
    public virtual bool Close(bool force)
    {
        var viewService = ServiceLocatorSingleton.Instance.GetInstance<IViewService>();
        return viewService.CloseView(this, force);
    }

    /// <summary>
    /// Raises the <see cref="E:ViewLoaded"/> event.
    /// </summary>
    /// <param name="e">
    /// The <see cref="System.EventArgs"/> instance containing the event 
    /// data.</param>
    protected void OnViewLoaded(EventArgs e)
    {
        if (viewLoaded != null)
        {
            viewLoaded(this, e);
        }
    }
    #endregion

    /// <summary>
    /// Gets a value indicating whether the control is in a designer.
    /// </summary>
    /// <value><c>true</c> if design time; otherwise, <c>false</c>.</value>
    protected bool DesignTime
    {
        get
        {
            return DesignerProperties.GetIsInDesignMode(this);
        }
    }

    #region IActiveAware and related

    void SetViewAwareAssociations(IViewModel oldViewModel, IViewModel newViewModel)
    {
        var oldViewAware = oldViewModel as IViewAware;
        var newViewAware = newViewModel as IViewAware;
        if (oldViewAware != null)
        {
            oldViewAware.DetachActiveAware();
        }

        if (newViewAware != null)
        {
            newViewAware.Attach(activeAwareUIElementAdapter);
        }
    }

    readonly ActiveAwareUIElementAdapter activeAwareUIElementAdapter;

    bool IActiveAware.IsActive
    {
        get
        {
            return activeAwareUIElementAdapter.IsActive;
        }
        set
        {
            activeAwareUIElementAdapter.IsActive = value;
        }
    }

    event EventHandler IActiveAware.IsActiveChanged
    {
        add
        {
            activeAwareUIElementAdapter.IsActiveChanged += value;
        }
        remove
        {
            activeAwareUIElementAdapter.IsActiveChanged -= value;
        }
    }

    #endregion
}

We see that the dependency property ViewModel, when changed, prompts the attachment of the ActiveAwareUIElement instance to the ViewModel. The mechanism for performing this is via the IViewAware implementation. If an IViewModel wishes to be aware of its view's state, in particular, when it becomes active or inactive; without having to have a direct reference to the view, then it can implement the IViewAware interface.

IViewAware Interface

/// <summary>
/// Provides for advanced presentation behaviour in a <see cref="IViewModel"/>s.
/// </summary>
public interface IViewAware
{
    /// <summary>
    /// Attaches the specified active aware instance so that changes in the
    /// <see cref="IActiveAware.IsActive"/>
    /// state can be monitored.
    /// </summary>
    /// <param name="activeAware">The active aware.</param>
    void Attach(IActiveAware activeAware);

    /// <summary>
    /// Detaches the active aware instance. Changes in the 
    /// <see cref="IActiveAware.IsActive"/>
    /// state will no longer be monitored.
    /// </summary>
    void DetachActiveAware();
}

The ViewModel base class implementation is provided next in full:

/// <summary>
/// A base implementation of the <see cref="IViewModel"/> interface.
/// </summary>
public abstract class ViewModelBase : IViewModel, INotifyPropertyChanged, IViewAware
{
    IActiveAware activeAwareInstance;

    protected ViewModelBase()
    {
        notifier = new PropertyChangeNotifier(this);
    }

    #region Title Property

    object title;

    [Description("The text to display on a tab.")]
#if !SILVERLIGHT
    [Browsable(true)]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
#endif        
    public object Title
    {
        get
        {
            return title;
        }
        set
        {
            notifier.Assign("Title", ref title, value);
        }
    }

    #endregion

    #region Property Change Notification
    readonly PropertyChangeNotifier notifier;

    protected PropertyChangeNotifier Notifier
    {
        get
        {
            return notifier;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged
    {
        add
        {
            notifier.PropertyChanged += value;
        }
        remove
        {
            notifier.PropertyChanged -= value;
        }
    }

    protected AssignmentResult Assign<TProperty>(
        string propertyName, ref TProperty property, TProperty newValue)
    {
        return notifier.Assign(propertyName, ref property, newValue);
    }

    #endregion

    #region Active Aware

    void IViewAware.Attach(IActiveAware activeAware)
    {
        ReplaceActiveAware(activeAware);
    }

    void IViewAware.DetachActiveAware()
    {
        ReplaceActiveAware(null);
    }

    void ReplaceActiveAware(IActiveAware activeAwareInstance)
    {
        if (this.activeAwareInstance != null)
        {
            this.activeAwareInstance.IsActiveChanged -= OnIsActiveChanged;
        }
        this.activeAwareInstance = activeAwareInstance;
        if (activeAwareInstance != null)
        {
            activeAwareInstance.IsActiveChanged += OnIsActiveChanged;
        }
    }

    bool lastActiveState;

    void OnIsActiveChanged(object sender, EventArgs e)
    {
        Notifier.NotifyChanged("Active", lastActiveState, Active);
        lastActiveState = Active;
    }

    /// <summary>
    /// Gets a value indicating whether this instance is being notified 
    /// of when it becomes active or inactive, 
    /// this may occur for example when its view gains focus or loses focus.
    /// </summary>
    /// <value><c>true</c> if monitoring the active state 
    /// of its view; otherwise, <c>false</c>.</value>
    public bool ActiveAware
    {
        get
        {
            return activeAwareInstance != null;
        }
    }

    /// <summary>
    /// Gets a value indicating whether this <see cref="ViewModelBase"/> 
    /// is active within the user interface.
    /// </summary>
    /// <value><c>true</c> if active; otherwise, <c>false</c>.</value>
    public bool Active
    {
        get
        {
            return activeAwareInstance != null ? activeAwareInstance.IsActive : false;
        }
    }

    #endregion

    public override string ToString()
    {
        return title != null ? title.ToString() : base.ToString();
    }
}

So there it is. Just one way to provide a ViewModel with active awareness. Please be aware that this code is preliminary and may be subject to change. The full source code will be available via the Calcium source download page soon.

Thanks to Kent Boogaart for his post on MVVM Infrastructure ActiveAwareCommand which the ActiveAwareUIElementAdapter was inspired from.