c# - How to bring window to front with wpf and using mvvm

Question

I have a window that essentially runs a timer. When the timer hits 0 I want to bring the window to the front so that it is visible and not hidden behind some other application.

From what I can gather I would simply call window.activate() to accomplish this but with mvvm my view model doesn't have a reference to window.

Answer 1

A "purist" MVVM solution is to use a behavior. Below is a behavior for a Window with an Activated property. Setting the property to true will activate the window (and restore it if it is minimized):

public class ActivateBehavior : Behavior<Window> {

  Boolean isActivated;

  public static readonly DependencyProperty ActivatedProperty =
    DependencyProperty.Register(
      "Activated",
      typeof(Boolean),
      typeof(ActivateBehavior),
      new PropertyMetadata(OnActivatedChanged)
    );

  public Boolean Activated {
    get { return (Boolean) GetValue(ActivatedProperty); }
    set { SetValue(ActivatedProperty, value); }
  }

  static void OnActivatedChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e) {
    var behavior = (ActivateBehavior) dependencyObject;
    if (!behavior.Activated || behavior.isActivated)
      return;
    // The Activated property is set to true but the Activated event (tracked by the
    // isActivated field) hasn't been fired. Go ahead and activate the window.
    if (behavior.AssociatedObject.WindowState == WindowState.Minimized)
      behavior.AssociatedObject.WindowState = WindowState.Normal;
    behavior.AssociatedObject.Activate();
  }

  protected override void OnAttached() {
    AssociatedObject.Activated += OnActivated;
    AssociatedObject.Deactivated += OnDeactivated;
  }

  protected override void OnDetaching() {
    AssociatedObject.Activated -= OnActivated;
    AssociatedObject.Deactivated -= OnDeactivated;
  }

  void OnActivated(Object sender, EventArgs eventArgs) {
    this.isActivated = true;
    Activated = true;
  }

  void OnDeactivated(Object sender, EventArgs eventArgs) {
    this.isActivated = false;
    Activated = false;
  }

}

The behavior requires a reference to System.Windows.Interactivity.dll. Fortunately, this is now available on NuGet in the Blend.Interactivity.Wpf package.

The behavior is attached to a Window in XAML like this:

<Window ...>
  <i:Interaction.Behaviors>
    <Behaviors:ActivateBehavior Activated="{Binding Activated, Mode=TwoWay}"/>
  </i:Interaction.Behaviors>

The view-model should expose a boolean Activated property. Setting this property to true will activate the window (unless it is already activated). As an added bonus it will also restore a minimized window.