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    <p>I know two ways how to do this.</p> <p><strong>1)</strong> Add new class, for example, <code>MainWindowViewModel</code> and add there 2 commands and an instance of <code>UserControlViewModel</code> (you haven't said the title, so I will call it in this way). Here is a part of example:</p> <pre><code>public class MainWindowViewModel { public UserControlViewModel ChildControlViewModel { get; set; } private Lazy&lt;RelayCommand&gt; nextCommand = new Lazy&lt;RelayCommand&gt;(() =&gt; new RelayCommand( () =&gt; this.ChildControlViewModel.CollectionView.MoveCurrentToNext(), () =&gt; this.ChildControlViewModel.CollectionView.CurrentPosition &lt; this.ChildControlViewModel.ItemTypes.Count - 1)); public ICommand NextCommand { get { return nextCommand.Value; } } //prev command... } </code></pre> <p>I have used the <code>Lazy</code> class, but the main idea is clear: the code is the same, except the call <code>this.ChildControlViewModel.CollectionView</code> instead of <code>CollectionView</code>.</p> <p><strong>2)</strong> Use the <code>Messenger</code> class.</p> <p>This way isn't so obvious and it has only one advantage: the viewmodels are loosely connected.</p> <pre><code>public class MainWindowViewModel { public const string NextCommandNotification = "NextCommand"; public const string PreviousCommandNotification = "PreviousCommand"; private bool isNextCommandEnabled; private bool isPreviousCommandEnabled; public MainWindowViewModel() { this.NextCommand = new RelayCommand( () =&gt; Messenger.Default.Send(new NotificationMessage&lt;MainWindowViewModel&gt;(this, NextCommandNotification)), () =&gt; this.isNextCommandEnabled); //prev command... Messenger.Default.Register&lt;NotificationMessage&lt;UserControlViewModel&gt;&gt;(this, msg =&gt; { if (msg.Notification == UserControlViewModel.CurrentItemChangedNotification) { this.isNextCommandEnabled = msg.Content.CollectionView.CurrentPosition &lt; msg.Content.ItemTypes.Count - 1; this.NextCommand.RaiseCanExecuteChanged(); //prev command... } }); } public ICommand NextCommand { get; private set; } //prev command... } public class UserControlViewModel { public const string CurrentItemChangedNotification = "CurrentItemChanged"; public UserControlViewModel() { Messenger.Default.Register&lt;NotificationMessage&lt;MainWindowViewModel&gt;&gt;(this, msg =&gt; { if (msg.Notification == MainWindowViewModel.NextCommandNotification) this.CollectionView.MoveCurrentToNext(); else if (msg.Notification == MainWindowViewModel.PreviousCommandNotification) this.CollectionView.MoveCurrentToPrevious(); }); this.CollectionView.CurrentChanged += (s,e) =&gt; Messenger.Default.Send(new NotificationMessage&lt;UserControlViewModel&gt;(this, CurrentItemChangedNotification)) } } </code></pre> <p>I'm not sure whether this code will work correctly. And it is not easy to explain.</p> <p>The <code>MainWindowViewModel</code> class send the message when a user press the button. The <code>UserControlViewModel</code> class process the message, change the position of the current item, and send the <code>CurrentItemChangedNotification</code> message. The <code>MainWindowViewModel</code> class process this message and updates the <code>CanExecute</code> part of the command.</p> <p>1st solution is better for me, but at the same time I use the <code>Messenger</code> class quite often. It depends on the situation.</p>
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    1. CO:-) This is great, thanks!
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