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POSilverlight MVVM binding updates fire in undesired order
 

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    <p><strong>Scenario</strong>: In a Silverlight 4 MVVM project, we have a <code>ListBox</code> control containing items, the selected item is two-way-bound to the appropriate property in the ViewModel. Another control (for example reasons, I've stripped it down to a single <code>TextBox</code>) is data bound to the selected item's content. The value should update on leave/focus lost.</p> <p><strong>Problem</strong>: When the value in the <code>TextBox</code> is changed and we leave that <code>TextBox</code> by pressing the Tab key, everything works as desired - the value is updated. However, if the user clicks on a different item in the <code>ListBox</code>, then the SelectedItem setter is fired before the content of <code>TextBox</code> setter is fired, leaving no chance to handle the user input.</p> <p><img src="https://imgur.com/NuEc0.png" alt="Screen"></p> <p>You can see in debugger, when adding breakpoints to the property setters, that the new <code>ListView</code> selection is applied first, before the <code>TextBox</code> update is processed.</p> <p><strong>Desired behavior</strong>: We need to know that the currently selected item was modified before the user has selected another item. It's not desired to have a custom update trigger which would notify on each key press (we know that's possible).</p> <p>Can you help?</p> <p><strong>Code (a very simple example):</strong></p> <p><strong>ViewModel</strong></p> <pre><code>public abstract class ViewModelBase : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; protected void OnPropertyChanged(string propertyName) { var handler = PropertyChanged; if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName)); } } public class ItemViewModel : ViewModelBase { private string _content; public ItemViewModel(string initContent) { _content = initContent; } public string Content { get { return _content; } set { if (_content != value) { _content = value; OnPropertyChanged("Content"); } } } } public class MainViewModel : ViewModelBase { private ObservableCollection&lt;ItemViewModel&gt; _items = new ObservableCollection&lt;ItemViewModel&gt;(); private ItemViewModel _selectedViewModel; public ObservableCollection&lt;ItemViewModel&gt; Items { get { return _items; } } public ItemViewModel SelectedItem { get { return _selectedViewModel; } set { if (_selectedViewModel != value) { _selectedViewModel = value; OnPropertyChanged("SelectedItem"); } } } } </code></pre> <p><strong>XAML</strong></p> <pre><code>&lt;Grid x:Name="LayoutRoot" Background="White"&gt; &lt;ListBox Height="100" HorizontalAlignment="Left" Margin="12,12,0,0" VerticalAlignment="Top" ItemsSource="{Binding Items}" SelectedItem="{Binding SelectedItem, Mode=TwoWay}" DisplayMemberPath="Content" Width="220" /&gt; &lt;TextBox Height="23" HorizontalAlignment="Left" Margin="12,118,0,0" Text="{Binding SelectedItem.Content, Mode=TwoWay}" VerticalAlignment="Top" Width="220" /&gt; &lt;/Grid&gt; </code></pre> <p><strong>XAML Code Behind</strong></p> <pre><code> public MvvmTestView() { InitializeComponent(); Loaded += new RoutedEventHandler(MvvmTestView_Loaded); } void MvvmTestView_Loaded(object sender, RoutedEventArgs e) { MainViewModel viewModel = new MainViewModel(); viewModel.Items.Add(new ItemViewModel("Hello StackOverflow")); viewModel.Items.Add(new ItemViewModel("Thanks to Community")); DataContext = viewModel; } </code></pre> <p><strong>UPDATE 1</strong> I present <a href="https://stackoverflow.com/questions/5338546/silverlight-mvvm-binding-updates-fire-in-undesired-order/5394399#5394399">a self designed solution</a> for you to check out, which will be probably be the accepted one, I still want to encourage you to make comments and give your hints. Thanks.</p>
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