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PO.NET: Difficulty with Events
 

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    <p>Perhaps I don't understand events fully.</p> <p>I'm building a Windows Phone 7 app in Silverlight.</p> <p>I have a <code>UserControl</code> that wraps a <code>ListBox</code>, called <code>EditableListBox</code>. The <code>ListBox</code> has a data template. The items in the list box are wrapped by <code>EditableListItem</code> objects.</p> <p>The data template is as follows:</p> <pre><code>&lt;DataTemplate&gt; &lt;Grid ManipulationCompleted="Grid_ManipulationCompleted"&gt; &lt;Grid.ColumnDefinitions&gt; &lt;ColumnDefinition Width="Auto"/&gt; &lt;ColumnDefinition Width="*" /&gt; &lt;/Grid.ColumnDefinitions&gt; &lt;Image Source="{Binding Path=IconSource}" Grid.Column="0" Width="96" Height="96" VerticalAlignment="Center" Visibility="{Binding Path=Editing, Converter={StaticResource visibilityConverter}}" /&gt; &lt;TextBlock Text="{Binding Path=Name}" Grid.Column="1" /&gt; &lt;/Grid&gt; &lt;/DataTemplate&gt; </code></pre> <p>I'm binding the <code>Visibility</code> to a property of each <code>EditableListItem</code>, so I need to implement <code>INotifyPropertyChanged</code> so updates to the backing items are reflected in the UI. (Right? Or is there a simpler way to do it?)</p> <p><code>EditableListItem</code>:</p> <pre><code>public class EditableListItem : INotifyPropertyChanged { private EditableListBox _parentListBox; public event PropertyChangedEventHandler PropertyChanged; public bool Editing { get { return _parentListBox.Editing; } } public EditableListItem(Section section, EditableListBox parentListBox) { _parentListBox = parentListBox; // after this line, _parentListBox.PropertyChanged is still null. // why is that? _parentListBox.PropertyChanged += PropertyChanged; _parentListBox.PropertyChanged += new PropertyChangedEventHandler(_parentListBox_PropertyChanged); } </code></pre> <p><code>EditableListBox</code>:</p> <pre><code>public partial class EditableListBox : UserControl, INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; // NotifyPropertyChanged will raise the PropertyChanged event, // passing the source property that is being updated. public void NotifyPropertyChanged(string propertyName) { if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); } } public void SetSectionsSource(ObservableCollection&lt;Section&gt; sectionsSource) { sectionsSource.CollectionChanged += new NotifyCollectionChangedEventHandler(sectionsSource_CollectionChanged); ContentListBox.ItemsSource = sectionsSource.Select(section =&gt; new EditableListItem(section, this) { Enabled = true }); //ContentListBox.ItemsSource.Add(new EditableListItem(new Section("Section", 3)) { Enabled = true }); } // ... private bool _editing; public bool Editing { get { return _editing; } set { _editing = value; NotifyPropertyChanged("Editing"); } } } </code></pre> <p>The <code>Editing</code> property is stored in <code>EditableListBox</code> - <code>EditableListItem</code> just forwards it. I wanted to attached <code>EditableListItem.PropertyChanged</code> to <code>EditableListBox.PropertyChanged</code> directly, but the following didn't work:</p> <pre><code> // after this line, _parentListBox.PropertyChanged is still null. // why is that? _parentListBox.PropertyChanged += PropertyChanged; </code></pre> <p>The following did work:</p> <pre><code>_parentListBox.PropertyChanged += new PropertyChangedEventHandler(_parentListBox_PropertyChanged); </code></pre> <p>Why is this? Is the first attempt totally invalid (if so, why does the compiler allow it?)?</p>
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