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POBuilding a Panel-like Custom Control with Attached Properties
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  1. POBuilding a Panel-like Custom Control with Attached Properties
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    copied!<p>I'm attempting to implement a WPF custom control that displays two collections of <code>FrameworkElement</code> objects in two separate panels. The control derives from <code>ItemsControl</code> and has a <code>Canvas</code> which has <code>IsItemsHost="True"</code> set. Additionally, I have a <code>Collection&lt;FrameworkElement&gt;</code> named <code>OtherItems</code> which I would like to display in a different <code>Canvas</code>. </p> <p>The custom control defines an attached property: <code>myControl.Position</code> which determines the X position of the item within its canvas. The control also defines a <code>MinValue</code> and <code>MaxValue</code> dependency properties. The control calculates uses the <code>MinValue</code>, <code>MaxValue</code> and <code>control.Position</code> properties to place the <code>UIElement</code>'s at the proper position:</p> <p><code>&lt;----------0----------&gt;</code> is the result of: MinValue=10, MaxValue=110, myControl.Position=60</p> <p>I can handle positioning for items that are in the <code>myControl.Items</code> container, but items that are in <code>myControl.OtherItems</code> have a null <code>Parent</code> reference:</p> <pre><code>public static readonly DependencyProperty PositionProperty = DependencyProperty.RegisterAttached("Position", typeof (double), typeof (myControl), new FrameworkPropertyMetadata(0.0, OnPositionChanged)); public static void OnPositionChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e) var element = sender as FrameworkElement; if (element != null) { var container = element.Parent as myControl; if (container != null) { container.PositionElement(element); } } } &lt;!--Within the myControl's template--&gt; &lt;Canvas IsItemsHost="True" x:Name="PART_ItemCanvas"/&gt; &lt;ItemsControl ItemsSource="{TemplateBinding OtherItems}"&gt; &lt;ItemsControl.ItemsPanel&gt; &lt;ItemsPanelTemplate&gt; &lt;Canvas x:Name="PART_OtherItemCanvas"/&gt; &lt;/ItemsPanelTemplate&gt; &lt;/ItemsControl.ItemsPanel&gt; &lt;/ItemsControl.ItemContainerStyle&gt; &lt;!-- As used --&gt; &lt;custom:myControl MaxValue="1000" MinValue="-250"&gt; &lt;custom:myControl.Items&gt; &lt;Button myControl.Position="0"/&gt; &lt;SomethingElse myControl.Position="25"/&gt; &lt;Etc myControl.Position="100"/&gt; &lt;/custom:myControl.Items&gt; &lt;custom:myControl.OtherItems&gt; &lt;Button myControl.Position="20"/&gt; &lt;Label myControl.Position="300"/&gt; &lt;/custom:myControl.OtherItems&gt; &lt;/custom:myControl&gt; </code></pre> <p>How can I handle the call to <code>OnPositionChanged</code> when I do not have access to the sender's parents? I'm assuming this is something that WPF layout controls do all the time, with attached properties like <code>Grid.Row</code> and <code>DockPanel.Dock</code>, but how do they manage it?</p>
 

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