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POIrregular event behavior - firing dependent on subscription to child control's equivalent event
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  1. POIrregular event behavior - firing dependent on subscription to child control's equivalent event
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    copied!<p>I encountered this behavior with WinRT's ListView.</p> <p><code>LayoutUpdated</code> event does fire less often if no event handler is attached to the inner <code>ScrollViewer</code>'s <code>LayoutUpdated</code> event.</p> <p>I marked the critical line near the bottom with <code>// === Critical line ===</code>. Removing the leading slashes will lead to <code>lv_LayoutUpdated</code> being called more often.</p> <p>This has implications since the event is fired far too seldom for it to be of any use to me (in the commented-out case) The problem is, that I cannot set the scroll position before the <code>ScrollViewer</code>'s mutation has come to an end. Routing through the <code>ScrollViewer</code>'s event seems silly, though.</p> <p>Is this a bug or is it explainable with bubbling strategies or sth.?</p> <p>EDIT: BTW, this page is not shown in the app's root frame, but in a frame of a different page if that makes any difference.</p> <p>EDIT2: I ended up disovering why the firing timing was useless (was doing the data binding async, which screwed with the standard event timings). The behavior persists, though - subscribing to <code>ScrollViewer</code>'s event leads to more publishing by the <code>ListView</code>'s event. Under what additional circumstances this ends up happening, I do not know. I am using Caliburn.Micro - so maybe their data source wiring does something to the control. E.g. one issue with CM is, that it swallows the first LayoutUpdated event of the view (if bound via Message:Attach atleast) - although this might only affect the view model.</p> <p>Anyway, I'll leave this question be for others to disover.</p> <p>This setup in the code-behind:</p> <pre><code> public FileListView() { this.InitializeComponent(); var lv = (ListView)FindName("Files"); lv.Loaded += lv_Loaded; lv.LayoutUpdated += lv_LayoutUpdated; } void lv_LayoutUpdated(object sender, object e) { var m = (FileListViewModel)DataContext; m.Offset = 100; } void lv_Loaded(object sender, RoutedEventArgs e) { var lv = (ListView)sender; var sv = lv.GetFirstDescendantOfType&lt;ScrollViewer&gt;(); if (sv == null) { return; } //sv.LayoutUpdated += sv_LayoutUpdated; // === Critical line === } void sv_LayoutUpdated(object sender, object e) { // No code here. Just the subscription suffices } </code></pre> <p>The XAML:</p> <pre><code> &lt;Page x:Class="Namespace1.FileListView" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="using:Poolar.Mobile.PsProject.WinRT.Views" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d"&gt; &lt;Grid HorizontalAlignment="Left" Height="768" VerticalAlignment="Top" Width="1366"&gt; &lt;Grid.RowDefinitions&gt; &lt;RowDefinition Height="61*"/&gt; &lt;RowDefinition Height="707*"/&gt; &lt;/Grid.RowDefinitions&gt; &lt;ListView x:Name="Files" HorizontalAlignment="Left" Height="446" VerticalAlignment="Top" Width="1366" Margin="10,10,-10,0" Grid.Row="1" /&gt; &lt;Button x:Name="GoBack" x:Uid="ButtonGoBack" Content="Button" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top"/&gt; &lt;/Grid&gt; &lt;/Page&gt; </code></pre>
 

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