Monday, 31 January 2011

Using DeferredLoadListBox in a Pivot Control

Recently I've been using the DeferredLoadListBox It's a fantastic way of improving the performance of your list views.




I started using the DeferredLoadListBox within a Pivot control but occasionally found that the app would randomly crash throwing the following exception:

All containers must have a Height set (ex: via ItemContainerStyle), though the heights need not all need to be the same.
This usually means you haven't set the properly. However I had ensured I had set the style height. I then got the source and started poking around.




What I found was although in the UnmaskItemContent method the container had a height if you inspected the ActualHeight property this was 0. I did a bit of research and found the following on MSDN:

ActualWidth and ActualHeight are calculated based on the Width/Height property values and the layout system.  There is no guarantee as to when these values will be "calculated"
So the problem appeared to be with timing.




After contacting the very helpful David Anson and exchanging a few ideas on solving the issue I decided to implement a retry counter. The concept being the first two times we find the container height is 0 drop out of the unmask method and try again later. If we still don't have a height on the third attempt an error must have occurred.




Implementing the counter was very simple and the most important thing is that this simple retry counter allows time for the height to be calculated and work as expected. The only minor side effect of this counter is that you can notice a slight pause in the UI however as this exception doesn't occur everytime this seems fair enough.




Find the modified source below and I hope it can be of use to you.



// Copyright (C) Microsoft Corporation. All Rights Reserved.
// This code released under the terms of the Microsoft Public License
// (Ms-PL, http://opensource.org/licenses/ms-pl.html).

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;

namespace Delay
{
///
/// Implements a subclass of ListBox based on a StackPanel that defers the
/// loading of off-screen items until necessary in order to minimize impact
/// to the UI thread.
///

public class DeferredLoadListBox : ListBox
{
private enum OverlapKind { Overlap, ChildAbove, ChildBelow };

private ScrollViewer _scrollViewer;
private ItemContainerGenerator _generator;
private bool _queuedUnmaskVisibleContent;
private bool _inOnApplyTemplate;

///
/// Handles the application of the Control's Template.
///

public override void OnApplyTemplate()
{
// Unhook from old Template elements
_inOnApplyTemplate = true;
ClearValue(VerticalOffsetShadowProperty);
_scrollViewer = null;
_generator = null;

// Apply new Template
base.OnApplyTemplate();

// Hook up to new Template elements
_scrollViewer = FindFirstChildOfType(this);
if (null == _scrollViewer)
{
throw new NotSupportedException("Control Template must include a ScrollViewer (wrapping ItemsHost).");
}
_generator = ItemContainerGenerator;
SetBinding(VerticalOffsetShadowProperty, new Binding { Source = _scrollViewer, Path = new PropertyPath("VerticalOffset") });
_inOnApplyTemplate = false;
}

///
/// Determines if the specified item is (or is eligible to be) its own item container.
///

/// The specified item.
/// true if the item is its own item container; otherwise, false.
protected override bool IsItemItsOwnContainerOverride(object item)
{
// Check container type
return item is DeferredLoadListBoxItem;
}

///
/// Creates or identifies the element used to display a specified item.
///

/// A DeferredLoadListBoxItem corresponding to a specified item.
protected override DependencyObject GetContainerForItemOverride()
{
// Create container (matches ListBox implementation)
var item = new DeferredLoadListBoxItem();
if (ItemContainerStyle != null)
{
item.Style = ItemContainerStyle;
}
return item;
}

///
/// Prepares the specified element to display the specified item.
///

/// The element used to display the specified item.
/// The item to display.
protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
{
// Perform base class preparation
base.PrepareContainerForItemOverride(element, item);

// Mask the container's content
var container = (DeferredLoadListBoxItem)element;
if (!DesignerProperties.IsInDesignTool)
{
container.MaskContent();
}

// Queue a (single) pass to unmask newly visible content on the next tick
if (!_queuedUnmaskVisibleContent)
{
_queuedUnmaskVisibleContent = true;
Dispatcher.BeginInvoke(() =>
{
_queuedUnmaskVisibleContent = false;
UnmaskVisibleContent();
});
}
}

private static readonly DependencyProperty VerticalOffsetShadowProperty =
DependencyProperty.Register("VerticalOffsetShadow", typeof(double), typeof(DeferredLoadListBox), new PropertyMetadata(-1.0, OnVerticalOffsetShadowChanged));
private static void OnVerticalOffsetShadowChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
{
// Handle ScrollViewer VerticalOffset change by unmasking newly visible content
((DeferredLoadListBox)o).UnmaskVisibleContent();
}

private void UnmaskVisibleContent()
{
// Capture variables
var count = Items.Count;

// Find index of any container within view using (1-indexed) binary search
var index = -1;
var l = 0;
var r = count + 1;
while (-1 == index)
{
var p = (r - l) / 2;
if (0 == p)
{
break;
}
p += l;
var c = (DeferredLoadListBoxItem)_generator.ContainerFromIndex(p - 1);
if (null == c)
{
if (_inOnApplyTemplate)
{
// Applying template; don't expect to have containers at this point
return;
}
// Should always be able to get the container
var presenter = FindFirstChildOfType(_scrollViewer);
var panel = (null == presenter) ? null : FindFirstChildOfType(presenter);
if (panel is VirtualizingStackPanel)
{
throw new NotSupportedException("Must change ItemsPanel to be a StackPanel (via the ItemsPanel property).");
}
else
{
throw new NotSupportedException("Couldn't find container for item (ItemsPanel should be a StackPanel).");
}
}
switch (Overlap(_scrollViewer, c, 0))
{
case OverlapKind.Overlap:
index = p - 1;
break;
case OverlapKind.ChildAbove:
l = p;
break;
case OverlapKind.ChildBelow:
r = p;
break;
}
}

if (-1 != index)
{
// Unmask visible items below the current item
for (var i = index; i < count; i++)
{
if (!UnmaskItemContent(i))
{
break;
}
}

// Unmask visible items above the current item
for (var i = index - 1; 0 <= i; i--)
{
if (!UnmaskItemContent(i))
{
break;
}
}
}
}

private bool UnmaskItemContent(int index)
{
var container = (DeferredLoadListBoxItem)_generator.ContainerFromIndex(index);
if (null != container)
{
// Return quickly if not masked (but periodically check visibility anyway so we can stop once we're out of range)
if (!container.Masked && (0 != (index % 16)))
{
return true;
}
// Check necessary conditions
if (0 == container.Height)
{
throw new NotSupportedException("All containers must have a Height set (ex: via ItemContainerStyle), though the heights need not all need to be the same.");
}
// If container overlaps the "visible" area (i.e. on or near the screen), unmask it
if (OverlapKind.Overlap == Overlap(_scrollViewer, container, 2 * _scrollViewer.ActualHeight))
{
container.UnmaskContent();
return true;
}
}
return false;
}

private static bool Overlap(double startA, double endA, double startB, double endB)
{
return (((startA <= startB) && (startB <= endA)) ||
((startB <= startA) && (startA <= endB)));
}

private static OverlapKind Overlap(FrameworkElement parent, FrameworkElement child, double padding)
{
// Get child bounds relative to parent
var transform = child.TransformToVisual(parent);
var bounds = new Rect(transform.Transform(new Point()), transform.Transform(new Point(/*child.ActualWidth*/ 0, child.ActualHeight)));
// Return kind of overlap
if (Overlap(0 - padding, parent.ActualHeight + padding, bounds.Top, bounds.Bottom))
{
return OverlapKind.Overlap;
}
else if (bounds.Top < 0)
{
return OverlapKind.ChildAbove;
}
else
{
return OverlapKind.ChildBelow;
}
}

private static T FindFirstChildOfType(DependencyObject root) where T : class
{
// Enqueue root node
var queue = new Queue();
queue.Enqueue(root);
while (0 < queue.Count)
{
// Dequeue next node and check its children
var current = queue.Dequeue();
for (var i = VisualTreeHelper.GetChildrenCount(current) - 1; 0 <= i; i--)
{
var child = VisualTreeHelper.GetChild(current, i);
var typedChild = child as T;
if (null != typedChild)
{
return typedChild;
}
// Enqueue child
queue.Enqueue(child);
}
}
// No children match
return null;
}
}
}

Saturday, 22 January 2011

Windows Phone 7: Security Exception on deactivate

This morning I have been finishing the tombstoning part of one of my Windows Phone applications I have been developing. Whilst testing I found that when the application deactivated or terminated that a SecurityException was thrown. Looking at the stack trace I noticed that it was occurring whilst trying to serialise some data to the Applications State store.




Now I knew I was putting some data into state so this wasn't to hard to find however, when I looked at what I was putting into state it was nothing more complicated than a custom type that exposed a collection of POCO classes. Why would this cause a security exception.




I decided to do a simple Google search for the exception "windows phone 7 security exception" and the second result looked similar: "c# - SecurityException was unhandled when using isolated storage" [http://stackoverflow.com/questions/4209280/securityexception-was-unhandled-when-using-isolated-storage]. So I had a look, and guessed that if you can't put internal classes into IsolatedStorage you also can't do it for ApplicationState, which makes sense when you think about it. So I made my type public and not internal and all was good in the world again.




Another week another tip.... I wonder what I will find out next week

Saturday, 15 January 2011

Windows Phone Development and Designer Exceptions

I've been building a few Windows Phone applications recently and have been learning lot's along the way and am hopefully releasing a couple of these in the near future. However on my current project I have been constantly hounded by the designer view of my xaml pages throwing exceptions.




Tonight I decided to finally look into why these occur, I have managed to ignore them previously due to the weird nature of them. My exception scenario consists of a page that contains a pivot control and inside one of the pivot items there is a user control. The user control contains a simple list view which has some data bound to it

Now I have always got past the exceptions as if you use the designer view for the user control it works fine, however as soon as I put it into a page or a pivot the designer starts kicking off.



The actual exception is as follows:

Could not load type 'System.Net.HttpUtility' from assembly 'System.Windows, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e'.
It then goes on to tell me the line number and location etc of this. At first glance this exception looks a bit weird as in the code view and if you run the application the exception is never thrown.

The first thing I did to try and resolve this issue was to find where System.Net.HttpUtility lives, in the desktop version of Silverlight it lives in System.Windows.Browser But as the Windows Phone version of Silverlight doesn't include System.Windows.Browser the development team moved the httputility type into the System.Net namespace and put it into the System.Windows assembly, http://msdn.microsoft.com/en-us/library/dd470087(v=vs.95).aspx#Assemblies. This explains why the project builds and runs etc, however it doesn't explain why the designer has such a bad time with it.




Sadly I couldn't find a source or information on this exception, my guess is the designer is loading an older version of the assembly, a missing designer tool update from the RTM or something similar {Edit see my update below}. However what I have found is that Silverlight does have another way of encoding and decoding URL strings. System.Uri.EscapeUriString, if you use this the phone and the designer all work happily. A minor change and a headache eased :) I'm hoping the next tools update fixes the issue but until then this seems the safest method of escaping url strings.



Update


As I oddly found this issue intriguing I decided to investigate further. I opened another Visual Studio attached it to the Visual Studio process I had my project in and turned on catch all exceptions. Then when the exception was thrown I looked at the AppDomains assembly list, System.AppDomain.CurrentDomain.GetAssemblies(), 55 in all were loaded. I then looked through this list for the assembly referenced in the exception. This assembly then had the following location value: Location "c:\\Program Files (x86)\\Microsoft Silverlight\\4.0.51204.0\\System.Windows.dll". This is the main Silverlight 4 runtime, NOT the Windows Phone version which is 3.7.x, to be certain I then opened the assembly in Reflector to be sure the System.Net.HttpUtility was missing and it was.


So as suspected the designer is loading the wrong assembly thus the error, the assembly it should be loading is in the referenced assemblies folder, C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\Silverlight\v4.0\Profile\WindowsPhone\System.Windows.dll , hopefully reporting this on Connect will result in a hotfix / update.