I have a scenario where in I populate a listbox with a 1000 items. I set the ItemsSource property with a source of data.
I have a requirement where I need to strike out an item of the listbox based on certain criteria, when the UI loads. I am using styles + attached properties to achieve the same by setting ContentTemplate of ListBoxItem in the callback method of attached property.
My problem is when I try to generate a ListBoxItem using ItemContainerGenerator.ContainerFromItem, for an item which is at the end of the list, I get null. As a result, I cannot strike out items of the listbox which are at the bottom of the list.
Has it got something to do with virtualization. I want to obtain all those ListBoxItems on load.
Is there any workaround?
Thanks
This is definitely caused by virtualization. This is exactly what UI virtualization is supposed to do - only create ListBoxItem objects for items that are visible on the screen. You can easily see that this is indeed the cause by setting VirtualizingStackPanel.IsVirtualizing = false on your ListBox and see that ItemContainerGenerator.ContainerFromItem no longer returns null.
You can set a style for your ListBoxItems in your ListBox that will have the logic to strike out the items as needed. This should also work when virtualization is enabled. For example:
<ListBox>
<ListBox.Resources>
<Style TargetType=ListBoxItem>
...
</Style>
</ListBox.Resources>
</ListBox>
Related
I have a ComboBox which uses ListBox to show items. ListBox’s ItemSource is bound with a CollectionViewSource.
Issue: Once I open ComboBox and scroll through the items and leave it in middle or at bottom. Once I reopen ComboBox, even though I refresh or reload ItemSource (CollectionViewSource), the Scrollbar remains at the same place where I left it last time. I want it to be as default (at top) each and every time I reload ItemSource.
Is there is any way of doing this in XAML itself? Another thing, I cannot use Behavior or Attach property. I want any template or style for this.
In order for a Style on a ListBox to embody some behavior for the <ScrollViewer>, you would need to use an Attached Property / Attached Behavior to control the ScrollViewer's "grabber" position. This is because your collection is bound to your ListBox and notifying when it is updated needs to drive a behavior that isn't natively on the ListBox. It may be possible to reset the scroll position with a <ControlTemplate> for the <ScrollViewer> itself, but I imagine it would be difficult as it would likely involve manipulating Transforms / StoryBoards based on DataTriggering with your ItemsSource, but again that may cause a dependency on needing to use an Attached property, which for some reason you can't use...
If you simply want to get a result now, and you don't care about testability or re-usability, I would handle the TargetUpdated event in the code-behind. It's ultimately what the Attached Behavior would end up doing. On the other hand, if you do care about re-usability then you need to evaluate and challenge why you can't use an Attached Behavior (they are also testable, too); an Attached Behavior would be also easier than trying to edit a ControlTemplate.
Here is the code-behind approach:
.xaml:
<ListBox x:Name="myListBox"
ItemsSource="{Binding MyItemsSource, NotifyOnTargetUpdated=True}"
TargetUpdated="ListBox_TargetUpdated"/>
.xaml.cs:
private void ListBox_TargetUpdated(object sender, DataTransferEventArgs e)
{
if (myListBox.Items.Count > 0)
myListBox.ScrollIntoView(myListBox.Items[0]);
}
Edit: On the flip-side, if you are using MVVM, you can do something like this SO post suggests and set IsSynchronizationWithCurrentItem="True" and when you refresh your ItemsSource, simply set your SelectedItem to the first in the list and handle the SelectionChanged event in your vm.
I have a list box that has a list of user controls. Each user control has 5 combo boxes. I want to be able to read the selected text of each combo box in each user control from the main application. However, when I change the selection in the combo box, the text property of the combo box in the user control doesn't change when I read it in the main application.
Code-behind:
radQueryParamList.Items.Add(new TCardQueryParameters());
Xaml (This is just a data template for how to display a TCardQueryParameters object):
<DataTemplate x:Key="TCardViewQueryParamDataTemplate">
<tcardqueryparam:TCardQueryParameters x:Name="TCardViewerParamUC" />
</DataTemplate>
<telerik:RadListBox Grid.Column="1" ItemTemplate="{StaticResource TCardViewQueryParamDataTemplate}" Name="radQueryParamList" VerticalAlignment="Top" HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch" Grid.ColumnSpan="3">
Where I loop over the list of user controls
string test = radTESTGACC.Text;//TEST combo box, Text property changes
//radQueryParamList is a listbox of user controls where TCardQueryParameters is the UC
foreach(TCardQueryParameters param in radQueryParamList.Items)
{
//Each UC has a radGACC combo box in it, and I am reading what the user
//selected for each user control here in the main app, but the text property
//never changes
String gacc = param.radGACC.Text; //Text property DOESN'T CHANGE
}
I thought that each instance of the user control would keep its own state and I would just be able to read what the user selected for that combo box, but that doesn't seem to be the case.
You have not bound the SelectedItem, SelectedValue, or SelectedIndex property of your internal ComboBox to anything so it maintains its selection.
An ItemTemplate is like a cookie cutter. It contains the definition of the object, but not the object itself. Properties specific to the object's state are lost unless they are bound to something on the DataContext behind the template.
This is important to note for two aspects.
First off, to improve performance WPF usually unloads items which are not visible, which often results in items being re-created from their template anytime they are reloaded. An example would be when you minimize an application to the taskbar, then maximize it again. This is usually better on performance and memory usage, however it does mean you have to be sure you store the state of items that were created with a Template somewhere.
And second, by default ListBoxes use something called virtualization. A simple way of explaining this would be this:
Suppose you have a ListBox of 100,000 items. In your ListBox, only 10 items can be visible at a time. WPF will render roughly 14 items (the 10 visible ones, and then a few extra for a scroll buffer so you don't see anything unusual while scrolling). When you scroll to new items, WPF just re-uses the existing items that are already rendered, and just replaces the DataContext behind those items.
As you can guess, it is far better on performance to render 14 UI items instead of 100k items.
So to answer your question, you will probably want to bind either SelectedItem, SelectedValue, or SelectedIndex of your TCardQueryParameters UserControl to a property on the DataContext (which in your case appears to be another different UserControl).
It should probably be noted that what you are essentially doing is creating a list of UserControls, assigning them to the ListBox, and then telling the ListBox that it should draw each UserControl with another separate UserControl. So although you are changing the SelectedItem in the template UserControl, that change is not being reflected to your ListBox.Items copy of the UserControl.
I'm guessing you probably don't want that, so would recommend removing your ItemTemplate completely.
Or better yet, create a new class object containing all the data for your UserControl, and adding that to your ListBox.Items. Then tell the ListBox to draw that item using a TCardQueryParameters UserControl as the ItemTemplate, like you have now.
I am using ItemsControl in my application, applied ItemTemplate and ItemsPanelTemplate to it with a ScrollViewer style. All is fine. Binding is working pretty fine but here the issue: I want to select an item in ItemsControl but I am not able to do so as we do in ListBox using IsSelected property in triggers.
How can I achieve item selection and IsMouseOver in ItemsControl similar to ListBox, is there any way, as I did not find any resource on net useful enough so thought to ask for some help here.
I've got a user control that I'm using as my DataTemplate for all the items in my ListBox. There's an animation in said UserControl that's pretty simple - it just expands a certain ListBox, and it works. The thing is, when I scroll, every Nth item ALSO has the ListBox expanded, where N depends on how big my browser is sized to (in other words, how many items the ListBox is holding at any one time.)
It seems as though new items getting loaded into the listbox as I scroll are tripping over this animation. Is there anything I can do about this?
If your outer ListBox contains only a few items then add this to its Xaml:-
<ListBox.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel />
</ItesmPanelTemplate>
</ListBox.ItemsPanel>
By default the ListBox uses a VirtualizingStackPanel which only contains concrete instances of ListBoxItem that are currently being displayed. Items a generated as needed by a ItemContainerGenerator which will recycle existing items. I suspect some in there the state of an ListBoxItem is not entirely cleared when re-used to display another item from the ItemsSource.
I have a WPF listview, and in one column the cell may contain one or more ListBoxes.
When I right-click a ListBox I'm building a context menu where each item has a DelegateCommand. Currently I'm setting the command parameter to a SelectedListBox property on the page viewmodel itself as my delegate command needs to know which ListBox has been right-clicked.
However this is leading to weird behaviour, which I'm assuming is because I'm binding multiple ListBoxes to the same page-level property (SelectedListBox).
The relevant XAML for the cell template for the listview is as follows:
<DataTemplate x:Key="MultipleListBoxCellTemplate">
<ListBox SelectedItem="{Binding Path=DataContext.SelectedListBox, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Page}}}" />
</DataTemplate>
Is there a better way to get which ListBox has been right-clicked to my viewmodel, or can anyone think of another approach? Much appreciated :)
When you are building the context menu, you know which list box was selected, yes? I would probably wrap that up in the ICommand you are binding the context item to. This way, each command knows exactly which ListBox it was created by and can get the selected item from there.
Alternately, you may be able to get around the issue with using SelectedItem by changing your binding to OneWayToSource so that the data only flows from the View back to the ViewModel. You may still have timing issues, which I suspect is your current problem, but depending on exactly what is going on, that may resolve it.