Why does ItemContainerGenerator return null?

I have a ListBox, and I nee开发者_StackOverflow中文版d to set its ControlTemplate to a Virtualizing WrapPanel which is a class that extends VirtualizingPanel, using a style that looks like this:

<Style TargetType="{x:Type ListBox}" x:Key="PhotoListBoxStyle">
                <Setter Property="Foreground" Value="White" />
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate  TargetType="{x:Type ListBox}" >
                            <s:VirtualizingVerticalWrapPanel>
                            </s:VirtualizingVerticalWrapPanel>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>

Now, in the private method of Virtualizing WrapPanel below I try to access this.ItemContainerGenerator, but I get null value, any idea what's the problem ??

private void RealizeFirstItem()
{
    IItemContainerGenerator generator = this.ItemContainerGenerator;
    GeneratorPosition pos = generator.GeneratorPositionFromIndex(0);

    using (generator.StartAt(pos, GeneratorDirection.Forward))
    {
        UIElement element = generator.GenerateNext() as UIElement;

         generator.PrepareItemContainer(element);

                    this.AddInternalChild(element);
    }
 }

I think I had a similar problem and this helped:

var necessaryChidrenTouch = this.Children;
IItemContainerGenerator generator = this.ItemContainerGenerator;

... for some reason you have to "touch" the children collection in order for the ItemContainerGenerator to initialize properly.

For Windows 8.1 Metro apps, the ItemContainerGenerator was depricated and will return null. New Apis:

ItemsControl.ItemContainerGenerator.ItemFromContainer = ItemsControl.ItemFromContainer

ItemsControl.ItemContainerGenerator.ContainerFromItem = ItemsControl.ContainerFromItem

ItemsControl.ItemContainerGenerator.IndexFromContainer = ItemsControl.IndexFromContainer

ItemsControl.ItemContainerGenerator.ContainerFromIndex = ItemsControl.ContainerFromIndex

http://msdn.microsoft.com/en-us/library/windows/apps/dn376326.aspx

Falck is mostly correct. Actually, you need to reference the 'InternalChildren' of the virtualized stack panel. The decompiled code for this property is:

    protected internal UIElementCollection InternalChildren
    {
        get
        {
            this.VerifyBoundState();
            if (this.IsItemsHost)
            {
                this.EnsureGenerator();
            }
            else if (this._uiElementCollection == null)
            {
                this.EnsureEmptyChildren(this);
            }
            return this._uiElementCollection;
        }
    }

The 'EnsureGenerator' does the work of making sure that a generator is available. Very poor 'just in time' design, IMO.

Most probably this is a virtualization-related issue so ListBoxItem containers get generated only for currently visible items (e.g. https://msdn.microsoft.com/en-us/library/system.windows.controls.virtualizingstackpanel(v=vs.110).aspx#Anchor_9)

I'd suggest switching to ListView instead of ListBox - it inherits from ListBoxand it supports ScrollIntoView() method which you can utilize to control virtualization;

targetListView.ScrollIntoView(itemVM);
DoEvents();
ListViewItem itemContainer = targetListView.ItemContainerGenerator.ContainerFromItem(itemVM) as ListViewItem;

(the example above also utilizes the DoEvents() static method explained in more detail here; WPF how to wait for binding update to occur before processing more code?)

There are a few other minor differences between the ListBox and ListView controls (What is The difference between ListBox and ListView) - which should not essentially affect your use case.

This is because you changed the Template of the Listbox, while u should have just changed the ItemsPanel:

    <ListBox>
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <s:VirtualizingVerticalWrapPanel />
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
    </ListBox>