MahApps.Metro - TreeViewItemHelper

Applies to TreeViewItem. One property, for the little triangle that opens and closes a node.

Property	Type	Default
`ToggleButtonStyle`	`Style`	`null`, set by the item style	style of the expand and collapse button

<TreeView>
    <TreeView.ItemContainerStyle>
        <Style BasedOn="{StaticResource MahApps.Styles.TreeViewItem}" TargetType="TreeViewItem">
            <Setter Property="mah:TreeViewItemHelper.ToggleButtonStyle" Value="{StaticResource BigExpander}" />
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

Set it through ItemContainerStyle rather than on the TreeView. The helper's own default is null; the MahApps TreeViewItem style is what puts MahApps.Styles.ToggleButton.TreeViewItem.ExpandCollapse there, and a style setter on the item beats a value inherited from the tree.

The style targets ToggleButton. Base your own on the built-in one unless you intend to draw the whole button yourself:

<Style x:Key="BigExpander"
       BasedOn="{StaticResource MahApps.Styles.ToggleButton.TreeViewItem.ExpandCollapse}"
       TargetType="{x:Type ToggleButton}">
    <Setter Property="Width" Value="24" />
    <Setter Property="Height" Value="24" />
</Style>

The colours of the row itself — selected, hovered, disabled — come from ItemHelper, which applies to TreeViewItem as well as to ListBoxItem.

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