Is there a way to jump to the ListHeader of a LongListSelector?

Question

I'm working on a Windows Phone 8 application and I wanted to implement a button that would allow me to jump to the top of the ListHeader of a LongListSelector (The ListHeader contains some radio buttons that I'm using to filter content). Is there any way to do this?

Answer 1

In your button handler, use this method to get the ViewPortControl from the LongListSelector's template.

    public static T FindChild<T>(DependencyObject parent, string childName) where T : DependencyObject
    {
        // Confirm parent and childName are valid. 
        if (parent == null)
        {
            return null;
        }

        T foundChild = null;

        int childrenCount = VisualTreeHelper.GetChildrenCount(parent);
        for (int i = 0; i < childrenCount; i++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(parent, i);
            // If the child is not of the request child type child
            var childType = child as T;
            if (childType == null)
            {
                // recursively drill down the tree
                foundChild = FindChild<T>(child, childName);

                // If the child is found, break so we do not overwrite the found child. 
                if (foundChild != null)
                {
                    break;
                }
            }
            else if (!string.IsNullOrEmpty(childName))
            {
                var frameworkElement = child as FrameworkElement;
                // If the child's name is set for search
                if (frameworkElement != null && frameworkElement.Name == childName)
                {
                    // if the child's name is of the request name
                    foundChild = (T)child;
                    break;
                }

                // Need this in case the element we want is nested
                // in another element of the same type
                foundChild = FindChild<T>(child, childName);
            }
            else
            {
                // child element found.
                foundChild = (T)child;
                break;
            }
        }

        return foundChild;
    }

Then use THIS to scroll to the top of the ViewPortControl.

Usage:

private void Button_OnClick(object sender, RoutedEventArgs e)
{
    var viewPort = FindChild<ViewportControl>(yourLongListSelector, "ViewportControl");
    viewPort.SetViewportOrigin(new Point(0,0));
}

In case you're wondering where the ViewPortControl came from, here's the LongListSelector's style.

<Style x:Key="LongListSelectorStyle1" TargetType="phone:LongListSelector">
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="Foreground" Value="{StaticResource PhoneForegroundBrush}"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="phone:LongListSelector">
                    <Grid Background="{TemplateBinding Background}" d:DesignWidth="480" d:DesignHeight="800">
                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="ScrollStates">
                                <VisualStateGroup.Transitions>
                                    <VisualTransition GeneratedDuration="00:00:00.5"/>
                                </VisualStateGroup.Transitions>
                                <VisualState x:Name="Scrolling">
                                    <Storyboard>
                                        <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="VerticalScrollBar"/>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="NotScrolling"/>
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                        <Grid Margin="{TemplateBinding Padding}">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="*"/>
                                <ColumnDefinition Width="auto"/>
                            </Grid.ColumnDefinitions>
                            <ViewportControl x:Name="ViewportControl" HorizontalContentAlignment="Stretch" VerticalAlignment="Top"/>
                            <ScrollBar x:Name="VerticalScrollBar" Grid.Column="1" Margin="4,0,4,0" Opacity="0" Orientation="Vertical"/>
                        </Grid>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>