[UWP]理解及扩展Expander

[ContentProperty(Name = "Content")][TemplatePart(Name = "PART_RootGrid", Type = typeof(Grid))][TemplatePart(Name = "PART_ExpanderToggleButton", Type = typeof(ToggleButton))][TemplatePart(Name = "PART_LayoutTransformer", Type = typeof(LayoutTransformControl))][TemplateVisualState(Name = "Expanded", GroupName = "ExpandedStates")][TemplateVisualState(Name = "Collapsed", GroupName = "ExpandedStates")][TemplateVisualState(Name = "LeftDirection", GroupName = "ExpandDirectionStates")][TemplateVisualState(Name = "DownDirection", GroupName = "ExpandDirectionStates")][TemplateVisualState(Name = "RightDirection", GroupName = "ExpandDirectionStates")][TemplateVisualState(Name = "UpDirection", GroupName = "ExpandDirectionStates")]public class Expander : ContentControl{    public Expander();    public string Header { get; set; }    public DataTemplate HeaderTemplate { get; set; }    public bool IsExpanded { get; set; }    public ExpandDirection ExpandDirection { get; set; }    public event EventHandler Expanded;    public event EventHandler Collapsed;    public void OnExpandDirectionChanged();    protected override void OnApplyTemplate();    protected virtual void OnCollapsed(EventArgs args);    protected virtual void OnExpanded(EventArgs args);}