为了实现统一的外观和提升用户体验，我们对之前的contentview2进行了优化，将其header部分独立出来，创建了一个可复用的控件headerview。这个新控件继承自control，并引入了header和attachelement属性。通过将headerview嵌入到其他控件的controltemplate中，取代原有的headercontentpresenter，我们成功统一了控件的外观。在controltemplate中的使用方式如下：
<headerview attachedelement="{Binding RelativeSource={RelativeSource Mode=TemplatedParent}}" header="{TemplateBinding Header}"></headerview>通过使用Mode=TemplatedParent的绑定，我们可以获取模板所属的控件。在HeaderView的实现中，我们监视AttachElement属性的变化，并订阅AttachElement的PointerEntered和PointerExited事件，以便在鼠标进入和离开时改变VisualState：
protected virtual void OnAttachedElementChanged(FrameworkElement oldValue, FrameworkElement newValue){
if (oldValue != null) {
oldValue.PointerEntered -= OnPointerEntered;
oldValue.PointerExited -= OnPointerExited;
}
if (newValue != null) {
newValue.PointerEntered += OnPointerEntered;
newValue.PointerExited += OnPointerExited;
}
}
private void OnPointerEntered(object sender, PointerRoutedEventArgs e){
_isPointerEntered = true;
UpdateVisualState();
}
private void OnPointerExited(object sender, PointerRoutedEventArgs e){
_isPointerEntered = false;
UpdateVisualState();
}通过上述步骤，我们实现了基本功能。
为了避免为每个拥有Header属性的控件单独修改ControlTemplate，我们引入了附加属性ExtendHeader到HeaderView中。在附加属性的PropertyChangedCallback函数中，我们检查目标对象是否具有Header属性，如果有，则将Header值设置到一个新的HeaderView对象中，并将目标控件的Header设置为这个HeaderView对象。代码如下：
/// <summary>
/// 从指定元素获取 ExtendHeader 依赖项属性的值。
/// </summary>
/// The element from which the property value is read.
/// <returns>ExtendHeader 依赖项属性的值</returns>
public static object GetExtendHeader(DependencyObject obj){
return (object)obj.GetValue(ExtendHeaderProperty);
}
/// <summary>
/// 将 ExtendHeader 依赖项属性的值设置为指定元素。
/// </summary>
/// The element on which to set the property value.
/// The property value to set.
public static void SetExtendHeader(DependencyObject obj, object value){
obj.SetValue(ExtendHeaderProperty, value);
}
/// <summary>
/// 标识 ExtendHeader 依赖项属性。
/// </summary>
public static readonly DependencyProperty ExtendHeaderProperty =
DependencyProperty.RegisterAttached("ExtendHeader", typeof(object), typeof(HeaderView), new PropertyMetadata(0, OnExtendHeaderChanged));
private static void OnExtendHeaderChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args){
HeaderView target = obj as HeaderView;
object oldValue = (object)args.OldValue;
object newValue = (object)args.NewValue;
if (oldValue == newValue)
return;
FrameworkElement element = obj as FrameworkElement;
if (element == null)
return;
var headerProperty = element.GetType().GetProperty("Header");
if (headerProperty == null)
return;
HeaderView view = new HeaderView
{
Header = newValue,
AttachedElement = element
};
headerProperty.SetValue(element, view);
}在XAML中使用这个附加属性非常简单，如下所示：
<textbox local:headerview.extendheader="TextBox Header" placeholdertext="Input some text..."></textbox>
同样，这个附加属性也可以在ControlTemplate中使用。例如，对于没有Header属性的ListBox，可以在其ControlTemplate中的HeaderView上绑定到HeaderView.ExtendHeader：
<listbox local:headerview.extendheader="ListBox Header">
<listbox.template>
<controltemplate targettype="ListBox">
<border borderbrush="{TemplateBinding BorderBrush}" borderthickness="{TemplateBinding BorderThickness}" x:name="LayoutRoot">
<stackpanel>
<headerview attachedelement="{Binding RelativeSource={RelativeSource Mode=TemplatedParent}}" header="{TemplateBinding local:HeaderView.ExtendHeader}"></headerview>
<scrollviewer automationproperties.accessibilityview="Raw" background="{TemplateBinding Background}" bringintoviewonfocuschange="{TemplateBinding ScrollViewer.BringIntoViewOnFocusChange}" horizontalscrollbarvisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" horizontalscrollmode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" isdeferredscrollingenabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" ishorizontalrailenabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" isverticalrailenabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" padding="{TemplateBinding Padding}" tabnavigation="{TemplateBinding TabNavigation}" verticalscrollbarvisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" verticalscrollmode="{TemplateBinding ScrollViewer.VerticalScrollMode}" x:name="ScrollViewer" zoommode="{TemplateBinding ScrollViewer.ZoomMode}">
<itemspresenter></itemspresenter>
</scrollviewer>
</stackpanel>
</border>
</controltemplate>
</listbox.template>
<listboxitem content="ListBoxItem 1"></listboxitem>
<listboxitem content="ListBoxItem 2"></listboxitem>
<listboxitem content="ListBoxItem 3"></listboxitem>
</listbox>![[UWP 自定义控件]了解模板化控件(6)：使用附加属性](https://img.php.cn/upload/article/001/503/042/175919622491546.jpg)
完整的HeaderView类的实现代码如下：
[TemplateVisualState(Name = NormalState, GroupName = CommonStates)]
[TemplateVisualState(Name = PointerOverState, GroupName = CommonStates)]
[TemplateVisualState(Name = NoHeaderState, GroupName = HeaderStates)]
[TemplateVisualState(Name = HasHeaderState, GroupName = HeaderStates)]
public class HeaderView : Control{
public const string CommonStates = "CommonStates";
public const string NormalState = "Normal";
public const string PointerOverState = "PointerOver";
public const string HeaderStates = "HeaderStates";
public const string NoHeaderState = "NoHeader";
public const string HasHeaderState = "HasHeader";
/// <summary>
/// 从指定元素获取 ExtendHeader 依赖项属性的值。
/// </summary>
/// The element from which the property value is read.
/// <returns>ExtendHeader 依赖项属性的值</returns>
public static object GetExtendHeader(DependencyObject obj){
return (object)obj.GetValue(ExtendHeaderProperty);
}
/// <summary>
/// 将 ExtendHeader 依赖项属性的值设置为指定元素。
/// </summary>
/// The element on which to set the property value.
/// The property value to set.
public static void SetExtendHeader(DependencyObject obj, object value){
obj.SetValue(ExtendHeaderProperty, value);
}
/// <summary>
/// 标识 ExtendHeader 依赖项属性。
/// </summary>
public static readonly DependencyProperty ExtendHeaderProperty =
DependencyProperty.RegisterAttached("ExtendHeader", typeof(object), typeof(HeaderView), new PropertyMetadata(0, OnExtendHeaderChanged));
private static void OnExtendHeaderChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args){
HeaderView target = obj as HeaderView;
object oldValue = (object)args.OldValue;
object newValue = (object)args.NewValue;
if (oldValue == newValue)
return;
FrameworkElement element = obj as FrameworkElement;
if (element == null)
return;
var headerProperty = element.GetType().GetProperty("Header");
if (headerProperty == null)
return;
HeaderView view = new HeaderView
{
Header = newValue,
AttachedElement = element
};
headerProperty.SetValue(element, view);
}
/// <summary>
/// 从指定元素获取 Header 依赖项属性的值。
/// </summary>
/// The element from which the property value is read.
/// <returns>Header 依赖项属性的值</returns>
public static object GetHeader(DependencyObject obj){
return (object)obj.GetValue(HeaderProperty);
}
/// <summary>
/// 将 Header 依赖项属性的值设置为指定元素。
/// </summary>
/// The element on which to set the property value.
/// The property value to set.
public static void SetHeader(DependencyObject obj, object value){
obj.SetValue(HeaderProperty, value);
}
/// <summary>
/// 标识 Header 依赖属性。
/// </summary>
public static readonly DependencyProperty HeaderProperty =
DependencyProperty.Register("Header", typeof(object), typeof(HeaderView), new PropertyMetadata(null, OnHeaderChanged));
private static void OnHeaderChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args){
object oldValue = (object)args.OldValue;
object newValue = (object)args.NewValue;
if (oldValue == newValue)
return;
HeaderView target = obj as HeaderView;
if (target != null)
{
target.OnHeaderChanged(oldValue, newValue);
}
}
/// <summary>
/// 标识 AttachedElement 依赖属性。
/// </summary>
public static readonly DependencyProperty AttachedElementProperty =
DependencyProperty.Register("AttachedElement", typeof(FrameworkElement), typeof(HeaderView), new PropertyMetadata(null, OnAttachedElementChanged));
private static void OnAttachedElementChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args){
HeaderView target = obj as HeaderView;
FrameworkElement oldValue = (FrameworkElement)args.OldValue;
FrameworkElement newValue = (FrameworkElement)args.NewValue;
if (oldValue != newValue)
target.OnAttachedElementChanged(oldValue, newValue);
}
public HeaderView(){
this.DefaultStyleKey = typeof(HeaderView);
}
/// <summary>
/// 获取或设置Header的值
/// </summary>
public object Header{
get { return (object)GetValue(HeaderProperty); }
set { SetValue(HeaderProperty, value); }
}
/// <summary>
/// 获取或设置AttachedElement的值
/// </summary>
public FrameworkElement AttachedElement{
get { return (FrameworkElement)GetValue(AttachedElementProperty); }
set { SetValue(AttachedElementProperty, value); }
}
private bool _isPointerEntered;
protected virtual void OnAttachedElementChanged(FrameworkElement oldValue, FrameworkElement newValue){
if (oldValue != null)
{
oldValue.PointerEntered -= OnPointerEntered;
oldValue.PointerExited -= OnPointerExited;
}
if (newValue != null)
{
newValue.PointerEntered += OnPointerEntered;
newValue.PointerExited += OnPointerExited;
}
}
protected virtual void OnHeaderChanged(object oldValue, object newValue){
UpdateVisualState();
}
protected override void OnApplyTemplate(){
base.OnApplyTemplate();
UpdateVisualState(false);
}
private void OnPointerEntered(object sender, PointerRoutedEventArgs e){
_isPointerEntered = true;
UpdateVisualState();
}
private void OnPointerExited(object sender, PointerRoutedEventArgs e){
_isPointerEntered = false;
UpdateVisualState();
}
internal virtual void UpdateVisualState(bool useTransitions = true){
if (_isPointerEntered)
VisualStateManager.GoToState(this, PointerOverState, useTransitions);
else
VisualStateManager.GoToState(this, NormalState, useTransitions);
if (Header == null)
VisualStateManager.GoToState(this, NoHeaderState, useTransitions);
else
VisualStateManager.GoToState(this, HasHeaderState, useTransitions);
}
} 
