依赖项属性

依赖项属性时WPF对.NET属性系统的扩展，用途在于提供一种方法来基于其他输入的值计算属性值，也就是说依赖项属性的值可能是通过以下途径获取：1.通过主题和用户首选项等系统属性；2.通过数据绑定或动画等来实时确定；3.通过资源和样式；4通过在元素树中的父子关系继承而来等。而且依赖项属性可通过独立验证、默认值、回调方法来修改属性值。

下面来看看定义一个依赖项属性的过程：

1. 定义依赖项属性

依赖项属性对象都是DependencyProperty类的实例，且考虑到依赖项属性可以被父子元素之间继承，即在不同类之间共享，所以依赖项属性都为静态字段。如下：

public static readonly DependencyProperty MarginProperty；

2 注册依赖项属性

通过以下代码注册依赖项属性:

FrameworkPropertyMetadata metadata = new FrameworkPropertyMetadata(

new Thickness(), FrameworkPropertyMetadataOption.AffectsMeasure);

MarginProperty = DependencyProperty.Register("Margin", 

typedef(Thickness), typeof(FrameworkElement), metadata,

new ValidateValueCallback(FrameworkElement.IsMarginValid));

注册依赖项属性要经过上面两步：

1.定义依赖项属性

创建FrameworkPropertyMetadata对象作为依赖项属性的默认值。上例中使用的构造函数中第一个参数为默认值，第二个参数为确定概述行的行为（如值更改发生的时间是否属性值继承等）。还有另一些重载的构造函数，有的含有指定属性值改变时的回调函数；

2. 注册依赖项属性

然后通过DependencyProperty.Register方法注册事件，第一个参数确定依赖项属性的名称，第二个参数确定依赖项属性值的类型，第三个参数确定依赖项属性的所有者，第四个参数确定默认值，最后一个参数确定验证属性的回调函数

3. 添加属性包装器

依赖项属性的存取器要使用DependencyObject中的GetValue和SetValue方法来存取，示例如下：

public Thickness Margin

{

set { SetValue(MarginProperty, value); }

get { return (Thickness)GetValue(MarginProperty); }

}

如上过程便可创建一个依赖项属性。

依赖项属性的一个重要功能是动态值识别，WPF确定一个属性的值会按下列顺序确定基本值：

1. 默认值

2. 继承而来的值

3. 来自主题样式的值

4. 来自项目样式的值

5. 本地值

而基本值未必是最后从属性中检索到的值，因为还存在其他改变属性值的提供者。所以WPF决定属性值得过程为：

1. 确定基本值

2. 如果属性值是个表达式，则对表达式进行求值

3. 如果属性是动画的目标，则应用动画

4. 调用回调函数修正属性值

下面使用一个完整的示例展示依赖项属性的使用。在WPF中没有通知栏控件，所以这儿使用WinForm中的NotifyIcon控件封装出一个WPF通知控件，具体代码如下：

public class NotificationAreaIcon : FrameworkElement
{
//对Forms.NotifyIcon控件封装
public static Forms.NotifyIcon notifyIcon;

//添加鼠标点击路由事件
public static readonly RoutedEvent MouseClickEvent = EventManager.RegisterRoutedEvent(
"MouseClick", RoutingStrategy.Bubble, typeof(MouseButtonEventHandler), typeof(NotificationAreaIcon));

//添加鼠标双击路由事件
public static readonly RoutedEvent MouseDoubleClickEvent = EventManager.RegisterRoutedEvent(
"MouseDoubleClick", RoutingStrategy.Bubble, typeof(MouseButtonEventHandler), typeof(NotificationAreaIcon));

//注册依赖项属性Icon
public static readonly DependencyProperty IconProperty =
DependencyProperty.Register("Icon", typeof(ImageSource), typeof(NotificationAreaIcon));

//注册依赖项属性Text
public static readonly DependencyProperty TextProperty =
DependencyProperty.Register("Text", typeof(string), typeof(NotificationAreaIcon));

//注册依赖项属性MenuItems
public static readonly DependencyProperty FormsContextMenuProperty =
DependencyProperty.Register("MenuItems", typeof(List<Forms.MenuItem>), typeof(NotificationAreaIcon), new PropertyMetadata(new List<Forms.MenuItem>()));

//初始化事件
protected override void OnInitialized(EventArgs e)
{
base.OnInitialized(e);

// Create and initialize the window forms notify icon based
notifyIcon = new Forms.NotifyIcon();
notifyIcon.Text = Text;
if (!DesignerProperties.GetIsInDesignMode(this))
{
notifyIcon.Icon = FromImageSource(Icon);
}
notifyIcon.Visible = FromVisibility(Visibility);

if (this.MenuItems != null && this.MenuItems.Count > 0)
{
notifyIcon.ContextMenu = 
new System.Windows.Forms.ContextMenu(this.MenuItems.ToArray());
}

notifyIcon.MouseDown += OnMouseDown;
notifyIcon.MouseUp += OnMouseUp;
notifyIcon.MouseClick += OnMouseClick;
notifyIcon.MouseDoubleClick += OnMouseDoubleClick;

Dispatcher.ShutdownStarted += OnDispatcherShutdownStarted;
}

private void OnDispatcherShutdownStarted(object sender, EventArgs e)
{
notifyIcon.Dispose();
}

public void DestroyIcon()
{
notifyIcon.Dispose();
}

private void OnMouseDown(object sender, Forms.MouseEventArgs e)
{
OnRaiseEvent(MouseDownEvent, new MouseButtonEventArgs(
InputManager.Current.PrimaryMouseDevice, 0, ToMouseButton(e.Button)));
}

private void OnMouseUp(object sender, Forms.MouseEventArgs e)
{
OnRaiseEvent(MouseUpEvent, new MouseButtonEventArgs(
InputManager.Current.PrimaryMouseDevice, 0, ToMouseButton(e.Button)));
}

private void OnMouseDoubleClick(object sender, Forms.MouseEventArgs e)
{
OnRaiseEvent(MouseDoubleClickEvent, new MouseButtonEventArgs(
InputManager.Current.PrimaryMouseDevice, 0, ToMouseButton(e.Button)));
}

private void OnMouseClick(object sender, Forms.MouseEventArgs e)
{
OnRaiseEvent(MouseClickEvent, new MouseButtonEventArgs(
InputManager.Current.PrimaryMouseDevice, 0, ToMouseButton(e.Button)));
}

private void OnRaiseEvent(RoutedEvent handler, MouseButtonEventArgs e)
{
e.RoutedEvent = handler;
RaiseEvent(e);
}

public List<Forms.MenuItem> MenuItems
{
get { return (List<Forms.MenuItem>)GetValue(FormsContextMenuProperty); }
set { SetValue(FormsContextMenuProperty, value); }
}

public ImageSource Icon
{
get { return (ImageSource)GetValue(IconProperty); }
set { SetValue(IconProperty, value); }
}

public string Text
{
get { return (string)GetValue(TextProperty); }
set { SetValue(TextProperty, value); }
}

public event MouseButtonEventHandler MouseClick
{
add { AddHandler(MouseClickEvent, value); }
remove { RemoveHandler(MouseClickEvent, value); }
}

public event MouseButtonEventHandler MouseDoubleClick
{
add { AddHandler(MouseDoubleClickEvent, value); }
remove { RemoveHandler(MouseDoubleClickEvent, value); }
}

#region Conversion members

private static Drawing.Icon FromImageSource(ImageSource icon)
{
if (icon == null)
{
return null;
}
Uri iconUri = new Uri(icon.ToString());
return new Drawing.Icon(Application.GetResourceStream(iconUri).Stream);
}

private static bool FromVisibility(Visibility visibility)
{
return visibility == Visibility.Visible;
}

private MouseButton ToMouseButton(Forms.MouseButtons button)
{
switch (button)
{
case Forms.MouseButtons.Left:
return MouseButton.Left;
case Forms.MouseButtons.Right:
return MouseButton.Right;
case Forms.MouseButtons.Middle:
return MouseButton.Middle;
case Forms.MouseButtons.XButton1:
return MouseButton.XButton1;
case Forms.MouseButtons.XButton2:
return MouseButton.XButton2;
}
throw new InvalidOperationException();
}

#endregion Conversion members
}

来自为知笔记(Wiz)