依赖附加属性的定义

基本过程：声明、注册、包装

依赖附加属性必须在依赖对象，附加属性不一定，关注的是被附加的对象是否是依赖对象

快捷方式：propa + tab

关键字：RegisterAttached

// 方法封装
public static int GetIndex(DependencyObject obj)
{
    return (int)obj.GetValue(IndexProperty);
}

public static void SetIndex(DependencyObject obj, int value)
{
    obj.SetValue(IndexProperty, value);
}

// 声明与注册
public static readonly DependencyProperty IndexProperty =
    DependencyProperty.RegisterAttached("Index", typeof(int), typeof(XHPanel), new PropertyMetadata(-1));

依赖附加属性的意义和作用

1. 用来辅助布局（布局控件与子控件的关系）

一般容器控件提供给子控件使用的，如果Grid.Row

使用方式：在容器控件中检查每个子项，从子项中获取附加属性的值 容器对象调用对应的GetValue方法，传入给被附加对象

a. 自定义控件的附加属性

父亲的属性：

// 方法封装
public static int GetIndex(DependencyObject obj)
{
    return (int)obj.GetValue(IndexProperty);
}

public static void SetIndex(DependencyObject obj, int value)
{
    obj.SetValue(IndexProperty, value);
}

// 声明与注册
public static readonly DependencyProperty IndexProperty =
    DependencyProperty.RegisterAttached("Index", typeof(int), typeof(XHPanel), new PropertyMetadata(-1));

使用方法：

可以在子控件中都使用Index属性

<local:XHPanel Test="12" ColumnSpace="20" RowSpace="20" Visibility="Collapsed">
  <Border Height="100" Background="Red" local:XHPanel.Index="1" />
  <Border Height="100" Background="Orange" local:XHPanel.Index="2" />
  <Border Height="100" Background="Yellow" local:XHPanel.Index="3"/>
  <Border Height="100" Background="Green" local:XHPanel.Index="4"/>
  <Border Height="100" Background="LightGreen" local:XHPanel.Index="5"/>
  <Border Height="100" Background="Blue" local:XHPanel.Index="6"/>
  <local:DependencyPropertyStudy local:XHPanel.Index="7" x:Name="dps" Background="Purple" Height="100"/>
</local:XHPanel>

在执行布局的时候，可以获取每个子控件的Index，进行不一样的布局：

获取部分的核心代码：

protected override Size ArrangeOverride(Size finalSize)
{
    for (int i = 1; i < newElements.Count+1; i++)
    {
        var index = GetIndex(item); // 获取对应对象的Index 附加属性值
    }
    // 根据index的不一样，进行不一样的布局
    .....
    return base.ArrangeOverride(finalSize);
}

2. 用来扩展绑定属性（单独应对对象功能）

一般对接特定对象，比如PasswordBox

使用方法：定义一个类，在类定义相关附加属性，利用这些附加属性进行数据与控件属性的转接

需求：PasswordBox中的password属性不可以绑定，不是依赖属性，但是我这边又需要动态赋值，所以，此时需要写个扩展属性:

核心代码如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;

namespace XH.PropertyLesson
{
    public class PWHelper
    {
        public static string GetPassword(DependencyObject obj)
        {
            return (string)obj.GetValue(PasswordProperty);
        }

        /// <summary>
        /// 设置Password
        /// </summary>
        /// <param name="obj">所附加的对象</param>
        /// <param name="value"></param>
        public static void SetPassword(DependencyObject obj, string value)
        {
            obj.SetValue(PasswordProperty, value);
        }

        // Using a DependencyProperty as the backing store for Password.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty PasswordProperty =
            DependencyProperty.RegisterAttached(
                "Password", 
                typeof(string), 
                typeof(PWHelper), 
                new PropertyMetadata(
                    "",
                    new PropertyChangedCallback(OnPasswordChanged)));

        // 1、可以获取到对应的控件 PasswordBox
        // 2、关联两个方向
        //    - 从绑定到控件
        //    - 从控件到绑定（针对拿到的控件进行PasswordChanged事件的绑定）

        /// <summary>
        /// Password修改回调函数
        /// </summary>
        /// <param name="DependencyObject">实际附加的对象</param>
        /// <param name="e"></param>
        /// <exception cref="NotImplementedException"></exception>
        private static void OnPasswordChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var control = (d as PasswordBox);
            if (control == null) return;

            control.Password = e.NewValue as string;
        }

        private static void Control_PasswordChanged(object sender, RoutedEventArgs e)
        {
            SetPassword((DependencyObject)sender, (sender as PasswordBox).Password);
        }

        public static object GetAttached(DependencyObject obj)
        {
            return (object)obj.GetValue(AttachedProperty);
        }

        public static void SetAttached(DependencyObject obj, object value)
        {
            obj.SetValue(AttachedProperty, value);
        }

        // 挂载事件
        public static readonly DependencyProperty AttachedProperty =
            DependencyProperty.RegisterAttached("Attached", typeof(object), typeof(PWHelper), 
                new PropertyMetadata(null,new PropertyChangedCallback(OnAttachedChanged)));

        private static void OnAttachedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var control = (d as PasswordBox);
            if (control == null) return;

            // 防止多次附加事件 可以先减 后加
            control.PasswordChanged -= Control_PasswordChanged; 
            control.PasswordChanged += Control_PasswordChanged; 
        }
    }
}

xaml使用代码：

<PasswordBox PasswordChar="*" Height="30" Width="100" 
  local:PWHelper.Password="{Binding }"
  local:PWHelper.Attached="1"/>

使用：