附加属性

附加属性，在XAML中是一种特殊的依赖属性，它允许给这个对象添加一个不属于它的值（另一个对象的值），从而改变另一个对象的值。它们通常用于扩展已有控件的功能，使其能够响应特定布局容器或服务提供的属性。简单来说，附加属性就像是“借用”其他类的属性来增强自身的功能。

比如 Grid.Row Grid.Column就属于附加属性。

如何自定义附加属性

PasswordBox的 Password不是依赖属性，不支持直接绑定，需要自定义附加属性。

PasswordBox的属性绑定步骤

• 新建PasswordBoxHelper类 使用propa 快捷键，然后按下 tab键，VS会自动创建附加属性的定义模板
  创建MyPwd以及IsBinding两个附加属性。
• MyPwd附加属性中，默认值为string.Empty 当值发生改变时触发回调函数OnPwdPropertyChanged
• 在OnPwdPropertyChanged 函数中将MyPwd（改变的值）传给PasswordBox的Password前端控件，并且需要设置光标位置
• IsBinding附加属性中，new PropertyMetadata(false, OnPropertyChanged)，默认值为false 当为true时，触发OnPropertyChanged
• OnPropertyChanged当为新值，给PasswordBox的PasswordChanged事件绑定方法OnPwdChanged
• OnPwdChanged方法主要是调用SetPwd方法，设置Pwd附加属性值。将PasswordBox的Password前端控件 –> MyPwd

代码示例

PasswordBoxHelper类

using System.Reflection;
using System.Windows;
using System.Windows.Controls;

namespace MVVMDemo
{
    public class PasswordBoxHelper
    {
        //附加属性 MyPwd
        public static string GetMyPwd(DependencyObject obj)
        {
            return (string)obj.GetValue(MyPwdProperty);
        }

        public static void SetMyPwd(DependencyObject obj, string value)
        {
            obj.SetValue(MyPwdProperty, value);
        }

        // Using a DependencyProperty as the backing store for MyPwd.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty MyPwdProperty =
            DependencyProperty.RegisterAttached("MyPwd", typeof(string), typeof(PasswordBoxHelper), new PropertyMetadata(string.Empty,OnPwdPropertyChanged));


        //属性是否改变  Pwd -> Passwordbox  
        private static void OnPwdPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            PasswordBox passwordBox = d as PasswordBox;
            if (passwordBox == null)
            {
                return;
            }
            passwordBox.Password = (string)e.NewValue;
            SetSelection(passwordBox, passwordBox.Password.Length,0 );
        }

        /// <summary>
        /// 设置光标位置
        /// </summary>
        /// <param name="passwordBox"></param>
        /// <param name="start">光标开始位置</param>
        /// <param name="length">选中长度</param>
        private static void SetSelection(PasswordBox passwordBox, int start, int length)
        {
            passwordBox.GetType()
                       .GetMethod("Select", BindingFlags.Instance | BindingFlags.NonPublic)
                       .Invoke(passwordBox, new object[] { start, length
         });
        }


        //附加属性 IsBinding
        public static bool GetIsBinding(DependencyObject obj)
        {
            return (bool)obj.GetValue(IsBindingProperty);
        }

        public static void SetIsBinding(DependencyObject obj, bool value)
        {
            obj.SetValue(IsBindingProperty, value);
        }

        // Using a DependencyProperty as the backing store for IsBinding.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty IsBindingProperty =
            DependencyProperty.RegisterAttached("IsBinding", typeof(bool), typeof(PasswordBoxHelper), new PropertyMetadata(false,OnPropertyChanged));

        //passwordbox -> mypwd
        private static void OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            PasswordBox passwordBox = d as PasswordBox;

            if (passwordBox == null)
                return;

            if ((bool)e.NewValue)
            {
                //SetMyPwd(passwordBox, passwordBox.Password);
                passwordBox.PasswordChanged += OnPwdChanged;
            }
            if ((bool)e.OldValue)
            {
                passwordBox.PasswordChanged -= OnPwdChanged;
            }
        }

        private static void OnPwdChanged(object sender, RoutedEventArgs e)
        {
            PasswordBox pwd = sender as PasswordBox;

            SetMyPwd(pwd, pwd.Password);
        }
    }
}

前端控件绑定

<PasswordBox x:Name="pwdbox" Grid.Row="1" Grid.Column="1"  FontSize="25" VerticalAlignment="Center" Width="200" HorizontalAlignment="Left" 
             local:PasswordBoxHelper.IsBinding="True"
             local:PasswordBoxHelper.MyPwd="{Binding Password,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
             />

<StackPanel Grid.Row="2" HorizontalAlignment="Center" VerticalAlignment="Center" Orientation="Horizontal" Grid.ColumnSpan="2">
    <TextBlock Grid.Row="1" Grid.Column="0" Text="密码: " FontSize="25" VerticalAlignment="Center" HorizontalAlignment="Right"/>
    <TextBox  Text="{Binding ElementName=pwdbox,Path=(local:PasswordBoxHelper.MyPwd)}" FontSize="25" Width="200" HorizontalAlignment="Right"/>
</StackPanel>

//双向绑定附加属性

local:PasswordBoxHelper.IsBinding="True"
             local:PasswordBoxHelper.MyPwd="{Binding Password,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"

//绑定源元素为：pwdbox的值，Path指用来绑定的属性路径，(...)表示附加属性
//XAML 引擎看到 Path=(local:PasswordBoxHelper.MyPwd)，就去找名为 pwdbox 的元素，读取它身上的 PasswordBoxHelper.GetMyPwd(pwdbox)；– 把这个值赋给当前控件的 Text。  

Text="{Binding ElementName=pwdbox,Path=(local:PasswordBoxHelper.MyPwd)}" 

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如果你对MVVM已经有个掌握，可以查看进阶文章：如何使用ReactiveUI框架快速实现MVVM模式。