依赖属性

Posted 2020-11-22 writecode

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　　Windows Presentation Foundation(WPF)提供了一组服务，这些服务可用于扩展公共语言运行时(CLR)属性的功能，这些服务通常统称为WPF属性系统。由WPF属性系统支持的属性称为依赖项属性(Dependency Property)。

　　基本上依赖属性都支持绑定(Binding)，但也并不是说，非依赖属性就不能使用绑定。例如我们常用的密码框(PasswordBox)为了保证安全性，它的Password属性就不是依赖属性(即不能以Binding的方式实现数据通知)，但是我们可以通过附加属性、行为的方式实现绑定。

　　首先，我们创建一个名为PasswordBoxHelper的类，为Password注册附加属性。

 1     public class PasswordBoxHelper
 2     {
 3         public static string GetPassword(DependencyObject obj)
 4         {
 5             return (string)obj.GetValue(PasswordProperty);
 6         }
 7 
 8         public static void SetPassword(DependencyObject obj, string value)
 9         {
10             obj.SetValue(PasswordProperty, value);
11         }
12 
13         // Using a DependencyProperty as the backing store for Password.  This enables animation, styling, binding, etc...
14         public static readonly DependencyProperty PasswordProperty =
15             DependencyProperty.RegisterAttached("Password", typeof(string), typeof(PasswordBoxHelper), new FrameworkPropertyMetadata(string.Empty, OnPasswordPropertyChanged));
16 
17         private static void OnPasswordPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
18         {
19             PasswordBox passwordBox = d as PasswordBox;
20             string password = (string)e.NewValue;
21 
22             if (passwordBox != null && !string.Equals(passwordBox.Password, password))
23             {
24                 passwordBox.Password = password;
25             }
26         }
27     }

　　然后，在创建一个名为PasswordBoxBehavior的类，继承自Behavior，为PasswordBox附加行为。

 1     public class PasswordBoxBehavior : Behavior<PasswordBox>
 2     {
 3         protected override void OnAttached()
 4         {
 5             base.OnAttached();
 6             AssociatedObject.PasswordChanged += AssociatedObject_PasswordChanged;
 7         }
 8 
 9         protected override void OnDetaching()
10         {
11             base.OnDetaching();
12             AssociatedObject.PasswordChanged -= AssociatedObject_PasswordChanged;
13         }
14 
15         private void AssociatedObject_PasswordChanged(object sender, RoutedEventArgs e)
16         {
17             PasswordBox passwordBox = sender as PasswordBox;
18 
19             string password = PasswordBoxHelper.GetPassword(passwordBox);
20 
21             if (passwordBox != null && passwordBox.Password != password)
22             {
23                 PasswordBoxHelper.SetPassword(passwordBox, passwordBox.Password);
24             }
25         }
26     }

　　最后，在界面上为我们在PasswordBoxHelper类中注册的Password设置属性绑定，同时为PasswordBox设置Behavior。

1 <PasswordBox x:Name="txtPwd" Width="180" VerticalContentAlignment="Center" Background="Transparent" MaxLength="8" 
　　　　　　　　  helper:PasswordBoxHelper.Password="{Binding Path=Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
2     <i:Interaction.Behaviors>
3         <helper:PasswordBoxBehavior />
4     </i:Interaction.Behaviors>
5 </PasswordBox>