如何绑定到 Silverlight 中的辅助属性

Question

为了便于讨论，这里有一个简单的 person 类，

public class Person : DependencyObject, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public static readonly DependencyProperty FirstNameProperty =
        DependencyProperty.Register( "FirstName",
                                     typeof ( string ),
                                     typeof ( Person ),
                                     null );
    public static readonly DependencyProperty LastNameProperty =
        DependencyProperty.Register( "LastName",
                                     typeof( string ),
                                     typeof( Person ),
                                     null );

    public string FirstName
    {
        get
        {
            return ( string ) GetValue( FirstNameProperty );
        }
        set
        {
            SetValue( FirstNameProperty, value );
            if(PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs( "FirstName" ));
        }
    }

    public string LastName
    {
        get
        {
            return ( string ) GetValue( LastNameProperty );
        }
        set
        {
            SetValue( LastNameProperty, value );
            if ( PropertyChanged != null )
                PropertyChanged( this, new PropertyChangedEventArgs( "LastName" ) );
        }
    }
}

我想创建一个像这样的只读属性，

public string FullName
    {
        get { return FirstName + " " + LastName; }
    }

在这种情况下绑定如何工作？我尝试添加 DependancyProperty 并引发全名的 PropertyChanged 事件。基本上，我只想拥有一个可以绑定的属性，只要名字或姓氏发生更改，该属性就会返回用户的全名。这是我使用的经过修改的最后一个类。

public class Person : DependencyObject, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public static readonly DependencyProperty FirstNameProperty =
        DependencyProperty.Register( "FirstName",
                                     typeof ( string ),
                                     typeof ( Person ),
                                     null );
    public static readonly DependencyProperty LastNameProperty =
        DependencyProperty.Register( "LastName",
                                     typeof( string ),
                                     typeof( Person ),
                                     null );
    public static readonly DependencyProperty FullNameProperty =
        DependencyProperty.Register( "FullName",
                                     typeof( string ),
                                     typeof( Person ),
                                     null );

    public string FirstName
    {
        get
        {
            return ( string ) GetValue( FirstNameProperty );
        }
        set
        {
            SetValue( FirstNameProperty, value );
            if ( PropertyChanged != null )
            {
                PropertyChanged( this, new PropertyChangedEventArgs( "FirstName" ) );
                PropertyChanged( this, new PropertyChangedEventArgs( "FullName" ) );
            }
        }
    }

    public string LastName
    {
        get
        {
            return ( string ) GetValue( LastNameProperty );
        }
        set
        {
            SetValue( LastNameProperty, value );
            if ( PropertyChanged != null )
            {
                PropertyChanged( this, new PropertyChangedEventArgs( "LastName" ) );
                PropertyChanged( this, new PropertyChangedEventArgs( "FullName" ) );
            }
        }
    }

    public string FullName
    {
        get { return GetValue( FirstNameProperty ) + " " + GetValue( LastNameProperty ); }
    }
}

Answer 1

我不确定您想在这里实现什么，但为什么您的 Person 类继承自 DependencyObject 以及为什么 FirstName 和 LastName DependencyProperties ？如果您只想将 Person 属性绑定到视图上的用户控件，那么让 Person 类实现 INotifyPropertyChanged 就足以使数据绑定起作用。您通常会将其绑定到作为依赖属性的用户控件的属性（例如，TextBlock 的 Text 属性）。

为您的 Person 类尝试一下：

using System.ComponentModel;
public class Person : INotifyPropertyChanged
{
    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    private void RaisePropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    #endregion

    private string _firstName;
    public string FirstName
    {
        get { return _firstName; }
        set
        {
            if (value != _firstName)
            {
                _firstName = value;
                RaisePropertyChanged("FirstName");
                RaisePropertyChanged("FullName");
            }
        }
    }

    private string _lastName;
    public string LastName 
    { 
        get { return _lastName; }
        set 
        {
            if (value != _lastName)
            {
                _lastName = value;
                RaisePropertyChanged("LastName");
                RaisePropertyChanged("FullName");
            }
        }
    }

    public string FullName 
    {
        get { return FirstName + " " + LastName; }
    }
}

并在您的视图中像这样使用它：

<Grid x:Name="LayoutRoot" Background="White" >
    <TextBlock Text="{Binding FullName}"/>
</Grid>

然后，在您的代码隐藏中，您可以像这样实例化它：

public partial class MainPage : UserControl
{
    public MainPage()
    {
        InitializeComponent();

        DataContext = new Person { FirstName = "John", LastName = "Doe" };
    }
}

HTH，
菲尔

Answer 2

首先，您现有的 FirstName 和 LastName 属性的实现是有缺陷的。 DependencyObject 已经有方法通知绑定值的更改，并且可以通过调用 Setter 方法之外的其他机制来更改值。

我的第一个问题是为什么 FirstName 和 LastName 依赖属性呢？对于此类课程来说，这似乎是一个奇怪的选择。菲尔的答案已经提供了我真正期望的正确答案。

但是，如果您的代码实际上是一种简化，并且实际上确实需要创建依赖项属性，那么应该如何完成：-

public class Person : DependencyObject, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public static readonly DependencyProperty FirstNameProperty =
        DependencyProperty.Register( "FirstName",
                                     typeof ( string ),
                                     typeof ( Person ),
                                     OnNamePropertyChanged);

    public static readonly DependencyProperty LastNameProperty =
        DependencyProperty.Register( "LastName",
                                     typeof( string ),
                                     typeof( Person ),
                                     OnNamePropertyChanged);

    private static void OnNamePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
      ((Person)d).OnNamePropertyChanged();
    }

    private void OnNamePropertyChanged()
    {
      if (PropertyChanged != null)
        PropertyChanged(this, new PropertyChangedEventArgs("FullName")));
    }

    public string FirstName
    {
      get { return GetValue(FirstNameProperty) as string; }
      set { SetValue(FirstNameProperty, value); }
    }

    public string LastName
    {
      get { return GetValue(LastNameProperty) as string; }
      set { SetValue(LastNameProperty, value); }
    }
    public string FullName
    {
      get { return FirstName + " " + LastName; }
    }
}