如何让我的 WPF 用户控件的依赖属性更新我的视图模型？

Question

我正在尝试创建一个具有要绑定的依赖属性的用户控件。在内部，我有一个绑定到这些相同属性的ComboBox，但绑定只能以一种方式工作。 ComboBox 从 ItemsSource 填充，但 SelectedItem 不会更新回我绑定到的视图模型。

一个简化的例子：

这是与用户控件绑定的视图模型：

public class PeopleViewModel : INotifyPropertyChanged
{
    public PeopleViewModel()
    {
        People = new List<string>( new [] {"John", "Alfred","Dave"});
        SelectedPerson = People.FirstOrDefault();
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private IEnumerable<string> _people;
    public IEnumerable<string> People 
    { 
        get { return _people; } 
        set
        {
            _people = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("People"));
            }
        }
    }

    private string _selectedPerson;
    public string SelectedPerson 
    {
        get { return _selectedPerson; }
        set 
        { 
            _selectedPerson = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("SelectedPerson"));
            }
        } 
    }
}

这是用户控件：

<UserControl x:Class="PeopleControlTest.PeopleControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" d:DesignHeight="56" d:DesignWidth="637">
<StackPanel >
    <ComboBox Margin="11"
              ItemsSource="{Binding BoundPeople, RelativeSource={RelativeSource AncestorType=UserControl}}"
              SelectedItem="{Binding BoundSelectedPerson, RelativeSource={RelativeSource AncestorType=UserControl}}"/>
</StackPanel>

后面有代码

public partial class PeopleControl : UserControl
{
    public PeopleControl()
    {
        InitializeComponent();
    }

    public static readonly DependencyProperty BoundPeopleProperty =
        DependencyProperty.Register("BoundPeople", typeof(IEnumerable<string>), typeof(PeopleControl), new UIPropertyMetadata(null));

    public static readonly DependencyProperty BoundSelectedPersonProperty =
        DependencyProperty.Register("BoundSelectedPerson", typeof(string), typeof(PeopleControl), new UIPropertyMetadata(""));

    public IEnumerable<string> BoundPeople
    {
        get { return (IEnumerable<string>)GetValue(BoundPeopleProperty); }
        set { SetValue(BoundPeopleProperty, value); }
    }

    public string BoundSelectedPerson
    {
        get { return (string)GetValue(BoundSelectedPersonProperty); }
        set { SetValue(BoundSelectedPersonProperty, value); }
    }
}

这就是我在主窗口中绑定用户控件的方式（将 windows 数据上下文设置为视图模型的实例）

<Window x:Class="PeopleControlTest.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
          xmlns:controls="clr-namespace:PeopleControlTest"
    Title="MainWindow" Height="350" Width="525">
    <Grid>
        <controls:PeopleControl 
            BoundPeople="{Binding People}" 
            BoundSelectedPerson="{Binding SelectedPerson}"/>
    </Grid>
</Window>

用户控件中的组合框填充了名称，但是当我选择不同的名称时，它不会更新回视图模型。知道我在这里缺少什么吗？

谢谢！

Answer 1

某些属性默认绑定双向（包括SelectedItem），但您的BoundSelectedPerson 没有。您可以设置绑定的模式：

   <controls:PeopleControl 
            BoundPeople="{Binding People}" 
            BoundSelectedPerson="{Binding SelectedPerson, Mode=TwoWay}"/>

或者您可以通过在DependencyProperty 上设置一个标志来默认设置为双向：

    public static readonly DependencyProperty BoundSelectedPersonProperty =
        DependencyProperty.Register("BoundSelectedPerson", typeof(string), typeof(PeopleControl), new FrameworkPropertyMetadata("",FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));