使用XAML绑定将自定义对象传递给UserControl

Question

我想通过XAML将自定义对象绑定到usercontrol：

我有一个MainPage和一个用户控件。模型对象：

public class Test
{
    public int Id { get; set; }
    public string Value { get; set; }
}

MainPage：

public sealed partial class MainPage : Page
{
    public MainPage()
    {
        this.InitializeComponent();
    }
}

public class MainPageViewModel : INotifyPropertyChanged
{
    public MainPageViewModel()
    {
        TestToto = new Test() { Id = 1, Value = "A value" };
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private Test _test;
    public Test TestToto
    {
        get { return _test; }
        set { _test = value; NotifyPropertyChanged("TestToto"); }
    }

    protected void NotifyPropertyChanged(String propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

和用户控件：

public sealed partial class MyUserControl1 : UserControl
{
    public MyUserControl1()
    {
        this.InitializeComponent();
        this.DataContext = this;
        Debug.WriteLine($"ID =====================> {PropertyTest}");
    }

    public Test PropertyTest
    {
        get { return (Test)GetValue(PropertyTestProperty); }
        set { SetValue(PropertyTestProperty, value); }
    }

    // Using a DependencyProperty as the backing store for PropertyTest.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty PropertyTestProperty =
        DependencyProperty.Register("PropertyTest", typeof(Test), typeof(MyUserControl1), new PropertyMetadata(default(Test)));

}

在我的MainPage.xaml中，我通过将dependencyproperty PropertyTest绑定到TestToto来创建MyUserControl1的实例（请参阅MainPage.xaml.cs）：

<Grid>
<TextBox Grid.Row="1"
             Text="{Binding TestToto.Value}" />

    <local:MyUserControl1 Grid.Row="2"
                          PropertyTest="{Binding DataContext.TestToto, ElementName=maPage}" />
</Grid>

这里maPage是MainPage页面的名称。然后在我的用户控件的XAML中我有：

<Grid>
    <TextBox x:Name="textBox"
             HorizontalAlignment="Left"
             Margin="30,47,0,0"
             TextWrapping="Wrap"
             Text="{Binding PropertyTest.Value}"
             VerticalAlignment="Top"
             Width="146" />
</Grid>

问题是PropertyTest DP始终为null：

第一个TextBox显示来自MainPage和第二个Textbox的Value属性应该显示相同的内容。

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提前致谢

此致