將WPF窗口DataContext設置為RelativeSource Self

Question

如果我在構造函數中將Window的DataContext設置this ， 以及在XAML 中將 {Binding RelativeSource={RelativeSource Self}}為I，那么我可以通過在其中放置一個斷點來看到DataContext引用了正確的對象實例（即MainWindow）。代碼隱藏的Loaded事件。 但是，Window的子元素exampleButton的Command綁定為null。 斷言失敗。

當我刪除XAML DataContext設置（僅依賴於構造函數設置）時，exampleButton的Command會正確使用DataContext。

為什么在XAML場景中exampleButton的Command綁定為null？

MainWindow.xaml

<Window x:Class="MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Example"
        SizeToContent="WidthAndHeight"
        x:Name="mainWindow"
        DataContext="{Binding RelativeSource={RelativeSource Self}}"
        Loaded="mainWindow_Loaded">
    <Button x:Name="exampleButton" Command="{Binding Path=ExampleCommand}" Content="Click"/>
</Window>

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    public ICommand ExampleCommand { get; }

    public MainWindow()
    {
        InitializeComponent();
        DataContext = this;
        ExampleCommand = new DelegateCommand(x => { throw new ApplicationException(); });
    }

    private void mainWindow_Loaded(object sender, RoutedEventArgs e)
    {
        Debug.Assert(mainWindow == this);
        Debug.Assert(mainWindow.DataContext == this);
        Debug.Assert(exampleButton.DataContext == this);
        Debug.Assert(exampleButton.Command == ExampleCommand); //<-- FAIL
    }
}

Answer 1

在InitializeComponent之前設置ExampleCommand和DataContext ：

 DataContext = this;
 ExampleCommand = new DelegateCommand(x => { throw new ApplicationException(); });
 InitializeComponent();

還要注意，使用DataContext="{Binding RelativeSource={RelativeSource Self}}"或DataContext = this;沒有區別DataContext = this; ，如果您在initializecomponent之前設置了datacontext。

Answer 2

為什么在XAML場景中exampleButton的Command綁定為null？

因為ExampleCommand屬性在InitializeComponent()方法返回並設置DataContext屬性時實際上具有null值。

如果要在此之后將屬性設置為新值，則該類必須實現INotifyPropertyChanged接口，以使目標屬性自動刷新：

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public MainWindow()
    {
        InitializeComponent();
        ExampleCommand = new RelayCommand<object>(x => { throw new ApplicationException(); });
    }

    private ICommand _exampleCommand;
    public ICommand ExampleCommand
    {
        get { return _exampleCommand; }
        set { _exampleCommand = value; NotifyPropertyChanged(); }
    }

    private void mainWindow_Loaded(object sender, RoutedEventArgs e)
    {
        Debug.Assert(exampleButton.DataContext == this);
        Debug.Assert(exampleButton.Command == ExampleCommand); //<-- FAIL
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}