c# - UserControl 的 RelativeSource 绑定(bind)

Question

我创建了一个 UserControl 用于在我的应用程序中显示超链接。

UserControl 的标记如下:

<UserControl x:Class="MVVMExample.View.UserControls.ActionLink"
             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="300" d:DesignWidth="300"
             DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <Grid>
        <TextBlock Margin="5">
                <Hyperlink Command="{Binding LinkCommand}" CommandParameter="{Binding LinkCommandParameter}">
                    <TextBlock Text="{Binding LinkText, UpdateSourceTrigger=PropertyChanged}"/>
                </Hyperlink>
        </TextBlock>
    </Grid>
</UserControl>

UserControl 的 DataContext 位于 CodeBehind 中，如下所示:

public partial class ActionLink : UserControl, INotifyPropertyChanged
{
    public static readonly DependencyProperty LinkTextProperty = DependencyProperty.Register(
        "LinkText", typeof (string), typeof (ActionLink), new PropertyMetadata(LinkTextChanged));

    public static readonly DependencyProperty LinkCommandParameterProperty = DependencyProperty.Register(
        "LinkCommandParameter", typeof (object), typeof (ActionLink),
        new PropertyMetadata(LinkCommandParameterChanged));

    public static readonly DependencyProperty LinkCommandProperty = DependencyProperty.Register(
        "LinkCommand", typeof (ICommand), typeof (ActionLink), new PropertyMetadata(LinkCommandChanged));

    public ActionLink()
    {
        InitializeComponent();
    }

    public object LinkCommandParameter
    {
        get { return GetValue(LinkCommandParameterProperty); }
        set { SetValue(LinkCommandParameterProperty, value); }
    }

    public string LinkText
    {
        get { return (string) GetValue(LinkTextProperty); }
        set
        {
            SetValue(LinkTextProperty, value);
            OnPropertyChanged();
        }
    }

    public ICommand LinkCommand
    {
        get { return (ICommand) GetValue(LinkCommandProperty); }
        set { SetValue(LinkCommandProperty, value); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private static void LinkTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ((ActionLink) d).LinkText = (string) e.NewValue;
    }

    private static void LinkCommandParameterChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ((ActionLink) d).LinkCommandParameter = e.NewValue;
    }

    private static void LinkCommandChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ((ActionLink) d).LinkCommand = (ICommand) e.NewValue;
    }

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

一切正常。

现在，如果我想将此 UserControl 与命令绑定(bind)一起使用，我必须执行以下操作:

<userControls:ActionLink LinkText="View customers" LinkCommand="{Binding DataContext.ViewCustomersCommand, RelativeSource={RelativeSource FindAncestor, AncestorType=Window}}"/>

如果我改用 Button，则不必提供 RelativeSource。有没有机会我也不必为自定义创建的 UserControl 的绑定(bind)属性提供 RelativeSource？

Answer 1

当你写作时

<userControls:ActionLink LinkCommand="{Binding ViewCustomersCommand}"/>

WPF 尝试建立到 UserControl 的 DataContext 中的 ViewCustomersCommand 属性的数据绑定(bind)，该属性通常继承自控件的父级，并持有对某个 View 模型对象的引用。这在这里不起作用，因为您已将 DataContext 显式设置为 UserControl 实例。

只要您的 UserControl 中具有可绑定(bind)属性(即依赖属性)，您就不应该设置它的 DataContext。如果这样做，您将始终必须明确指定绑定(bind)源对象，因为不再继承 DataContext。

所以删除

DataContext="{Binding RelativeSource={RelativeSource Self}}"

从您的 UserControl 的 XAML 设置并在其所有内部绑定(bind)中设置一个 RelativeSource:

<Hyperlink
    Command="{Binding LinkCommand,
              RelativeSource={RelativeSource AncestorType=UserControl}}"
    CommandParameter="{Binding LinkCommandParameter,
                       RelativeSource={RelativeSource AncestorType=UserControl}}">
    <TextBlock
        Text="{Binding LinkText,
               RelativeSource={RelativeSource AncestorType=UserControl}}"/>
</Hyperlink>