3

我的自定义面板控件的逻辑子级没有引发DataContextChanged的​​问题。我把它缩小到这个:

从向导生成的 WPF 应用程序开始,我添加:

    private void Window_Loaded( object sender, RoutedEventArgs e )
    {
        var elt = new FrameworkElement();
        this.AddLogicalChild( elt );
        DataContext = 42;
        Debug.Assert( (int)elt.DataContext == 42 );
    }

据我了解,这是有效的,因为DataContext是一个可继承的依赖属性

现在,我在 Window (this) 及其逻辑子项上为DataContextChanged添加事件处理程序:

    this.DataContextChanged += 
        delegate { Debug.WriteLine( "this:DataContextChanged" ); };
    elt.DataContextChanged += 
        delegate { Debug.WriteLine( "elt:DataContextChanged" ); };

如果我运行它,只会执行第一个事件处理程序为什么是这样?如果不是 AddLogicalChild( elt ) 我执行以下操作:

this.Content = elt;

两个处理程序都将执行。但在我的情况下,这不是一个选项 - 我将 FrameworkContentElements 添加到我的控件中,这些控件不应该是可视子项。

这里发生了什么?除了 AddLogicalChild() 之外,我还应该做更多的事情来使它工作吗?

(幸运的是,有一个相当简单的解决方法——只需将元素的 DataContext 绑定到窗口的 DataContext 即可)

BindingOperations.SetBinding( elt, FrameworkElement.DataContextProperty, 
            new Binding( "DataContext" ) { Source = this } );

谢谢你。

4

2 回答 2

7

您还需要覆盖该LogicalChildren属性:

protected override System.Collections.IEnumerator LogicalChildren
{
    get { yield return elt; }
}

当然,您也希望返回由基本实现定义的任何逻辑子代。

于 2009-04-07T08:23:38.583 回答
1

如果有人遇到类似问题,我想为 Kent 的回答添加一些建议:

如果您创建具有多个内容对象的自定义控件,您应该确保:

  • 内容对象通过 AddLogicalChild() 添加到 LogicalTree
  • 创建您自己的 Enumerator 类并在覆盖的 LogicalChildren 属性中返回该类的实例

如果您不将内容对象添加到逻辑树,您可能会遇到诸如无法解析与 ElementNames 的绑定之类的问题(ElementName 由 FindName 解析,而 FindName 又使用 LogicalTree 来查找元素)。

更危险的是,我的经验是,如果您错过将对象添加到逻辑树中,则 ElementName 解析在某些情况下有效,而在其他情况下则无效。

如果您不覆盖 LogicalChildren,则不会像上面描述的那样更新 DataContext。

这里有一个简单的 SplitContainer 的简短示例:

拆分容器:

    public class SplitContainer : Control
{
    static SplitContainer()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(SplitContainer), new FrameworkPropertyMetadata(typeof(SplitContainer)));
    }

    /// <summary>
    /// Identifies the <see cref="Child1"/> property.
    /// </summary>
    public static readonly DependencyProperty Child1Property =
        DependencyProperty.Register(nameof(Child1), typeof(object), typeof(SplitContainer), new FrameworkPropertyMetadata(Child1PropertyChangedCallback));

    /// <summary>
    /// Left Container
    /// </summary>
    public object Child1
    {
        get { return (object)GetValue(Child1Property); }
        set { SetValue(Child1Property, value); }
    }

    /// <summary>
    /// Identifies the <see cref="Child2"/> property.
    /// </summary>
    public static readonly DependencyProperty Child2Property =
        DependencyProperty.Register(nameof(Child2), typeof(object), typeof(SplitContainer), new FrameworkPropertyMetadata(Child2PropertyChangedCallback));

    /// <summary>
    /// Right Container
    /// </summary>
    public object Child2
    {
        get { return (object)GetValue(Child2Property); }
        set { SetValue(Child2Property, value); }
    }

    private static void Child1PropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var splitContainer = (SplitContainer)d;

        if (e.OldValue != null)
        {
            splitContainer.RemoveLogicalChild(e.OldValue);
        }
        if (e.NewValue != null)
        {
            splitContainer.AddLogicalChild(((object)e.NewValue));
        }
    }

    private static void Child2PropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var splitContainer = (SplitContainer)d;

        if (e.OldValue != null)
        {
            splitContainer.RemoveLogicalChild(e.OldValue);
        }
        if (e.NewValue != null)
        {
            splitContainer.AddLogicalChild(((object)e.NewValue));
        }
    }

    protected override IEnumerator LogicalChildren
    {
        get
        {
            return new SplitContainerLogicalChildrenEnumerator(this);
        }
    }
}

SplitContainerLogicalChildrenEnumerator:

    internal class SplitContainerLogicalChildrenEnumerator : IEnumerator
{
    private readonly SplitContainer splitContainer;
    private int index = -1;

    public SplitContainerLogicalChildrenEnumerator(SplitContainer splitContainer)
    {
        this.splitContainer = splitContainer;
    }

    public object Current
    {
        get
        {
            if (index == 0)
            {
                return splitContainer.Child1;
            }
            else if (index == 1)
            {
                return splitContainer.Child2;
            }
            throw new InvalidOperationException("No child for this index available");
        }
    }

    public bool MoveNext()
    {
        index++;
        return index < 2;
    }

    public void Reset()
    {
        index = -1;
    }
}

样式(例如在 Themes/generic.xaml 中):

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                xmlns:scm="clr-namespace:System.ComponentModel;assembly=PresentationFramework"
                xmlns:sys="clr-namespace:System;assembly=mscorlib"
                xmlns:local="clr-namespace:SplitContainerElementNameProblem">
<Style TargetType="{x:Type local:SplitContainer}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:SplitContainer}">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="*" />
                    </Grid.ColumnDefinitions>
                    <ContentPresenter Grid.Column="0"
                                      Content="{TemplateBinding Child1}" />
                    <ContentPresenter Grid.Column="1"
                                      Content="{TemplateBinding Child2}" />
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

演示每个绑定都可以正常工作的示例:

XAML:

<Window x:Class="SplitContainerElementNameProblem.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:SplitContainerElementNameProblem"
    mc:Ignorable="d"
    Title="MainWindow" Height="450" Width="800">
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>
    <Grid Grid.Row="0">
        <TextBox x:Name="text1" Text="abc" />
    </Grid>
    <local:SplitContainer Grid.Row="1">
        <local:SplitContainer.Child1>
            <TextBox x:Name="text2"
                     Text="{Binding ElementName=text1, Path=Text}" />
        </local:SplitContainer.Child1>
        <local:SplitContainer.Child2>
            <StackPanel>
                <TextBox x:Name="text3"
                         Text="{Binding ElementName=text2, Path=Text}" />
                <TextBox x:Name="text4"
                         Text="{Binding MyName}" />
            </StackPanel>
        </local:SplitContainer.Child2>
    </local:SplitContainer>
</Grid>

XAML.cs

    public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        DataContext = this;

        MyName = "Bruno";
    }

    public string MyName
    {
        get;
        set;
    }
}
于 2021-05-20T07:45:45.043 回答