c# - 子属性更改时，FreezableCollection 不提供更改通知

Question

我有一个 FreezableCollection，我想监视其子属性的更改。这是代码的一个小节:

public class FieldHeading : DependencyObject
{
    public static readonly DependencyProperty LayoutProperty = DependencyProperty.Register("Layout", typeof(FieldHeadingLayout), typeof(FieldHeading),
        new FrameworkPropertyMetadata(FieldHeadingLayout.Above,
        FrameworkPropertyMetadataOptions.AffectsRender |
        FrameworkPropertyMetadataOptions.AffectsMeasure |
        FrameworkPropertyMetadataOptions.AffectsParentMeasure));

    public FieldHeadingLayout Layout
    {
        get { return (FieldHeadingLayout) GetValue(LayoutProperty); }
        set { SetValue(LayoutProperty, value); }
    }

}

public class FieldPanel : FrameworkElement
{
    private static readonly DependencyProperty FieldHeadingProperty = DependencyProperty.Register("FieldHeading", typeof(FreezableCollection<FieldHeading>), typeof(FieldPanel),
        new FrameworkPropertyMetadata(null,
            FrameworkPropertyMetadataOptions.AffectsMeasure |
            FrameworkPropertyMetadataOptions.AffectsParentMeasure |
            FrameworkPropertyMetadataOptions.AffectsRender, HeadingChanged));

    private static void HeadingChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Debug.WriteLine("Hello");
    }

    public FreezableCollection<FieldHeading> FieldHeadings
    {
        get
        { return (FreezableCollection<FieldHeading>) GetValue(FieldHeadingProperty); }
        set { SetValue(FieldHeadingProperty, value);}
    }

    public FieldPanel()
    {
        AddVisual(_contentVisual = new DrawingVisual());
        FieldHeadings = new FreezableCollection<FieldHeading>();
    }
}

然后我们为其中一个 FieldHeadings 的 Layout 分配一个新值，不会生成更改通知。显然我错过了一些重要的东西。永远不会调用 HeadingChanged。

有关 FreezableCollection 的 MSDN 帮助，可在此处找到:FreezableCollection , 状态:

事件已更改... 当 Freezable 或 它包含的对象 被修改时发生。 (继承自 Freezable。)

在此先感谢您的帮助。

~卡梅伦

Answer 1

实际上，您可以做您想做的事。这就是为什么 FreezableCollection<T>存在!您需要做的就是更改 FieldHeading从 Freezable 派生而不是 DependencyObject并且对集合中项目的更改将发出相同的更改通知，就好像整个项目已被替换一样。

这是一个非常有用但鲜为人知的 WPF 功能。

From Charles Petzold himself ,

These freezable collection classes fire change notifications whenever items are added to or removed from the collection, of course, but also when a dependency property of any item within the collection changes. This is an extremely powerful mechanism.

这是一个演示如何使用 FreezableCollection<> 的小示例.我在 Visual Studio 中创建了一个新的 WPF 项目。以下是 MainWindow.xaml.cs 的 XAML 及其背后的 C# 代码:

<Window x:Class="FreezableCollection.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:FreezableCollection"
        x:Name="Root"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <DataTemplate DataType="{x:Type local:MyFreezable}">
            <CheckBox IsChecked="{Binding IsNice}" Content="Check me!"/>
        </DataTemplate>
    </Window.Resources>
    <Grid>
        <ListBox ItemsSource="{Binding ElementName=Root, Path=MyFreezables}" />
    </Grid>
</Window>

using System.Windows;

namespace FreezableCollection
{
    public partial class MainWindow : Window
    {
        public static readonly DependencyProperty MyFreezablesProperty =
            DependencyProperty.Register("MyFreezables", typeof(MyFreezableCollection), typeof(MainWindow), new FrameworkPropertyMetadata(null, FreezablesChangedCallback));

        public MyFreezableCollection MyFreezables
        {
            get => (MyFreezableCollection)GetValue(MyFreezablesProperty);
            set => SetValue(MyFreezablesProperty, value);
        }

        public MainWindow()
        {
            InitializeComponent();
            MyFreezables = new MyFreezableCollection { new MyFreezable() };
        }

        private static void FreezablesChangedCallback(object sender, DependencyPropertyChangedEventArgs args)
        {
            MessageBox.Show("Changed!");
        }
    }

    public class MyFreezableCollection : FreezableCollection<MyFreezable>
    {
        protected override Freezable CreateInstanceCore() => new MyFreezableCollection();
    }

    public class MyFreezable : Freezable
    {
        public static readonly DependencyProperty IsNiceProperty =
            DependencyProperty.Register("IsNice", typeof(bool), typeof(MyFreezable), new PropertyMetadata(false));

        public bool IsNice
        {
            get => (bool)GetValue(IsNiceProperty);
            set => SetValue(IsNiceProperty, value);
        }

        protected override Freezable CreateInstanceCore() => new MyFreezable();
    }
}

示例显示一个包含单个 CheckBox 的列表.单击它以切换 IsNice数据绑定(bind)中项目的属性 MyFreezableCollection .即使没有向列表中添加任何内容或从列表中删除任何内容，也会调用依赖属性更改回调。