更改 ItemsSource 集合中的属性时如何重绘 ItemsControl ItemsPanel？

Question

我创建了一个自定义的 WeightedUniformGrid 类，我将其用作 ItemsControl 中的 ItemsPanel。每个网格元素都使用 ItemsSource 集合中对象中的 Weight 属性进行加权。

但是当我在视图模型中更改权重属性时，它不会立即显示在视图中。我必须更改要使用新值绘制的 WeightedUniformGrid 的窗口大小。

如何获取 ItemsSource 集合中的属性更改以导致 ItemsControl 被重绘？我在想也许在 WeightedUniformGrid 中添加一个 DependencyProperty，这将包括 AffectsArrange 和 AffectsMeasure。但我不确定我可以将它绑定到什么。

我已经搜索了很长时间，并且有一些类似的问题（例如this one）。但我无法根据我的需要调整其中的任何一个。

MainWindow.xaml：

<ItemsControl ItemsSource="{Binding Path=ElementGroupCollection}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <local:WeightedUniformGrid Rows="2" />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <Border BorderBrush="Black" BorderThickness="1"
                    RenderOptions.EdgeMode="Aliased">
                <Viewbox Stretch="Uniform">
                    <TextBlock Text="{Binding Path=Number}" Foreground="Black" FontSize="20" 
                                    HorizontalAlignment="Center" VerticalAlignment="Center"/>
                </Viewbox>
            </Border>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

MainWindowVM.cs：

public class MainWindowVM
{
    public MainWindowVM()
    {
        _elementGroupCollection = new ObservableCollection<ElementGroup>()
        {
            new ElementGroup(1, 60),
            new ElementGroup(2, 150),
            new ElementGroup(3, 90),
            new ElementGroup(4, 80),
            new ElementGroup(5, 60),
            new ElementGroup(6, 160)
        };
    }

    private ObservableCollection<ElementGroup> _elementGroupCollection;
    public ObservableCollection<ElementGroup> ElementGroupCollection
    {
        get { return _elementGroupCollection; }
    }
}

ElementGroup.cs：

public class ElementGroup : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    public ElementGroup(int number, double? weight)
    {
        Number = number;
        Weight = (weight >= 0) ? weight : null;
    }

    public int Number { get; }

    private double? _weight;
    public double? Weight
    {
        get { return _weight; }
        set { SetNotify(ref _weight, value); }
    }

    public void SetNotify<T>(ref T storage,
                             T value,
                             [CallerMemberName] string propertyName = null)
    {
        storage = value;
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

WeightedUniformGrid.cs：

public class WeightedUniformGrid : UniformGrid
{

    protected override Size MeasureOverride(Size constraint)
    {
        var size = base.MeasureOverride(constraint);
        double elementsPerRow = Math.Ceiling((double)Children.Count / Rows);
        double elementHeight = size.Height / Rows;
        double unweightedElementWidth = size.Width / elementsPerRow;
        for (int i = 0; i < Children.Count; ++i)
        {
            var child = (FrameworkElement)Children[i];
            var dc = child.DataContext;
            int rowNumber = (int)Math.Floor(i / elementsPerRow);
            double? weight = dc.GetType().GetProperty("Weight")?.GetValue(dc, null) as double?;
            if (weight == null) { weight = 100; }
            double weightedElementWidth = unweightedElementWidth * (double)weight / 100;
            child.Measure(new Size(weightedElementWidth, elementHeight));
        }
        return size;
    }

    protected override Size ArrangeOverride(Size arrangeSize)
    {
        var size = base.ArrangeOverride(arrangeSize);
        int elementsPerRow = (int)Math.Ceiling((double)Children.Count / Rows);
        double elementHeight = size.Height / Rows;
        double unweightedElementWidth = size.Width / elementsPerRow;
        double[] accumulatedWidthPerRow = new double[Rows];
        for (int i = 0; i < Children.Count; ++i)
        {
            var child = (FrameworkElement)Children[i];
            var dc = child.DataContext;
            int rowNumber = i / elementsPerRow;
            double? weight = dc.GetType().GetProperty("Weight")?.GetValue(dc, null) as double?;
            if (weight == null) { weight = 100; }
            double weightedElementWidth = unweightedElementWidth * (double)weight / 100;
            child.Arrange(new Rect(new Point(accumulatedWidthPerRow[rowNumber], rowNumber * elementHeight),
                                    new Point(accumulatedWidthPerRow[rowNumber] + weightedElementWidth, (rowNumber + 1) * elementHeight)));
            accumulatedWidthPerRow[rowNumber] += weightedElementWidth;
        }
        return size;
    }
}

Answer 1

您的 WeightedUniformGrid 不得直接访问其子元素的 DataContext 中的 Weight 属性。除此之外，这是一种不好的做法，它不会正常工作，因为当视图模型项的权重发生变化时，没有强制布局传递的机制。

应该有一个绑定到权重的附加属性。附加属性的 FrameworkPropertyMetadataOptions 将强制布局传递。

public class WeightedUniformGrid : UniformGrid
{
    public static readonly DependencyProperty WeightProperty =
        DependencyProperty.RegisterAttached(
            "Weight", typeof(double), typeof(WeightedUniformGrid),
            new FrameworkPropertyMetadata(double.NaN,
                FrameworkPropertyMetadataOptions.AffectsParentMeasure |
                FrameworkPropertyMetadataOptions.AffectsParentArrange));

    public static double GetWeight(UIElement element)
    {
        return (double)element.GetValue(WeightProperty);
    }

    public static void SetWeight(UIElement element, double value)
    {
        element.SetValue(WeightProperty, value);
    }

    protected override Size MeasureOverride(Size constraint)
    {
        var size = base.MeasureOverride(constraint);
        double elementsPerRow = Math.Ceiling((double)Children.Count / Rows);
        double elementHeight = size.Height / Rows;
        double unweightedElementWidth = size.Width / elementsPerRow;
        for (int i = 0; i < Children.Count; ++i)
        {
            var child = Children[i];
            int rowNumber = (int)Math.Floor(i / elementsPerRow);

            // get attached property value
            double weight = GetWeight(child);
            if (double.IsNaN(weight)) { weight = 100; }

            double weightedElementWidth = unweightedElementWidth * weight / 100;
            child.Measure(new Size(weightedElementWidth, elementHeight));
        }
        return size;
    }

    protected override Size ArrangeOverride(Size arrangeSize)
    {
        var size = base.ArrangeOverride(arrangeSize);
        int elementsPerRow = (int)Math.Ceiling((double)Children.Count / Rows);
        double elementHeight = size.Height / Rows;
        double unweightedElementWidth = size.Width / elementsPerRow;
        double[] accumulatedWidthPerRow = new double[Rows];
        for (int i = 0; i < Children.Count; ++i)
        {
            var child = Children[i];
            int rowNumber = i / elementsPerRow;

            // get attached property value
            double weight = GetWeight(child);
            if (double.IsNaN(weight)) { weight = 100; }

            double weightedElementWidth = unweightedElementWidth * (double)weight / 100;
            child.Arrange(new Rect(new Point(accumulatedWidthPerRow[rowNumber], rowNumber * elementHeight),
                                    new Point(accumulatedWidthPerRow[rowNumber] + weightedElementWidth, (rowNumber + 1) * elementHeight)));
            accumulatedWidthPerRow[rowNumber] += weightedElementWidth;
        }
        return size;
    }
}

在 ItemContainerStyle 中绑定附加属性：

<ItemsControl ItemsSource="{Binding Path=ElementGroupCollection}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <local:WeightedUniformGrid Rows="2" />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <ItemsControl.ItemContainerStyle>
        <Style TargetType="ContentPresenter">
            <Setter Property="local:WeightedUniformGrid.Weight" Value="{Binding Weight}"/>
        </Style>
    </ItemsControl.ItemContainerStyle>
    ...
</ItemsControl>