WPF转换器和ObservableCollections [英] WPF Converters and ObservableCollections

问题描述

我将ObservableCollection绑定到一个控件，该控件具有一个转换器，可以根据集合是否具有任何值来更改其可见性:

I'm binding an ObservableCollection to a control which has a converter to change its visibility depending on if the collection has any values or not:

简化示例:

XAML:

<Window.Resources>
    <local:MyConverter x:Key="converter"/>
</Window.Resources>

<Grid x:Name="grid">
    <Rectangle Height="100" Width="200" Fill="CornflowerBlue"
                Visibility="{Binding Converter={StaticResource converter}}"/>
    <Button Content="click" 
            HorizontalAlignment="Left" VerticalAlignment="Top" 
            Click="Button_Click"/>
</Grid>

C#:

ObservableCollection<string> strings;

public MainWindow()
{
    InitializeComponent();

    strings = new ObservableCollection<string>();
    grid.DataContext = strings;
}

private void Button_Click(object sender, RoutedEventArgs e)
{
    strings.Add("new value");
}

绑定集合时，在有值的情况下可见Rectangle，而在集合为空时则不可见.但是，如果集合为空，而我在运行时添加了一个值，则不会出现Rectangle(甚至不会触发转换器的Convert方法).我是否缺少某些东西，或者只是想问太多IValueConverter?

When the collection is bound, the Rectangle is visible when there are values and not when the collection is empty. However, if the collection is empty and I add a value at runtime, the Rectangle does not appear (the converter's Convert method isn't even fired). Am I missing something or just trying to ask too much of IValueConverter?

推荐答案

好的，所以这就是我使用MultiValueConverter

OK, so here's how I got around the problem using a MultiValueConverter

转换器现在看起来像:

public object Convert(
    object[] values, 
    Type targetType, 
    object parameter, 
    System.Globalization.CultureInfo culture)
{
    ObservableCollection<string> strings = 
        values[0] as ObservableCollection<string>;

    if (strings == null || !strings.Any())
        return Visibility.Collapsed;
    else
        return Visibility.Visible;
}

public object[] ConvertBack(
    object value, 
    Type[] targetTypes, 
    object parameter, 
    System.Globalization.CultureInfo culture)
{
    throw new NotImplementedException();
}

XAML现在看起来像:

And the XAML now looks like:

<Rectangle Height="100" Width="200" Fill="CornflowerBlue">
    <Rectangle.Visibility>
        <MultiBinding Converter="{StaticResource converter}">
            <Binding Path="."/>
            <Binding Path="Count"/>
        </MultiBinding>
    </Rectangle.Visibility>
</Rectangle>

C#保持不变:)

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