wpf - 绑定(bind)到 Storyboard 上的附加行为

Question

我为 Storyboard 创建了一个附加的依赖属性，目的是让我能够在 Storyboard Completed 事件触发时调用 ViewModel 上的方法:

public static class StoryboardExtensions
{
    public static ICommand GetCompletedCommand(DependencyObject target)
    {
        return (ICommand)target.GetValue(CompletedCommandProperty);
    }

    public static void SetCompletedCommand(DependencyObject target, ICommand value)
    {
        target.SetValue(CompletedCommandProperty, value);
    }

    public static readonly DependencyProperty CompletedCommandProperty =
        DependencyProperty.RegisterAttached(
            "CompletedCommand",
            typeof(ICommand),
            typeof(StoryboardExtensions),
            new FrameworkPropertyMetadata(null, OnCompletedCommandChanged));

    static void OnCompletedCommandChanged(DependencyObject target, DependencyPropertyChangedEventArgs e)
    {
        Storyboard storyboard = target as Storyboard;
        if (storyboard == null) throw new InvalidOperationException("This behavior can be attached to Storyboard item only.");
        storyboard.Completed += new EventHandler(OnStoryboardCompleted);
    }

    static void OnStoryboardCompleted(object sender, EventArgs e)
    {                        
        Storyboard item = ... // snip
        ICommand command = GetCompletedCommand(item);
        command.Execute(null);
    }
}

然后我尝试在 XAML 中使用它，使用绑定(bind)语法:

<Grid>
    <Grid.Resources>
        <Storyboard x:Key="myStoryboard" my:StoryboardExtensions.CompletedCommand="{Binding AnimationCompleted}">
            <DoubleAnimation Storyboard.TargetProperty="Opacity" From="1" To="0" Duration="0:0:5" />
        </Storyboard>

        <Style x:Key="myStyle" TargetType="{x:Type Label}">
            <Style.Triggers>
                <DataTrigger 
                 Binding="{Binding Path=QuestionState}" Value="Correct">
                    <DataTrigger.EnterActions>
                        <BeginStoryboard Storyboard="{StaticResource myStoryboard}" />
                    </DataTrigger.EnterActions>
                </DataTrigger>
            </Style.Triggers>
        </Style>

    </Grid.Resources>
    <Label x:Name="labelHello" Grid.Row="0" Style="{StaticResource myStyle}">Hello</Label>
</Grid>

这失败了，但有以下异常(exception):

System.Windows.Markup.XamlParseException occurred Message="Cannot convert the value in attribute 'Style' to object of type 'System.Windows.Style'. Cannot freeze this Storyboard timeline tree for use across threads. Error at object 'labelHello' in markup file 'TestWpfApp;component/window1.xaml'

有没有办法让绑定(bind)语法与 Storyboard 的附加 ICommand 属性一起使用？

Answer 1

这是设计使然。如果您有一个可卡住的对象放入样式中，则样式将被卡住以允许跨线程访问。但是您的绑定(bind)本质上是一个表达式，这意味着它不能被卡住，因为数据绑定(bind)是单线程的。

如果您需要这样做，请将触发器放在样式之外的框架元素下，而不是样式中。您可以在 Grid.Triggers 部分执行此操作。这确实有点糟糕，因为您的样式不再完整并且您必须复制触发器，但它是 WPF 中的“设计”功能。

MSDN 社交论坛上的完整答案是 here .