如何在从可视化树中删除元素之前执行一些代码？

Question

我正在尝试为 WPF 中的用户控件实现一些淡入和淡出动画。对于淡入动画，我可以使用 Loaded 事件来完成此操作。

    public sealed partial class NowPlayingView : UserControl
    {
        public Duration AnimationDuration
        {
            get { return (Duration)GetValue(AnimationDurationProperty); }
            set { SetValue(AnimationDurationProperty, value); }
        }

        public static readonly DependencyProperty AnimationDurationProperty =
            DependencyProperty.Register("AnimationDuration", typeof(Duration), typeof(NowPlayingView), new PropertyMetadata(Duration.Automatic));

        public NowPlayingView()
        {
            Opacity = 0;
            InitializeComponent();
            Loaded += NowPlayingView_Loaded;
            Unloaded += NowPlayingView_Unloaded;

        }

        private void NowPlayingView_Unloaded(object sender, RoutedEventArgs e)
        {
            DoubleAnimation animation = new(1.0, 0.0, AnimationDuration);
            BeginAnimation(OpacityProperty, animation);
        }

        private void NowPlayingView_Loaded(object sender, RoutedEventArgs e)
        {
            DoubleAnimation animation = new (0.0, 1.0, AnimationDuration);
            BeginAnimation(OpacityProperty, animation);
        }
    }

我尝试使用

Unloaded

事件来实现淡出效果，却发现该事件是在 UserControl 从可视化树中删除之后（当 UserControl 不再可见或不可访问时）触发的。有没有办法在 UserControl“关闭”之前运行一些代码，例如

OnClosing

的

Window

事件？

编辑：

为了了解更多上下文，

UserControl

充当更复杂窗口的组件。只要属性

NowPlayingViewModel

不为空，它就会被激活，当为空时，它就会被停用（我这样做是为了隐藏用户控件）。当我将 ViewModel 设置为 null 时，我想要运行淡出动画，并且我希望保持代码隐藏与其他 ViewModel 逻辑分离。

<!-- Now playing View-->
<ContentControl Grid.RowSpan="3" Grid.ColumnSpan="2" Content="{Binding NowPlayingViewModel}">
    <ContentControl.Resources>
        <DataTemplate DataType="{x:Type viewmodels:NowPlayingViewModel}">
            <views:NowPlayingView AnimationDuration="00:00:00.8" />
        </DataTemplate>
    </ContentControl.Resources>
</ContentControl>

根据我的测试，到目前为止，我找不到任何好的解决方案，尽管我愿意接受导致类似行为的建议。

Answer 1

Closing

事件..但是您可以在加载 UserControl 时获取父窗口并在那里实现淡出行为..

首先，删除

Unloaded += NowPlayingView_Unloaded;

然后，稍微修改一下

Loaded

代码..

private Window ParentWindow
{
    get
    {
        DependencyObject parentDepObj = this;
        do
        {
            parentDepObj = VisualTreeHelper.GetParent(parentDepObj);
            if (parentDepObj is Window parentWindow) return parentWindow;
        } while (parentDepObj != null);

        return null;
    }
}

private void NowPlayingView_Loaded(object sender, RoutedEventArgs e)
{
    DoubleAnimation animation = new(0.0, 1.0, AnimationDuration);
    BeginAnimation(OpacityProperty, animation);
    var parentWindow = this.ParentOfType<Window>();
    parentWindow.Closing += WindowClosing;
}

private void WindowClosing(object sender, CancelEventArgs args)
{
    var pw = ParentWindow;
    pw.Closing -= WindowClosing;
    args.Cancel = true;
    var anim = new(1.0, 0.0, AnimationDuration);
    anim.Completed += (s, _) => pw.Close();
    BeginAnimation(OpacityProperty, anim);
}

---

可选注释

。您可以通过简单的调用来替换 ParentWindow 属性的 getter

private Window ParentWindow => this.ParentOfType<Window>();

其中

ParentOfType

是一些公共静态类Utilities中的扩展函数..

public static T ParentOfType<T>(this DependencyObject child) where T : DependencyObject
{
    var parentDepObj = child;
    do
    {
        parentDepObj = VisualTreeHelper.GetParent(parentDepObj);
        if (parentDepObj is T parent) return parent;
    } while (parentDepObj != null);

    return null;
}

Answer 2

    public sealed partial class NowPlayingView : UserControl
    {
        private bool _animating = false;

        public bool ShowWindow
        {
            get { return (bool)GetValue(ShowWindowProperty); }
            set { SetValue(ShowWindowProperty, value); }
        }

        public static readonly DependencyProperty ShowWindowProperty =
            DependencyProperty.Register("ShowWindow", typeof(bool), typeof(NowPlayingView), new PropertyMetadata(false, OnShowWindowChanged));

        private static void OnShowWindowChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is NowPlayingView userControl && (bool)e.NewValue == true)
                userControl.Show();            
        }

        public NowPlayingView()
        {
            InitializeComponent();
            Opacity = 0;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            _animating = true;

            Hide();

            _animating = false;
        }

        private void Show()
        {
            Visibility = Visibility.Visible;

            DoubleAnimation animation = new()
            {
                From = 0.0,
                To = 1.0,
                FillBehavior = FillBehavior.HoldEnd,
                Duration = new(TimeSpan.FromMilliseconds(600))
            };
            Storyboard storyboard = new();

            storyboard.Children.Add(animation);
            Storyboard.SetTarget(animation, this);
            Storyboard.SetTargetProperty(animation, new PropertyPath(OpacityProperty));
            storyboard.Begin();
        }

        private void Hide()
        {
            DoubleAnimation animation = new()
            {
                From = 1.0,
                To = 0.0,
                FillBehavior = FillBehavior.Stop,
                Duration = new(TimeSpan.FromMilliseconds(600))
            };
            Storyboard storyboard = new();

            storyboard.Children.Add(animation);
            Storyboard.SetTarget(animation, this);
            Storyboard.SetTargetProperty(animation, new PropertyPath(OpacityProperty));
            storyboard.Completed += delegate 
            { 
                Visibility = Visibility.Collapsed; 
            };
            storyboard.Begin();
        }
    }

我创建了一个依赖属性

ShowWindow

。每当该属性更改为

true

时，都会触发 Storyboard 进行淡入动画。对于淡出，我只是连接了一个按钮 OnClick 事件，但我可以在 DependencyPropertyChanged 方法中轻松执行此操作。通过这种方式，我可以使代码保持一定程度的解耦，并通过绑定到

ShowWindow

上的

UserControl

依赖属性的属性直接从其 ViewModel 控制 UserControl 的动画。