我有一个绑定到 ObservableCollection 的 WPF ListBox。当我向其中添加项目时，我想要一些“引起注意”到“新来者”的动画。有许多使用 DataTemplate 触发器和 FrameworkElment.Loaded 事件执行此操作的示例。效果很好，看起来很酷。
但是，滚动列表时也会触发相同的动画。这是由于 ListBox 的虚拟化功能，它会根据需要“加载”新项目。
有没有办法构造一个 ListBox，使它只在 ObservableCollection 发生变化时触发“加载”动画，而在滚动时不触发？
附：禁用虚拟化不是一种选择
【问题讨论】：
标签： wpf
为了触发动画，我创建了附加的依赖属性 AddNewItem (Boolean)：
public class MyDependencyClass : DependencyObject
{
public static readonly DependencyProperty AddNewItemProperty;
public static void SetAddNewItem(DependencyObject DepObject, bool value)
{
DepObject.SetValue(AddNewItemProperty, value);
}
public static bool GetAddNewItem(DependencyObject DepObject)
{
return (bool)DepObject.GetValue(AddNewItemProperty);
}
static MyDependencyClass()
{
PropertyMetadata MyPropertyMetadata = new PropertyMetadata(false);
AddNewItemProperty = DependencyProperty.RegisterAttached("AddNewItem",
typeof(bool),
typeof(MyDependencyClass),
MyPropertyMetadata);
}
}
然后创建我们的依赖属性的边框（警告添加一个新项目）：
<Border x:Name="WarningBorder" Style="{StaticResource HideBorderStyle}" local:MyDependencyClass.AddNewItem="False" Height="33" Background="#44515B" BorderThickness="0" Margin="65,0,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Opacity="0">
<TextBlock x:Name="WarningText" FontFamily="./#Segoe UI" TextAlignment="Center" Margin="0,5,0,0" FontSize="18" Foreground="Gainsboro" Text="You add new item" />
</Border>
样式 HideBorderStyle 包含在属性 AddNewItem=True 时触发的 DataTrigger。 HideBorderStyle 列表：
<Style x:Key="HideBorderStyle" TargetType="{x:Type Border}">
<Style.Triggers>
<DataTrigger Binding="{Binding ElementName=WarningBorder, Path=(local:MyDependencyClass.AddNewItem), Mode=OneWay}" Value="True">
<DataTrigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetProperty="Width" BeginTime="0:0:0" From="0.0" To="427.0" Duration="0:0:0.5" />
<DoubleAnimation Storyboard.TargetProperty="Opacity" BeginTime="0:0:0" From="0.0" To="1.0" Duration="0:0:1.0" />
<DoubleAnimation Storyboard.TargetProperty="Opacity" BeginTime="0:0:5" From="1.0" To="0.0" Duration="0:0:1.0" />
<DoubleAnimation Storyboard.TargetProperty="Width" BeginTime="0:0:5" From="427.0" To="0.0" Duration="0:0:1.0" />
</Storyboard>
</BeginStoryboard>
</DataTrigger.EnterActions>
</DataTrigger>
</Style.Triggers>
</Style>
假设我们有一个 SampleListBox 和 AddButton：
<Grid>
<ListBox x:Name="SampleListBox" Width="400" Height="200" BorderThickness="1" BorderBrush="Black" DisplayMemberPath="Name" Background="AliceBlue" Loaded="SampleListBox_Loaded" />
<Button Content="Add" Width="50" Height="30" VerticalAlignment="Bottom" Click="AddButton_Click" />
</Grid>
AddButton_Click 列表：
private void AddButton_Click(object sender, RoutedEventArgs e)
{
PersonListBox.Add(new Person()
{
Name = "NewItem",
});
SampleListBox.ItemsSource = PersonListBox;
}
在 SampleListBox_Loaded 中，我初始化 SampleListBox 的数据并分配一个处理程序 NotifyCollectionChangedEventHandler，当集合更改时调用该处理程序。 SampleListBox_Loaded 事件列表：
private void SampleListBox_Loaded(object sender, RoutedEventArgs e)
{
PersonListBox.Add(new Person()
{
Name = "Peter Orange",
Age = 32,
Sample = "Sample",
});
SampleListBox.ItemsSource = PersonListBox;
PersonListBox.CollectionChanged += new NotifyCollectionChangedEventHandler(PersonListBox_CollectionChanged);
}
在 PersonListBox_CollectionChanged 中，我为依赖属性 AddNewItem 设置了值 True。 PersonListBox_CollectionChanged 列表：
private void PersonListBox_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
if (e.Action == NotifyCollectionChangedAction.Add)
{
MyDependencyClass.SetAddNewItem(WarningBorder, true);
}
}
现在，当一个项目被添加到集合中时，动画就会被触发。如果添加另一个元素动画不会触发。所以她工作了，你需要为以后的显示动画折叠值：
private void ResetButton_Click(object sender, RoutedEventArgs e)
{
MyDependencyClass.SetAddNewItem(WarningBorder, false);
}
在这种情况下，您需要决定何时重置动画的值。
编辑：
根据我之前的示例为添加的项目设置动画。但是现在将给 ListBoxItem 分配带有 DataTrigger 的样式：
<ListBox x:Name="SampleListBox" local:MyDependencyClass.AddNewItem="False" local:MyDependencyClass.InitCountOfList="0" Width="400" Height="200" BorderThickness="1" BorderBrush="Black" DisplayMemberPath="Name" Background="AliceBlue" Loaded="SampleListBox_Loaded">
<ListBox.ItemContainerStyle>
<Style TargetType="{x:Type ListBoxItem}">
<Style.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding ElementName=SampleListBox, Path=(local:MyDependencyClass.AddNewItem), Mode=OneWay}" Value="True" />
<Condition Binding="{Binding ElementName=SampleListBox, Converter={StaticResource BoolToInitItemsConverter}}" Value="True" />
</MultiDataTrigger.Conditions>
<MultiDataTrigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetProperty="Opacity" BeginTime="0:0:0" From="0.0" To="1.0" Duration="0:0:1.0" />
</Storyboard>
</BeginStoryboard>
</MultiDataTrigger.EnterActions>
</MultiDataTrigger>
</Style.Triggers>
</Style>
</ListBox.ItemContainerStyle>
</ListBox>
为了正确工作，动画（尤其是在程序开始时）是根据另一个属性 - InitCountOfList (Int) 创建的：
public class MyDependencyClass : DependencyObject
{
public static readonly DependencyProperty AddNewItemProperty, InitCountOfListProperty;
#region AddNewItem
public static void SetAddNewItem(DependencyObject DepObject, bool value)
{
DepObject.SetValue(AddNewItemProperty, value);
}
public static bool GetAddNewItem(DependencyObject DepObject)
{
return (bool)DepObject.GetValue(AddNewItemProperty);
}
#endregion
#region InitCountOfList
public static void SetInitCountOfList(DependencyObject DepObject, int value)
{
DepObject.SetValue(InitCountOfListProperty, value);
}
public static int GetInitCountOfList(DependencyObject DepObject)
{
return (int)DepObject.GetValue(InitCountOfListProperty);
}
#endregion
#region Constructor of DependencyProperty
static MyDependencyClass()
{
PropertyMetadata BoolPropertyMetadata = new PropertyMetadata(false);
PropertyMetadata IntPropertyMetadata = new PropertyMetadata(0);
AddNewItemProperty = DependencyProperty.RegisterAttached("AddNewItem",
typeof(bool),
typeof(MyDependencyClass),
BoolPropertyMetadata);
InitCountOfListProperty = DependencyProperty.RegisterAttached("InitCountOfList",
typeof(int),
typeof(MyDependencyClass),
IntPropertyMetadata);
}
#endregion
}
数据在 Window_ContentRendered 事件中初始化：
private void Window_ContentRendered(object sender, EventArgs e)
{
PersonListBox.Add(new Person()
{
Name = "Peter Orange",
});
SampleListBox.ItemsSource = PersonListBox;
// Set the initial number of collection
MyDependencyClass.SetInitCountOfList(SampleListBox, SampleListBox.Items.Count);
}
Handler NotifyCollectionChangedEventHandler 在 SampleListBox_Loaded 事件中赋值：
private void SampleListBox_Loaded(object sender, RoutedEventArgs e)
{
PersonListBox.CollectionChanged += new NotifyCollectionChangedEventHandler(PersonListBox_CollectionChanged);
}
Handler 和 AddButton_Click 保持不变：
private void PersonListBox_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
if (e.Action == NotifyCollectionChangedAction.Add)
{
MyDependencyClass.SetAddNewItem(SampleListBox, true);
}
}
private void AddButton_Click(object sender, RoutedEventArgs e)
{
PersonListBox.Add(new Person()
{
Name = "NewItem",
});
}
转换器BoolToInitItemsConverter，如果在开始时没有添加元素，则返回true：
/// <summary>
/// Return true, if count of collection > initial
/// </summary>
public class BoolToInitItemsConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
ListBox MyListBox = value as ListBox;
int InitCountOfList = MyDependencyClass.GetInitCountOfList(MyListBox);
if (MyListBox.Items.Count > InitCountOfList)
{
// Added a new element
return true;
}
// First run and we do not want
// to run the animation
return false;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return DependencyProperty.UnsetValue as object;
}
}
现在动画工作只针对添加的项目。
【讨论】：