WPF中Popup控件的使用

Popup表示具有内容的弹出窗口，其主要属性为：

1. Child：获取或设置 Popup控件的内容。
2. IsOpen：获取或设置一个值，该值指示Popup 是否可见
3. Placement:获取或设置 Popup 控件打开时的控件方向，并指定Popup 控件在与屏幕边界重叠时的控件行为
4. PlacementTarget：获取或设置当打开 Popup 控件时该控件相对于其放置的元素。
5. PopupAnimation：获取或设置Popup 控件的打开和关闭动画。
6. StaysOpen：获取或设置一个值，该值指示当 Popup 控件焦点不再对准时，是否关闭该控件。

Popup主要事件为：

1. Opened：当IsOpen 属性更改为 true 时发生。

   private void PopupOpening(object sender, EventArgs e)
   {
       //Code to execute when Popup opens
   }
   

2. Closed：当IsOpen 属性更改为 false 时发生。

   private void PopupClosing(object sender, EventArgs e)
   {
       //Code to execute when Popup closes
   }
   

我们使用Popup实现一个功能：当文本框获取到焦点时，弹出下拉框，里面选择水果种类，具体效果如下图所示：

相关的界面代码为：

  <Window.Resources>
        <x:Array x:Key="MyArray" Type="system1:String">
            <system1:String>西瓜</system1:String>
            <system1:String>葡萄</system1:String>
            <system1:String>芒果</system1:String>
            <system1:String>猕猴桃</system1:String>
        </x:Array>
    </Window.Resources>
    <Grid Margin="20">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"></ColumnDefinition>
            <ColumnDefinition Width="100"></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <TextBox Grid.Column="0"  Height="30" BorderThickness="1" 
                 VerticalContentAlignment="Center"
                 Padding="10,0"
                 x:Name="TestTextBox"
                 ToolTip="{StaticResource MyArray}">
        </TextBox>
        <Button Grid.Column="1" Content="搜索" Margin="10,0" Height="30" Click="ButtonBase_OnClick"></Button>
        <Popup Grid.Column="0"
               AllowsTransparency="True"
               PopupAnimation="Slide"
               PlacementTarget="{Binding ElementName=TestTextBox}"
               Placement="Bottom"
               Width="{Binding ElementName=TestTextBox, Path=ActualWidth}"
               IsOpen="{Binding ElementName=TestTextBox, Path=IsKeyboardFocused, Mode=OneWay}">
            <ListBox ItemsSource="{Binding ElementName=TestTextBox, Path=ToolTip}"
                     SelectedItem="{Binding ElementName=TestTextBox, Path=Text, Mode=OneWayToSource}"></ListBox>
        </Popup>
    </Grid>

为了，让Popup控件能够跟随目标控件移动，需要增加一个工具类，具体如下所示：

 public class PopupHelper
    {
        /// <summary>
        /// 附加属性：跟随放置控件而移动
        /// </summary>
        public static readonly DependencyProperty PopupPlacementTargetProperty = DependencyProperty.RegisterAttached("PopupPlacementTarget", typeof(DependencyObject), typeof(PopupHelper), new PropertyMetadata(null, OnPopupPlacementTargetChanged));

        public static DependencyObject GetPopupPlacementTarget(DependencyObject obj)
        {
            return (DependencyObject)obj.GetValue(PopupPlacementTargetProperty);
        }
        public static void SetPopupPlacementTarget(DependencyObject obj, DependencyObject value)
        {
            obj.SetValue(PopupPlacementTargetProperty, value);
        }

        private static void OnPopupPlacementTargetChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is System.Windows.Controls.Primitives.Popup pop && e.NewValue is DependencyObject placementTarget)
            {
                var window = Window.GetWindow(placementTarget);
                if (window != null)
                {
                    window.LocationChanged += delegate
                    {
                        var mi = typeof(System.Windows.Controls.Primitives.Popup).GetMethod("UpdatePosition", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
                        mi?.Invoke(pop, null);
                    };
                }
            }
        }
    }

该附加属性的使用如下所示：

 <Popup local:PopupHelper.PopupPlacementTarget="{Binding ElementName=TestTextBox}"></Popup>