WPF - 按鈕下的中心ContextMenu

Question

我正在嘗試將按鈕下的ContextMenu居中。我試圖根據ContextMenu的高度來設置VerticalOffset屬性，這樣無論項目的數量如何，菜單的頂部始終位於相同的位置。我似乎無法獲得菜單的高度（可能是因爲它尚未繪製？）。我的代碼如下，我怎樣才能獲得ContextMenu的高度？

private void Button_Click(object sender, RoutedEventArgs e) 
{ 
    Button PopUpButton = (sender as Button); 
    PopUpButton.ContextMenu.IsEnabled = true; 
    PopUpButton.ContextMenu.PlacementTarget = PopUpButton; 
    PopUpButton.ContextMenu.Placement = System.Windows.Controls.Primitives.PlacementMode.Center; 
    PopUpButton.ContextMenu.VerticalOffset = ((PopUpButton.ContextMenu.Height/2) + 25); 
    PopUpButton.ContextMenu.IsOpen = true; 
} 

Answer 1

如果你想水平居中和放置ContextMenu下Button你必須綁定VerticalOffset到ContextMenu的ActualHeight和使用Converter，將計算所需的垂直偏移。你可以通過Button的高度作爲ConverterParameter使它更通用。

private void Button_Click(object sender, RoutedEventArgs e) 
    { 
     Button PopUpButton = (sender as Button); 
     PopUpButton.ContextMenu.IsEnabled = true; 
     PopUpButton.ContextMenu.PlacementTarget = PopUpButton; 
     PopUpButton.ContextMenu.Placement = System.Windows.Controls.Primitives.PlacementMode.Center; 

     Binding b = new Binding("ActualHeight") 
     { 
      Source = PopUpButton.ContextMenu, 
      Converter = new HeightToOffsetConverter(), 
      ConverterParameter = PopUpButton.ActualHeight, 
     }; 

     PopUpButton.ContextMenu.SetBinding(ContextMenu.VerticalOffsetProperty, b); 
     PopUpButton.ContextMenu.IsOpen = true; 
    } 

    public class HeightToOffsetConverter : IValueConverter 
    { 
     public object Convert(object value, Type targetType, 
      object parameter, CultureInfo culture) 
     { 
      double popupHeight = (double)value; 
      double buttonHeight = (double)parameter; 

      return popupHeight/2 + buttonHeight/2; 
     } 

     public object ConvertBack(object value, Type targetType, 
      object parameter, CultureInfo culture) 
     { 
      return Binding.DoNothing; 
     } 
    } 

你也可以（並在我看來你應該）從XAML設置此綁定。