我的c＃ComboBox在選擇一個項目後不會調用getter。爲什麼？

Question

我有一個與SelectedItem組合框。如果我選擇一個項目，我的設置器會進行一些計算，也許我想將值重設爲原來的值。不幸的是，我的觀點並沒有更新。

我有以下組合框：

<ComboBox BorderThickness="0" VerticalAlignment="Center" Margin="2,0" 
    DisplayMemberPath="Name" 
    ItemsSource="{Binding ItemsVS.View}" 
    SelectedItem="{Binding SelectedItem, Mode=TwoWay}" > 

</ComboBox> 

這裏是我的ViewModel的屬性：

private CollectionViewSource _itemsVS; 
public CollectionViewSource ItemsVS 
{ 
    get { return _itemsVS; } 
    set 
    { 
     _itemsVS = value; 
     if(PropertyChanged!=null) 
      PropertyChanged(this, new PropertyChangedEventArgs("ItemsVS")); 
    } 
} 


private ItemViewModel _selectedItem; 
public ItemViewModel SelectedItem 
{ 
    get 
    { 
     // after setting the old value in the setter the getter is not called 
     // and the view is not refreshed 
     return _selectedItem; 
    } 
    set 
    { 
     var old = _selectedItem; 
     _selectedItem = value; 
     // After some logic I want to have the old value!!!!!!!!!!!!!! 
     _selectedItem = old; 
     if (PropertyChanged != null) 
      PropertyChanged(this, new PropertyChangedEventArgs("SelectedItem")); 


    } 
} 

Answer 1

如果你想使ComboBox設置新值後，回讀電流值，您需要爲您的Binding添加「無操作」Converter，該功能無效。這是一個有用的小技巧，因爲綁定通常不會檢查實際應用的源值是否與綁定提供的新值匹配。添加轉換器會強制它檢查。

public sealed class NoOpConverter : IValueConverter 
{ 
    public static readonly NoOpConverter Instance = new NoOpConverter(); 

    public object Convert(
     object value, 
     Type targetType, 
     object parameter, 
     CultureInfo culture) 
    { 
     return value; 
    } 

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

<ComboBox SelectedItem="{Binding Path=SelectedItem, 
           Mode=TwoWay, 
           Converter={x:Static NoOpConverter.Instance}}" 
      ... />