在 DependencyProperty 的 CoerceValueCallback 中取消更新时如何更新绑定的源?

问题描述 投票:0回答:2

我有一个控件(在下面的示例中名为

GridView
)和一个视图模型,它们通过其
SelectedValue
属性上的 2 路绑定进行绑定。我想通过使用
GridView
来禁止
CoerceValueCallback
控件中该属性的某些值。

当绑定将该无效值(本例中为 42)推入

DependencyProperty
时,
CoerceValue
方法会丢弃该值,并且控件按预期保留其先前的值。不幸的是,由于
DependencyProperty
尚未更改,因此绑定不会更新视图模型中的源值。

取消属性值更改时如何用控件中的值更新视图模型中的属性?

class ViewModel : INotifyPropertyChanged
{
    public int? SelectedValue
    {
        get { return _selectedValue; }
        set
        {
            _selectedValue = value;
            RaisePropertyChanged("SelectedValue");
        }
    }
    private int? _selectedValue;

    public event PropertyChangedEventHandler PropertyChanged;
    private void RaisePropertyChanged(string name)
    {
        var propertyChanged = PropertyChanged;
        if (propertyChanged != null)
        {
            propertyChanged(this, new PropertyChangedEventArgs(name));
        }
    }
}

class GridView : FrameworkElement
{
    public object SelectedValue
    {
        get { return (object)GetValue(SelectedValueProperty); }
        set { SetValue(SelectedValueProperty, value); }
    }

    public static readonly DependencyProperty SelectedValueProperty =
        DependencyProperty.Register("SelectedValue", typeof(object), typeof(GridView),
            new PropertyMetadata(null, null, new CoerceValueCallback(CoerceValue)));

    private static object CoerceValue(DependencyObject d, object baseValue)
    {
        // forbid value 42 and return previous value
        if (Equals(baseValue, 42))
        {
            return d.GetValue(GridView.SelectedValueProperty);
        }
        return baseValue;
    }
}

    [STAThread]
    static void Main()
    {
        ViewModel vm = new ViewModel();
        GridView grid = new GridView();
        Binding binding =  new Binding("SelectedValue") { Source = vm, Mode = BindingMode.TwoWay };
        grid.SetBinding(GridView.SelectedValueProperty, binding);

        vm.SelectedValue = 12;
        Console.WriteLine("should be 12: {0} = {1}", grid.SelectedValue, vm.SelectedValue);
        grid.SelectedValue = 23;
        Console.WriteLine("should be 23: {0} = {1}", grid.SelectedValue, vm.SelectedValue);
        vm.SelectedValue = 42;
        Console.WriteLine("should still be 23: {0} = {1}", grid.SelectedValue, vm.SelectedValue);
    }

我已经尝试过

    var binding = BindingOperations.GetBindingExpression(d,
                      GridView.SelectedValueProperty);
    binding.UpdateSource();

CoerceValue
方法中无济于事。我什至用
Dispatcher.CurrentDispatcher.BeginInvoke(updateSource, DispatcherPriority.DataBind);
尝试了前两行,这是在类似的堆栈溢出问题中建议的。

还有其他想法吗?

更新

@Berryl 建议我应该在视图模型中执行该逻辑,我同意他的观点......只是我不能。我将解释我在上面的代码中尝试简化的完整用例。

GridView 不是我的控件,而是我继承的第 3 方数据网格。当用户编辑记录时,我们会弹出一个窗口,用户在其中编辑记录,然后视图模型刷新数据并使用其 ID 重新选择旧记录。一切工作正常,直到用户使用网格的过滤功能。如果对记录的编辑会因过滤器而隐藏该行,则会发生以下情况:

  1. ValueList
    属性中查看模型集
  2. 网格读取新列表
  3. 网格过滤列表
  4. 视图模型使用
    SelectedValue
     中的记录设置 
    ValueList
  5. 属性
  6. 网格读取新的
    SelectedValue
    但将其丢弃,因为它已被过滤掉
  7. 网格的 SelectedValue = 23,视图模型的 SelectedValue = 42
  8. 用户双击所选行(他看到 23)来编辑记录
  9. 映射到双击的
    ICommand
    在视图模型中被调用
  10. 视图模型启动编辑窗口,其中包含来自其
    SelectedValue
    属性的记录
  11. 用户编辑了记录 42,但认为它是 23
wpf mvvm binding
2个回答
1
投票

我在很久以前的电子邮件中发现了这个解决方案,并认为我应该分享它(进行一些清理)。这需要当时致电 Telerik 和 Microsoft 提供支持。

基本上,我无法从 CoerceValueCallback 取消属性更新的原因是:

  1. 在依赖属性的逻辑中强制值之前更新绑定
  2. CoerceValueCallback 的这种行为是经过设计考虑的

现在是关于如何修复它的长答案:

static GridView()
{
    SelectedItemProperty.OverrideMetadata(typeof(GridView), new FrameworkPropertyMetadata(null, CoerceSelectedItemProperty));
}

// Rewriting SelectedItem coercion logic because of the following issue (we had support calls to both Telerik and Microsoft)
// When
//   - a filter is applied on the grid
//   - user refreshes after modifying a record
//   - the record he has changed would be filtered out by the the grid filter
//   - the view model sets the SelectedItem to the modified record (which is hidden now)
//
// Telerik's coercion method resets the selected item to a visible row but because WPF's
// binding occurs before the coercion method, the view model is not updated.
// Even a call to UpdateSource() on the binding does not work in this case because the binding
// is already updating the target while this happens so it does nothing when you call it.
private static object CoerceSelectedItemProperty(DependencyObject d, object baseValue)
{
    // Call normal Coercion method because we don't want to rewrite Telerik's logic
    // and keep result to return it at the end.
    object returnValue = SelectedItemProperty.GetMetadata(typeof(RadGridView)).CoerceValueCallback(d, baseValue);
    var control = (GridView)d;

    // If coerce returned something other than DependencyProperty.UnsetValue we can use it to push it back to
    // the binding source because it is of the right type and the right value.
    // The only case when we can use control.SelectedItem is when coerce returned UnsetValue otherwise the
    // view model is always one click late.
    object officialValue = returnValue == DependencyProperty.UnsetValue
                                ? control.SelectedItem
                                : returnValue;

    var binding = control.GetBindingExpression(SelectedItemProperty);
    var source = binding.ResolvedSource;
    var property = source.GetType().GetProperty(binding.ResolvedSourcePropertyName);
    property.SetValue(source, officialValue, null);
    return returnValue;
}

0
投票

我在这里发布了类似问题的解决方案:使用不同的 vlaue 从目标更新源

简短:神奇的线条是

await Task.Yield();

使用此功能,您可以暂时将控制权交还给 UI 线程,使其能够处理挂起的更新,例如重绘 UI 或处理由 INotifyPropertyChanged 触发的属性更改

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