收集更改后，组合框丢失选择 [英] Combo-box loses selection after collection changes

问题描述

我有一个WPF组合框绑定到一个obvserable集合（OC）：

I have a WPF combo box bound to an obvserable collection (OC):

    <ComboBox Name="cbCombination" ItemsSource="{Binding Combinations}" 
              SelectedIndex="0" />

在其他地方，在设置为数据上下文的对象中：

Elsewhere, in the object set as data context:

    public ObservableCollection<Combination> Combinations { get; set; }

组合覆盖其ToString，一切都是peachy：组合框的下拉菜单显示所有组合项在组合OC。组合框的选择框显示第一个组合的值。

Combination overrides its ToString and everything is peachy: The combo-box's drop-down displays all Combination items in the Combinations OC. The combo-box's selection box displays the value of the first Combination.

现在，数据上下文对象必须更改它的组合OC：

Now, the data-context object has to change the values in its Combinations OC:

    var combinationsList = CombinationsManager.CombinationsFor(someParam);
    this.Combinations.Clear();
    foreach (var combination in combinationsList)
        this.Combinations.Add(combination);
    NotifyPropertyChanged(@"Combinations");

这会导致组合框的选择框显示一个空字符串。 （下拉菜单已关闭，但当我将其下拉时，会显示正确的新组合，因此已绑定到更新的集合）。

This causes the combo-box's selection box shows an empty string. (The drop-down is closed. However, when I make it drop down, it does show the correct new Combinations, so it is bound to the updated collection).

我试图捕获SourceUpdated和（在我的配置中）TargetUpdated事件（设置SelectedIndex在那里），但我的事件处理程序没有调用！

I tried to capture both SourceUpdated and (in my dispair) TargetUpdated events (thining of setting the SelectedIndex there), but my event handlers didn't get called!

所以我的问题是：当一个WPF ComboBox被绑定的observable集合改变时，如何刷新它的选择框的值？

更新：

我完全忘记了，我不知道是否重要，但组合框在UserControl内。 UserControl的XAML如下所示：

I've totally forgotten to mention, and I don't know whether it's important, but the combo-box is within a UserControl. The UserControl's XAML looks like this:

<UserControl x:Class="...CombinationsToolBar"
         .... mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">
<ToolBarTray Name="toolBarTray1" >
    <ToolBar Name="toolBar1">
        <ComboBox Name="cbCombination" 
         ItemsSource="{Binding Path=Combinations, NotifyOnSourceUpdated=True, 
                                                                     IsAsync=True}"
            SelectedIndex="0" IsEditable="False"  
            SelectionChanged="CbCombinationSelectionChanged"
            SourceUpdated="CbCombinationSourceUpdated"
            TargetUpdated="CbCombinationTargetUpdated"></ComboBox>
    </ToolBar>
</ToolBarTray>

在UserControl代码我有CbCombinationSelectionChanged，CbCombinationSourceUpdated和CbCombinationTargetUpdated的断点。

In the UserControl's code I have breakpoints on CbCombinationSelectionChanged, CbCombinationSourceUpdated and CbCombinationTargetUpdated.

当首次加载包含用户控件的表单时，CbCombinationSelectionChanged触发一次。

The CbCombinationSelectionChanged fires once when the form containing the user control is first loaded. It is indeed called a second time when the Combinations collection is cleared, as @asktomsk said.

源更新和目标更新不会被触发 - CbCombinationSourceUpdated和CbCombinationTargetUpdated不会被调用。

The source updated and target updated are not triggered - CbCombinationSourceUpdated and CbCombinationTargetUpdated are not called.

由于组合框位于用户控件内，并且组合集合在视图模型中，并且由于视图模型无法访问组合框，因此

As the combo box is inside a usercontrol and the Combinations collection is within the view model, and as the view model doesn't have access to the combo box, I have no opportunity to set the selected index of the combo unless the events fire.

：（

推荐答案

问题出现在 this.Combinations.Clear（）;

it，它设置 SelectedItem = null 和 SelectedIndex = -1

所以你应该重新设置选择。如果你有权访问你的ComboBox，然后在填充组合列表后写入 cbCombination.SelectedIndex = 0; 。

当然你也可以将SelectedItem / SelectedIndex绑定到代码中的某些属性。

So you should set selection again. If you have an access to your ComboBox then just write this cbCombination.SelectedIndex = 0; after filling Combinations list.
Of course you can bind SelectedItem/SelectedIndex to some property in code behind too.

BTW

也不需要调用 已经实现 INotifyPropertyChanged 填充集合后，code> NotifyPropertyChanged（@Combinations code>。

BTW
Also it is not required to call NotifyPropertyChanged(@"Combinations"); after filling the collection because Combinations already implements INotifyPropertyChanged.

更新

> ObservableCollection 已更改，请在您的 UserControl 代码中订阅 CollectionChanged

To detect that your ObservableCollection was changed, subscribe to CollectionChanged event in your UserControl code behind. Make sure that you subscribed before collection changed!

Combinations.CollectionChanged += (s, e) =>
{
    if (cbCombination.Items.Count > 0)
        cbCombination.SelectedIndex = 0;
};

另一个建议

当你不需要一个更聪明的逻辑，而不仅仅是在组合框中选择零索引时，上述方法是有效的。
但是通常你需要一个更复杂的逻辑来选择一些项目。在这种情况下，您可以向模型中添加新属性：

Approach above is works when you do not need a smarter logic than just select zero index in combobox. But often you will need a more complex logic to select some item. In this case you may add new property to your model:

public Combination SelectedCombination
{
    get{ return _selectedCombination; }
    set
    {
        _selectedCombination = value;
        NotifyPropertyChanged("SelectedCombination");
    }
}

并将此属性绑定到您的组合框：

and bind this property to your combobox:

<ComboBox Name="cbCombination" ItemsSource="{Binding Combinations}" 
          SelectedIndex="0" SelectedItem={Bindings SelectedCombination} />

在这种情况下，您可以在填充Combinations集合时选择任何项目，并且将在组合框中自动选择：

In this case you can select any item when filling the Combinations collection and it will be automatically selected in combobox:

var combinationsList = CombinationsManager.CombinationsFor(someParam);
this.Combinations.Clear();
foreach (var combination in combinationsList)
    this.Combinations.Add(combination);

if (Combinations.Count > 0)
    SelectedCombination = Combinations[0];

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