可编辑组合框绑定和更新源触发器

Question

要求

我想要 ComboBox 用户可以在其中输入一些文本或从下拉列表中选择文本。当用户在键入后按 Enter 或当项目只是从下拉列表中 selected 时，应该更新绑定源（在我的情况下最好的视图行为）。

问题

当设置UpdateSourceTrigger=PropertyChange（默认）时，源更新将在每个字符后触发，这不是很好，因为属性 setter 调用是昂贵的；
设置UpdateSourceTrigger=LostFocus后，select从下拉列表中选择项目将需要更多操作才能真正失去焦点，这对用户来说不是很友好（点击后需要额外点击到 select 项）。

我尝试使用 UpdateSourceTrigger=Explicit，但效果不佳：

<ComboBox IsEditable="True" VerticalAlignment="Top" ItemsSource="{Binding List}"
          Text="{Binding Text, UpdateSourceTrigger=LostFocus}"
          SelectionChanged="ComboBox_SelectionChanged"
          PreviewKeyDown="ComboBox_PreviewKeyDown" LostFocus="ComboBox_LostFocus"/>

public partial class MainWindow : Window
{
    private string _text = "Test";
    public string Text
    {
        get { return _text; }
        set
        {
            if (_text != value)
            {
                _text = value;
                MessageBox.Show(value);
            }
        }
    }

    public string[] List
    {
        get { return new[] { "Test", "AnotherTest" }; }
    }

    public MainWindow()
    {
        InitializeComponent();
        DataContext = this;
    }

    private void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (e.AddedItems.Count > 0)
            ((ComboBox)sender).GetBindingExpression(ComboBox.TextProperty).UpdateSource();
    }

    private void ComboBox_PreviewKeyDown(object sender, KeyEventArgs e)
    {
        if(e.Key == Key.Enter)
            ((ComboBox)sender).GetBindingExpression(ComboBox.TextProperty).UpdateSource();
    }

    private void ComboBox_LostFocus(object sender, RoutedEventArgs e)
    {
        ((ComboBox)sender).GetBindingExpression(ComboBox.TextProperty).UpdateSource();
    }

}

此代码有 2 个问题：

当从下拉菜单中 selected 项目时，源会更新为之前 selected 值，为什么？
当用户开始输入内容然后单击下拉按钮从列表中选择内容时 - 源再次更新（由于失去焦点？），如何避免这种情况？

我有点害怕陷入困境 XY problem 所以我发布了原始需求（也许我走错方向了？）而不是要求帮助我解决上述问题之一。

Answer 1

我建议保留 UpdateSourceTrigger=PropertyChanged，并延迟组合框以帮助缓解昂贵的 setter/update 问题。延迟将导致 PropertyChanged 事件在触发前等待您指定的毫秒数。

更多关于延迟的信息：http://www.jonathanantoine.com/2011/09/21/wpf-4-5-part-4-the-new-bindings-delay-property/

希望有人会为您提出更好的解决方案，但这至少应该让您暂时继续前进。

Answer 2

您根据特定事件更新源代码的方法是正确的，但是 ComboBox 更新内容的方式还有更多需要考虑的地方。此外，您可能希望将 UpdateSourceTrigger 设置为 LostFocus，这样您就没有太多的更新案例需要处理。

您还应该考虑将代码移动到可重复使用的附件属性，以便将来可以将其应用于其他地方的组合框。正好我以前也创造过这样的属性

/// <summary>
/// Attached properties for use with combo boxes
/// </summary>
public static class ComboBoxBehaviors
{
    private static bool sInSelectionChange;

    /// <summary>
    /// Whether the combo box should commit changes to its Text property when the Enter key is pressed
    /// </summary>
    public static readonly DependencyProperty CommitOnEnterProperty = DependencyProperty.RegisterAttached("CommitOnEnter", typeof(bool), typeof(ComboBoxBehaviors),
        new PropertyMetadata(false, OnCommitOnEnterChanged));

    /// <summary>
    /// Returns the value of the CommitOnEnter property for the specified ComboBox
    /// </summary>
    public static bool GetCommitOnEnter(ComboBox control)
    {
        return (bool)control.GetValue(CommitOnEnterProperty);
    }

    /// <summary>
    /// Sets the value of the CommitOnEnterProperty for the specified ComboBox
    /// </summary>
    public static void SetCommitOnEnter(ComboBox control, bool value)
    {
        control.SetValue(CommitOnEnterProperty, value);
    }

    /// <summary>
    /// Called when the value of the CommitOnEnter property changes for a given ComboBox
    /// </summary>
    private static void OnCommitOnEnterChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        ComboBox control = sender as ComboBox;
        if (control != null)
        {
            if ((bool)e.OldValue)
            {
                control.KeyUp -= ComboBox_KeyUp;
                control.SelectionChanged -= ComboBox_SelectionChanged;
            }
            if ((bool)e.NewValue)
            {
                control.KeyUp += ComboBox_KeyUp;
                control.SelectionChanged += ComboBox_SelectionChanged;
            }
        }
    }

    /// <summary>
    /// Handler for the KeyUp event attached to a ComboBox that has CommitOnEnter set to true
    /// </summary>
    private static void ComboBox_KeyUp(object sender, KeyEventArgs e)
    {
        ComboBox control = sender as ComboBox;
        if (control != null && e.Key == Key.Enter)
        {
            BindingExpression expression = control.GetBindingExpression(ComboBox.TextProperty);
            if (expression != null)
            {
                expression.UpdateSource();
            }
            e.Handled = true;
        }
    }

    /// <summary>
    /// Handler for the SelectionChanged event attached to a ComboBox that has CommitOnEnter set to true
    /// </summary>
    private static void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (!sInSelectionChange)
        {
            var descriptor = DependencyPropertyDescriptor.FromProperty(ComboBox.TextProperty, typeof(ComboBox));
            descriptor.AddValueChanged(sender, ComboBox_TextChanged);
            sInSelectionChange = true;
        }
    }

    /// <summary>
    /// Handler for the Text property changing as a result of selection changing in a ComboBox that has CommitOnEnter set to true
    /// </summary>
    private static void ComboBox_TextChanged(object sender, EventArgs e)
    {
        var descriptor = DependencyPropertyDescriptor.FromProperty(ComboBox.TextProperty, typeof(ComboBox));
        descriptor.RemoveValueChanged(sender, ComboBox_TextChanged);

        ComboBox control = sender as ComboBox;
        if (control != null && sInSelectionChange)
        {
            sInSelectionChange = false;

            if (control.IsDropDownOpen)
            {
                BindingExpression expression = control.GetBindingExpression(ComboBox.TextProperty);
                if (expression != null)
                {
                    expression.UpdateSource();
                }
            }
        }
    }
}

这里是在xaml中设置属性的例子：

<ComboBox IsEditable="True" ItemsSource="{Binding Items}" Text="{Binding SelectedItem, UpdateSourceTrigger=LostFocus}" local:ComboBoxBehaviors.CommitOnEnter="true" />

我认为这将为您提供所需的行为。您可以按原样随意使用它，也可以根据自己的喜好对其进行修改。

行为实现存在一个问题，如果您开始键入一个现有值（并且不按回车键），然后从下拉列表中选择相同的值，在这种情况下源不会更新，直到您按 enter、更改焦点或选择不同的值。我确信这可以解决，但我花时间解决这个问题还不够，因为它不是正常的工作流程。

Answer 3

我有一个类似的问题，我在 .cs 代码中处理了它。这不是 XAML 方式，但它完成了。首先，我打破了绑定，然后手动将值双向传播。

<ComboBox x:Name="Combo_MyValue" 
                      ItemsSource="{Binding Source={StaticResource ListData}, XPath=MyContextType/MyValueType}"
                      DisplayMemberPath="@Description"
                      SelectedValuePath="@Value"
                      IsEditable="True"
                      Loaded="Combo_MyValue_Loaded"
                      SelectionChanged = "Combo_MyValue_SelectionChanged"
                      LostFocus="Combo_MyValue_LostFocus"
                      />


    private void Combo_MyValue_Loaded(object sender, RoutedEventArgs e)
    {
        if (DataContext != null)
        {
            Combo_MyValue.SelectedValue = ((MyContextType)DataContext).MyValue;
        }
    }

    private void Combo_MyValue_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if( e.AddedItems.Count == 0)
        {
            // this is a custom value, we'll set it in the lost focus event
            return;
        }
        // this is a picklist value, get the value from itemsource
        XmlElement selectedItem = (XmlElement)e.AddedItems[0];
        string selectedValue = selectedItem.GetAttribute("Value");
        ((PumpParameters)DataContext).MyValue = selectedValue;
    }

    private void Combo_MyValue_LostFocus(object sender, RoutedEventArgs e)
    {
        if( Combo_MyValue.IsDropDownOpen || Combo_MyValue.SelectedIndex != -1)
        {
            // not a custom value
            return; 
        }
        // custom value
        ((MyContextType)DataContext).MyValue = Combo_MyValue.Text;
    }

Answer 4

我遇到了同样的问题。我对 ComboBox.Text 属性进行了绑定，包括 ValidationRules。如果在从列表中选择某些内容时立即更新源，这似乎是更好的用户体验，但如果在框中输入内容，那么我不希望在输入完成之前进行验证。

通过让绑定的 UpdateSourceTrigger="LostFocus"，我得到了一个令人满意的解决方案。我创建了一个附加行为，它在发布 SelectionChanged 事件时强制更新绑定源（在 TextBox 中键入时不发布）。如果您愿意，可以将此事件处理程序放入代码隐藏而不是附加行为或附加属性 class.

protected void ComboBox_SelectionChanged(Object sender, SelectionChangedEventArgs e)
{
    // Get the BindingExpression object for the ComboBox.Text property.
    // We'll use this to force the value of ComboBox.Text to update to the binding source
    var be = BindingOperations.GetBindingExpression(comboBox, ComboBox.TextProperty);
    if (be == null) return;
    // Unfortunately, the code of the ComboBox class publishes the SelectionChanged event
    // immediately *before* it transfers the value of the SelectedItem to its Text property.
    // Therefore, the ComboBox.Text property does not yet have the value
    // that we want to transfer to the binding source.  We use reflection to invoke method
    // ComboBox.SelectedItemUpdated to force the update to the Text property just a bit early.
    // Method SelectedItemUpdated encapsulates everything that we need--it is exactly what
    // happens from method ComboBox.OnSelectionChanged.
    var method = typeof(ComboBox).GetMethod("SelectedItemUpdated",
                                    BindingFlags.NonPublic | BindingFlags.Instance);
    if (method == null) return;
    method.Invoke(comboBox, new Object[] { });
    // Now that ComboBox.Text has the proper value, we let the binding object update
    // its source.
    be.UpdateSource();
}

可编辑组合框绑定和更新源触发器

Editable ComboBox binding and update source trigger

c#

wpf

binding

combobox

要求

问题