Silverlight：Combobox中的默認值

Question

我想在組合框中顯示默認文本。例如，「選擇一個人」的消息。你能幫我解決嗎？

請注意，我使用數據綁定從domaincontext

謝謝!!

Answer 1

爲了達到這個目的，我使用了派生的ExtendedComboBox類，它擴展了內置的ComboBox類。你可以在my blog post或以下找到這個類的源代碼。

後你這個類添加到您的項目，您可以使用此XAML代碼顯示默認值：

<local:ExtendedComboBox ItemsSource="{Binding ...Whatever...}" NotSelectedText="Select item..." /> 

而且，這裏是test page與此控件。我認爲第二個組合框就是你需要的。

這個類的全碼：

[TemplateVisualState(Name = ExtendedComboBox.StateNormal, GroupName = ExtendedComboBox.GroupItemsSource)] 
[TemplateVisualState(Name = ExtendedComboBox.StateNotSelected, GroupName = ExtendedComboBox.GroupItemsSource)] 
[TemplateVisualState(Name = ExtendedComboBox.StateEmpty, GroupName = ExtendedComboBox.GroupItemsSource)] 
public class ExtendedComboBox : ComboBox 
{ 
    public const string GroupItemsSource = "ItemsSourceStates"; 
    public const string StateNormal = "Normal"; 
    public const string StateNotSelected = "NotSelected"; 
    public const string StateEmpty = "Empty"; 

    private ContentPresenter selectedContent; 

    public ExtendedComboBox() 
    { 
     this.DefaultStyleKey = typeof(ComboBox); 
    } 

    public override void OnApplyTemplate() 
    { 
     base.OnApplyTemplate(); 
     this.selectedContent = this.GetTemplateChild("ContentPresenter") as ContentPresenter; 

     // This event can change the NotSelected state 
     this.SelectionChanged += (s, e) => this.SetTextIfEmpty(); 

     // Set a state at start 
     this.SetTextIfEmpty(); 
    } 

    // This method can change the Empty state 
    protected override void OnItemsChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e) 
    { 
     base.OnItemsChanged(e); 
     this.SetTextIfEmpty(); 
    } 

    /// <summary> 
    /// Text if the SelectedItem property is null. 
    /// </summary> 
    public string NotSelectedText 
    { 
     get { return (string)GetValue(NotSelectedTextProperty); } 
     set { SetValue(NotSelectedTextProperty, value); } 
    } 

    public static readonly DependencyProperty NotSelectedTextProperty = 
     DependencyProperty.Register("NotSelectedText", typeof(string), typeof(ExtendedComboBox), new PropertyMetadata(" ")); 

    /// <summary> 
    /// Text if there are no items in the ComboBox at all. 
    /// </summary> 
    public string EmptyText 
    { 
     get { return (string)GetValue(EmptyTextProperty); } 
     set { SetValue(EmptyTextProperty, value); } 
    } 

    public static readonly DependencyProperty EmptyTextProperty = 
     DependencyProperty.Register("EmptyText", typeof(string), typeof(ExtendedComboBox), new PropertyMetadata(null)); 

    /// <summary> 
    /// Changes the state of this control and updates the displayed text. 
    /// </summary> 
    protected void SetTextIfEmpty() 
    { 
     if (this.selectedContent == null || !(this.selectedContent.Content is TextBlock)) 
      return; 
     var text = this.selectedContent.Content as TextBlock; 

     if (this.SelectedItem == null && this.Items != null && this.Items.Count > 0) 
     { 
      text.Text = this.NotSelectedText; 
      VisualStateManager.GoToState(this, ExtendedComboBox.StateNotSelected, true); 
     } 
     else if (this.SelectedItem == null) 
     { 
      text.Text = this.EmptyText ?? this.NotSelectedText; 
      VisualStateManager.GoToState(this, ExtendedComboBox.StateEmpty, true); 
     } 
     else VisualStateManager.GoToState(this, ExtendedComboBox.StateNormal, true); 
    } 
} 

Answer 2

只是這樣做：

theComboBox.SelectedItem = yourDataItem; 

另外，設置所選指數：

theComboBox.SelectedIndex = 0; 

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編輯

如果的ItemSource綁定，要覆蓋組合的DataContextChanged和然後使用上述其中一行設置索引/選定項目。

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但是，如果你不想默認的文本可以進行選擇，你會做沿着this線的東西。