Handle ComboBox TextChanged/DropDownOpened in ViewModel

Question

I want to handle the DropDownOpened event of ComboBox in my ViewModel. How can I do that.

Basically my ItemSource of Comboxbox is bound to a collection. This collection can change, I do not know when it changes. So what I want to do is, I want to repopulate ComboBox every time User Clicks on the combobox (to open the dropdown). How can I do that in ViewModel.

Many Thanks in advance.

`collection can change, I do not know when it changes. ` The collection has an event [`CollectionChanged`](http://msdn.microsoft.com/en-us/library/system.collections.specialized.inotifycollectionchanged.aspx). [Sample](http://stackoverflow.com/questions/4587448/collectionchanged-sample). This is what you need? — Anatoliy Nikolaev, Aug 15 '13 at 11:50

score 0 · Accepted Answer · answered Aug 15 '13 at 12:06

In my opinion, the best way to handle UI events in WPF using view models is to implement Attached Properties. Here is an example that I use to monitor the LostFocus event... instead of passing the EventArgs object through to the view model, I am exposing a basic instance of ICommand (which you can implement in your view model) that will be called when the focus is lost from the relating TextBox:

public static DependencyProperty OnLostFocusProperty = DependencyProperty.
RegisterAttached("OnLostFocus", typeof(ICommand), typeof(TextBoxProperties), new 
UIPropertyMetadata(null, OnLostFocusChanged));

public static ICommand GetOnLostFocus(DependencyObject dependencyObject)
{
    return (ICommand)dependencyObject.GetValue(OnLostFocusProperty);
}

public static void SetOnLostFocus(DependencyObject dependencyObject, ICommand value)
{
    dependencyObject.SetValue(OnLostFocusProperty, value);
}

public static void OnLostFocusChanged(DependencyObject dependencyObject, 
DependencyPropertyChangedEventArgs e)
{
    TextBox textBox = dependencyObject as TextBox;
    if (e.OldValue == null && e.NewValue != null) textBox.LostFocus += TextBox_LostFocus;
    else if (e.OldValue != null && e.NewValue == null) textBox.LostFocus -= TextBox_LostFocus;
}

private static void TextBox_LostFocus(object sender, RoutedEventArgs e)
{
    TextBox textBox = sender as TextBox;
    ICommand command = GetOnLostFocus(textBox);
    if (command != null && command.CanExecute(textBox)) command.Execute(textBox);
    e.Handled = false;
}

You would use it like this:

<TextBox Text="{Binding SomeValue}" Attached:TextBoxProperties.OnLostFocus="{Binding 
YourCommandName}" />

I'm sure that you can make a few changes to be able to apply it to your situation. If you are not familiar with Attached Properties, please view the Attached Properties Overview page at MSDN.

Handle ComboBox TextChanged/DropDownOpened in ViewModel

1 Answers1