Windows Presentation Foundation
02May
LostFocus (TextBox) vs. Buttons IsDefault-Property
If you bind the Text-Property of a TextBox to something, the "something" is updated when the TextBox loses focus. This is the Default-UpdateSourceTrigger defined in the Metadata for the TextBox.TextProperty. In a Data Binding you can specify another UpdateSourceTrigger, like e.g. PropertyChanged.
If a TextBox has LostFocus as UpdateSourceTrigger, which is the default, you can get problems if the TextBox is inside of a Dialog that contains a OK-Button with the IsDefault-Property set to true. When the TextBox has the Focus, the user can press Enter and the Button is clicked, but the TextBox still keeps the Focus. The LostFocus-Event doesn't occur, so the Source of a Data Binding isn't updated with the value of the TextBoxs Text-Property. You need to update the source explicit. Let's take an example and use the Person-class of my previous post:
For such a scenario, you have to update the source explicit. You could do this in the Click-Eventhandler. To update the source, you have to get the BindingExpression for the TextProperty. You get the BindingExpression by calling the GetBindingExpression-Method on the TextBox. Pass in the Text-Property as parameter. On the BindingExpression call the UpdateSource-Method to update the Person-instance explicitly with the value defined in the Text-Property of the TextBox. With the Eventhandler below the Person-Instance would always be actualized when the Button is clicked. The MessageBox would display the right value:
public class Person : INotifyPropertyChanged
{
private string _name;
public string Name
{
get { return _name; }
set
{
_name = value;
if (PropertyChanged != null)
PropertyChanged(this,
new PropertyChangedEventArgs("Name"));
}
}
public event PropertyChangedEventHandler PropertyChanged;
}<Window x:Class="LostFocusTest.Window1" ...
xmlns:local="clr-namespace:LostFocusTest"
Title="Focus/IsDefault" Width="200" SizeToContent="Height">
<StackPanel>
<TextBox Margin="5" Text="{Binding Name}"/>
<Button Content="OK" IsDefault="True"
Click="Button_Click"/>
</StackPanel>
</Window>public partial class Window1 : Window
{
private Person _pers;
public Window1()
{
InitializeComponent();
_pers = new Person();
_pers.Name = "Thomas";
this.DataContext = _pers;
}
private void Button_Click(object sender, RoutedEventArgs e)
{
MessageBox.Show(_pers.Name);
}
}private void Button_Click(object sender, RoutedEventArgs e)
{
TextBox textBox = Keyboard.FocusedElement as TextBox;
if (textBox != null)
{
BindingExpression be =
textBox.GetBindingExpression(TextBox.TextProperty);
if (be != null)
be.UpdateSource();
}
MessageBox.Show(_pers.Name);
}