Keyevents-DataBinding

Question

Hi,
<TextBox Height="23" HorizontalAlignment="Left" Margin="120,122,0,0" Name="PostCode" VerticalAlignment="Top" Width="120" Text="{Binding Path=PostCode}" Keyboard.GotKeyboardFocus="{Binding
Path=ChangeStatus}"/>
      I have a textbox and i want to bind textbox's GotKeyboardFocus to a function(Command/Function) in a viewmodel?Can you let me know how can i do this? As when i try to bind it to a command i got the follwoing error
Error    1    GotKeyboardFocus="{Binding Path=ChangeStatus}" is not valid. '{Binding Path=ChangeStatus}' is not a valid event handler method name. Only instance methods on the generated or code-behind class are valid.
Line 32 Position 161.    C:\senthil-code\Prismtwopage\ModuleTwo\three.xaml    32    161    ModuleTwo
Thanks,
Senthil

Answer 1

Hi Senthil,
In MVVM pattern, if want to add event handler for GotKeyboardFocus
event, we can use Expression Blend interaction triggers and the System.Windows.Interactivity.InvokeCommandAction
class.
Firstly, we need to add a reference to System.Windows.Interactivity.dll
Then, we can create a class called RelayCommand which implements ICommand interface. This class acts as Enhancement for the ICommad and extracts the boilerplate code to separate class:
public class RelayCommand : ICommand
private Action<object> execute;
private Predicate<object> canExecute;
private event EventHandler CanExecuteChangedInternal;
public RelayCommand(Action<object> execute)
: this(execute, DefaultCanExecute)
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
if (execute == null)
throw new ArgumentNullException("execute");
if (canExecute == null)
throw new ArgumentNullException("canExecute");
this.execute = execute;
this.canExecute = canExecute;
public event EventHandler CanExecuteChanged
add
CommandManager.RequerySuggested += value;
this.CanExecuteChangedInternal += value;
remove
CommandManager.RequerySuggested -= value;
this.CanExecuteChangedInternal -= value;
public bool CanExecute(object parameter)
return this.canExecute != null && this.canExecute(parameter);
public void Execute(object parameter)
this.execute(parameter);
public void OnCanExecuteChanged()
EventHandler handler = this.CanExecuteChangedInternal;
if (handler != null)
//DispatcherHelper.BeginInvokeOnUIThread(() => handler.Invoke(this, EventArgs.Empty));
handler.Invoke(this, EventArgs.Empty);
public void Destroy()
this.canExecute = _ => false;
this.execute = _ => { return; };
private static bool DefaultCanExecute(object parameter)
return true;
ViewModel:
public class MainWindowViewModel
private ICommand gotFocusExecuteCommand { get; set; }
private bool canExecute = true;
public bool CanExecute
get
return this.canExecute;
set
if (this.canExecute == value)
return;
this.canExecute = value;
public ICommand GotFocusExecuteCommand
get
return gotFocusExecuteCommand;
set
gotFocusExecuteCommand = value;
public MainWindowViewModel()
gotFocusExecuteCommand = new RelayCommand(TextBox_GotKeyboardFocus, param => this.canExecute);
private void TextBox_GotKeyboardFocus(object obj)
MessageBox.Show("Here");
public void ChangeCanExecute(object obj)
canExecute = !canExecute;
XAML:
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
xmlns:local="clr-namespace:WpfTextBoxMVVM.ViewModel"
Title="MainWindow" Height="150" Width="525">
<Window.DataContext>
<local:MainWindowViewModel />
</Window.DataContext>
<StackPanel>
<TextBox Text="textBox1" Height="23" Width="120"/>
<TextBox Height="23" Name="textBox2" VerticalAlignment="Top" Width="120" Text="TextBox2" >
<i:Interaction.Triggers>
<i:EventTrigger EventName="GotKeyboardFocus" >
<i:InvokeCommandAction Command="{Binding GotFocusExecuteCommand}" />
</i:EventTrigger>
</i:Interaction.Triggers>
</TextBox>
</StackPanel>
We are trying to better understand customer views on social support experience, so your participation in this interview project would be greatly appreciated if you have time. Thanks for helping make community forums a great place.
Click
HERE to participate the survey.