MVVM, Data binding command - silverlight

Question

I've started using databinding with MVVM pattern in Silverlight. There's one problem for me.
I have a listbox, when I select an item there one property in the View-Model have to change it's value. I implemented DelegateCommand and made custom CommandListBox class as follows
public class DelegateCommand<T> : ICommand
{
private readonly Action<T> _execute;
private readonly Predicate<T> _canExecute;
public DelegateCommand(Action<T> execute)
: this(execute, x => true)
{
}
public DelegateCommand(Action<T> execute, Predicate<T> canExecute)
{
if (canExecute == null) throw new ArgumentNullException("canExecute");
if (execute == null) throw new ArgumentNullException("execute");
_execute = execute;
_canExecute = canExecute;
}
public void Execute(object parameter)
{
_execute((T)parameter);
}
public bool CanExecute(object parameter)
{
return _canExecute((T)parameter);
}
public event EventHandler CanExecuteChanged;
public void RaiseCanExecuteChanged()
{
CanExecuteChanged.DynamicInvoke(this);//.Raise(this);
}
}
public class DelegateCommand : DelegateCommand<object>
{
public DelegateCommand(Action execute)
: base(execute != null ? x => execute() : (Action<object>)null)
{
}
public DelegateCommand(Action execute, Func<bool> canExecute)
: base(execute != null ? x => execute() : (Action<object>)null,
canExecute != null ? x => canExecute() : (Predicate<object>)null)
{
}
}
public class CommandListBox : ListBox
{
public CommandListBox()
{
SelectionChanged += (sender, e) =>
{
if (Command != null && Command.CanExecute(CommandParameter))
Command.Execute(CommandParameter);
};
}
public static DependencyProperty CommandProperty =
DependencyProperty.Register("Command",
typeof(ICommand), typeof(CommandListBox),
new PropertyMetadata(null, CommandChanged));
private static void CommandChanged(DependencyObject source, DependencyPropertyChangedEventArgs args)
{
var treeList = source as CommandListBox;
if (treeList == null) return;
treeList.RegisterCommand(args.OldValue as ICommand, args.NewValue as ICommand);
}
private void RegisterCommand(ICommand oldCommand, ICommand newCommand)
{
if (oldCommand != null)
oldCommand.CanExecuteChanged -= HandleCanExecuteChanged;
if (newCommand != null)
newCommand.CanExecuteChanged += HandleCanExecuteChanged;
HandleCanExecuteChanged(newCommand, EventArgs.Empty);
}
private void HandleCanExecuteChanged(object sender, EventArgs args)
{
if (Command != null)
IsEnabled = Command.CanExecute(CommandParameter);
}
public ICommand Command
{
get { return GetValue(CommandProperty) as ICommand; }
set { SetValue(CommandProperty, value); }
}
public static DependencyProperty CommandParameterProperty =
DependencyProperty.Register("CommandParameter",
typeof(object), typeof(CommandListBox),
new PropertyMetadata(null));
public object CommandParameter
{
get { return GetValue(CommandParameterProperty); }
set { SetValue(CommandParameterProperty, value); }
}
}
I'm catching the event, when the selected item in listbox is changed but I don't know how to take it's value.
Could you help me please?

Answer 1

Your CommandListBox derives from a ListBox, it exposes a SelectionChanged event, you're attaching to that event already.
SelectionChanged += (sender, e) =>
{
if (Command != null && Command.CanExecute(CommandParameter))
Command.Execute(CommandParameter);
};
In order to obtain the currently selected item you can do something like this :
SelectionChanged += (sender, e) =>
{
CommandListBox source = sender as CommandListBox; // This is the sender
if(source != null) // just to be sure
{
var value = source.SelectedItem;
}
if (Command != null && Command.CanExecute(CommandParameter))
Command.Execute(CommandParameter);
};
Does this help ?

Answer 2

You can bind your listbox's SelectedItem property to an appropriate property in your ViewModel.