I'm just learning Silverlight and looking at MVVM and Commanding.
Ok, so I have seen the basic RelayCommand implementation:
public class RelayCommand : ICommand
{
private readonly Action _handler;
private bool _isEnabled;
public RelayCommand(Action handler)
{
_handler = handler;
}
public bool IsEnabled
{
get { return _isEnabled; }
set
{
if (value != _isEnabled)
{
_isEnabled = value;
if (CanExecuteChanged != null)
{
CanExecuteChanged(this, EventArgs.Empty);
}
}
}
}
public bool CanExecute(object parameter)
{
return IsEnabled;
}
public event EventHandler CanExecuteChanged;
public void Execute(object parameter)
{
_handler();
}
}
How can I pass a parameter down with a Command using this?
I've seen that you can pass a CommandParameter
like this:
<Button Command="{Binding SomeCommand}" CommandParameter="{Binding SomeCommandParameter}" ... />
In my ViewModel, I need to create the Command, but RelayCommand
is expecting an Action
delegate. Can I implement RelayCommand<T>
using Action<T>
- if so, how do I do it and how to I use it?
Can anyone give me any practical examples on CommandParameters with MVVM that don't involve using 3rd-party libraries (e.g. MVVM Light) as I want to understand it fully before using existing libraries.
Thanks.
public class Command : ICommand
{
public event EventHandler CanExecuteChanged;
Predicate<Object> _canExecute = null;
Action<Object> _executeAction = null;
public Command(Predicate<Object> canExecute, Action<object> executeAction)
{
_canExecute = canExecute;
_executeAction = executeAction;
}
public bool CanExecute(object parameter)
{
if (_canExecute != null)
return _canExecute(parameter);
return true;
}
public void UpdateCanExecuteState()
{
if (CanExecuteChanged != null)
CanExecuteChanged(this, new EventArgs());
}
public void Execute(object parameter)
{
if (_executeAction != null)
_executeAction(parameter);
UpdateCanExecuteState();
}
}
Is the Base Class for Commands
And this is your Command Property in ViewModel:
private ICommand yourCommand; ....
public ICommand YourCommand
{
get
{
if (yourCommand == null)
{
yourCommand = new Command( //class above
p => true, // predicate to check "CanExecute" e.g. my_var != null
p => yourCommandFunction(param1, param2));
}
return yourCommand;
}
}
in XAML set Binding to Command Property like:
<Button Command="{Binding Path=YourCommand}" .../>