Need help understanding MVVM Tutorial, RelayCommand & Use of lambdas

Question

I am reading Josh Smith' WPF Apps With The Model-View-ViewModel Design Pattern tutorial

i don't understand what the below code is trying to do.
First, the syntax reminds me properties, but with add/remove instead.

But what is CommandManager.RequerySuggested ?

It delegates the event subscription to the CommandManager.RequerySuggested event. This ensures that the WPF commanding infrastructure asks all RelayCommand objects if they can execute whenever it asks the built-in commands

//Figure 3 The RelayCommand Class
public class RelayCommand : ICommand 
{ 
#region Fields 
    readonly Action<object> _execute; 
    readonly Predicate<object> _canExecute; 
#endregion // Fields
#region Constructors 
public RelayCommand(Action<object> execute) : this(execute, null) 
{ } 
public RelayCommand(Action<object> execute, Predicate<object> canExecute) 
{ 
    if (execute == null) throw new ArgumentNullException("execute"); 
   _execute = execute; 
   _canExecute = canExecute; 
} 
#endregion // Constructors 
#region ICommand Members 
[DebuggerStepThrough] 
public bool CanExecute(object parameter) 
{ 
    return _canExecute == null ? true : _canExecute(parameter); 
} 
public event EventHandler CanExecuteChanged 
{ 
    add    { CommandManager.RequerySuggested += value; } 
    remove { CommandManager.RequerySuggested -= value; } 
} 
public void Execute(object parameter) 
{ _execute(parameter); } 
#endregion // ICommand Members }

Also, save command is configured with lambdas. 1st, there are 2 param variables. Will they conflict? i cannot just do something like RelayCommand(this.Save(), this.CanSave) or is there no such syntax.

_saveCommand = new RelayCommand(param => this.Save(),
                                param => this.CanSave );

Answer 1

1. CommandManager.RequerySuggested += value means that if the function for CanExecute can resolve to both true and false depending on some conditions.

   WPF will disable the Button/MenuItem ( CommandButtonBase ) if it evaluates to false and enable whenever the condition evaluates to true .
   If you don't have those two lines, WPF will ask the command only once (when the Button/MenuItem is loaded and will not update after that unless you do it manually.

2. The two parameters (lambda-expressions) are of type Action<object> and a Predicate<object> respectively. So, they cannot, by definition, conflict ( params is just a name - and as the two functions have different scope - they don't conflict).

   If you have a method with the right signature, you can use that in the constructor

   • private void Save(object obj)
     and
     private bool CanSave(object obj)

   respectively, but you shouldn't have the () at the end - so new RelayCommand(this.Save,this.CanSave) should work.