3

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 );
Gennady Vanin Геннадий Ванин
  • 10,136
  • 11
  • 75
  • 106
Jiew Meng
  • 84,767
  • 185
  • 495
  • 805

1 Answers1

3

  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.

Gennady Vanin Геннадий Ванин
  • 10,136
  • 11
  • 75
  • 106
Goblin
  • 7,970
  • 3
  • 36
  • 40
  • 1 thing i dont understand about `CanExecuteChanged` is the add/remove. it looks like to me a getter/setter, when i add an event handler for `CanExecuteChanged` i also add it to the `CommandManager.RequerySuggested` event. same for remove? but you say value can evaluate to true/false. so what is the add/remove actually? also assuming value is bool, how will `CommandManager` know which control to disable? – Jiew Meng Aug 23 '10 at 01:54
  • The add/remove is what happens everytime something adds a command.CanExecuteChanged += SomeRoutine (and -= of course). So, everytime the WPF engine adds a handler to the command (that's what happens when you add it to a button's Command property behind the scenes), we add a hook to CommandManager - this hook contains the value (ie. the button) and the Command - and then afterwards when CommandManager decides enough has changed that CanExecute could be different - it tells the Button to ask the Command for the new CanExecute - thus dis-/enabling the button accodringly. – Goblin Aug 23 '10 at 07:33