Need help understanding MVVM Tutorial, RelayCommand & Use of lambdas

Question

i am reading this tutorial

i don't understand what the below code is trying to do. 1st, 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

public event EventHandler CanExecuteChanged
{
    add { CommandManager.RequerySuggested += value; }
    remove { CommandManager.RequerySuggested -= value; }
}

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

My thoughts

command is nothing but the function which is written in a class and you need to call that function

WPF just go ahead with Command

1- When you bind any command to your Control say ( Button ) , with the help of CanExcute method , wpf can enable / disable your ui control.

use this link to get more insigh, you will definatly understand by reading below artical,if still doubt than just leave a reply , i will try to explain in detail then

http://joshsmithonwpf.wordpress.com/2008/06/17/allowing-commandmanager-to-query-your-icommand-objects/

Answer 2

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 and a Predicate 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.