Monitoring sources for events in a M-V-VM world

My dear colleague Matteo yesterday came up with a question, that per se is fairly easy, imagine a scenario where you have a OuterViewModel that exposes an InnerViewModel and a command, such as:

class InnerViewModel : INotifyPropertyChanged
{
   
}

class OuterViewModel : INotifyPropertyChanged
{
    public OuterViewModel()
    {
        //your own logic to setup the command and the inner view model
    }

    public InnerViewModel Inner{ get; set; }
    public ICommand DoSomething{ get; }
}

Now Matteo wants that, each time a property exposed by the InnerViewModel changes, the command evaluation status is re-evaluated. As we said the solution is simple, just attach an handler to the PropertyChanged event of the InnerViewModel and re-evaluate the command.
So far…so good…with a lot a pollution around Smile

Why pollution?

The ICommand interface does not expose a method to request the command re-evaluation;
You have to remember to attach the event on the inner view model and it is not natural because the focus, during development, is on the command not on the source of the change notification; this is the typical situation where you want a command to react to something that has happened…nothing more;

Radical to the rescue
Let’s se what you can achieve using the Radical framework:

class OuterViewModel : INotifyPropertyChanged
{
    public OuterViewModel()
    {
        //your own logic to setup the inner view model

        this.DoDomething = DelegateCommand.Create()
                   .OnCanExecute( o => return true ) //your own validation logic
                   .OnExecute( o => {  } ) //your own execution logic
                   .AddMonitor( PropertyObserver.ForAllPropertiesOf( this.InnerViewModel );
    }

    public InnerViewModel Inner{ get; set; }
    public ICommand DoSomething{ get; }
}

Or even easier using the brand new extension method:

class OuterViewModel : INotifyPropertyChanged
{
    public OuterViewModel()
    {
        //your own logic to setup the inner view model

        this.DoDomething = DelegateCommand.Create()
                   .OnCanExecute( o => return true ) //your own validation logic
                   .OnExecute( o => {  } ) //your own execution logic
                   .Observe( this.InnerViewModel );
    }

    public InnerViewModel Inner{ get; set; }
    public ICommand DoSomething{ get; }
}

easy peasy lemon squeezy Smile

, implementing your own monitors is really straightforward, just implement the IMonitor interface or even easier inherit from the AbstractMonitor/AbstractMonitor base classes.
Side notes:

under the hood the DelegateCommand utilizes the weak event pattern;
all the presented stuff is available for the full FX, for Silverlight 4 and for the Phone;

Do you want to give it a try? nuget it: Radical on nuget and Radical.Windows on nuget. (Radical contains the observers stuff and Radical.Windows the DelegateCommand)
.m