Sunday, August 4, 2013

delegate command with(out) CommandManager

Now I feel you can implement a basic delegate command with or without CommandManager. (My sample wpf application description is in another blog post.)

A) The original RelayCommand by Josh Smith uses CommandManager. With CommandManager, we developers don't need to add InvalidateRequerySuggested() after some VM state change, since CommandManager will take care of that.

One consequence is, the CommandManager fires a hell lot of activity via (static) event CommandManager.RequerySuggested. Since our custom CanExecuteMyCommand() callback is in the inv list, it gets called every time.

B) Some prefer a home-grown delegate command without CommandManager. Event firing is better understood and less opaque than with CommandManager. However, we do need to remember to add something like MyCommand.RecheckStatus() in many places.

Here's my guess -- Since the inv list is no longer provided by CommandManager, we must provide it in our delegate command class. There's one inv list per command instance. By contrast, the CommandManager has one application-wide inv list for all the command instances. The inv list is behind the application-wide static event.

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