How do I trigger an update of CanExecute in a custom implementation of ICommand

I've created an AsyncCommand that returns false for CanExecute while the command is running, which is great except I can't seem to update the CanExecute so it never gets disabled.

In this article it says I would need to use CommandManager.RequerySuggested but that it's not available in Xamarin Forms.
http://www.wpfsharp.com/2015/04/28/binding-to-icommand-with-xamarin-forms/

I'd love to just override the Xamarin Forms Command like the author of the article does but unfortunately that means I won't have access to the instance of the command object to check if the command is in the middle of execution.

How does Xamarin Forms Command handle the whole updating CanExecute?