Sometimes, clever little patterns like that are useful to get stuff done in time. However, it that case it definitely smells a lot. Also, the global singleton stinks.
To me this is a violation of the Interface Segregation Principle. I think the author went the right way to fix it, but not far enough.
He added a parameter to segregate the device registration operations from other operations. It works, but the segregation is artificial : you still need a Device object (which seems to be used by a lot of classes, another smell, god object ?)
Instead of asking for an entire Device object, register_device should ask for a different type, specific to that operation, like « NewDevice », « UnregisteredDevice », etc. This object would depend on very specific information only available at the device creation. Now, if another developer tried to call register_device, he would need to create a weird object that he never heard of, with parameters he cannot even provide.
Is an object really a god object if lots of people use it? I thought it was the opposite way around: a large object that contains too much functionality, but this device could be a tiny single purpose object for all we know, that a lot of clients need to use.
