Observer pattern or Callback?

Question

I have to do a design of a DownloadManager, but my main question is related to the notifications that a Download can send to the DownloadManager<

Answer 1

The observer is more flexible/scalable. Is not that weird after all the usage you mentioned for the observer pattern. Patters are after all just guidlines, if you need to change it a bit to fit your needs then go for it.

Consider the case when you have multiple DownloadManagers (maybe this is not a valid use case for your particular situation). You will need to register both of them. If you have even more then you will have to register all of them. Pretty long list, plus you need to implement listeners management in the Downloads