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 like onUpdate() to update a progress bar, onError(), onFinish(), etc. Somehow the DownloadManager has to receive this notifications from its Downloads.

I've thought 2 possible ways:

• Observer pattern
• Callbacks

Observer pattern

Basically there are 1 Observable and N Observers. In my case the DownloadManager has te be an Observer and the Downloads the Observables, so the relation is N Observables 1 Observer, just the opposite.

The advantage is to centralize all the possible notifications in one method, the notify() or update() (from java) method from the Observers, in my case only the DownloadManager. I can pass a param to the notify() method with the code of the notification.

Disadvantage? I'm using an oop pattern for a thing that can be done easily with a callback. Also, N observables 1 observer it's something weird, at least with the observer pattern because this pattern was done for 1 observable N observers, so I really won't be using the observer pattern.

Callback

Very similar to the observer pattern. The DownloadManager implements a "listener" (interface). This listener implements the notification functions onFinish(), onUpdate(), etc. Then this listener must be registered in all Downloads, so when a Download finishes it will call listener.onFinish(). Additionally I can pass parameters to this methods from the Downloads, like in the observer pattern.

Advantage: Easily usage. Disadvantage: None.

I will probably use a callback because in my opinion it makes no sense to use an observer pattern for 1 observer N observables.

And you, which option will use?

Answer 1

There is also one option, which is opposite to Observer/Callback approach. You can turn clients of DownloadManager from passive to active players of the show. Instead of waiting for a message from the manager they will regularly request its status.

This way your download process will look much smoother to the end-user. And you will be able to control it better.

Of course, you will have to use two threads. Or you will have to teach the DownloadManager to work in small steps, returning control to its client, instead of running one unbreakable piece of work.