Question

So the task is the following:
1)I have a track ID, I need to ask the server for all the track data
2)parse response (here I also have an album ID)
3)now I have an album ID, I need to ask the server for all the album data
4)parse response (here I also have an artist ID)
5)now I have an artist ID, I need to ask the server for all the artist data

I wonder what is the right way to do this with gcd. 3 dispatch_sync-s inside dispatch_async?
I want all this to be one operation, run in the background, so at first I thought about NSOperation, but all callbacks, parsing, saving to core data need to happen on background thread, so I’d have to create a separate run loop for callbacks to make sure it will not be killed before I get a response and will not block ui.

so the question is how should I use gcd here, or is it better to go with nsoperation and a runloop thread for callbacks? thanks

Answer 1

I would suggest using NSOperation and callbacks executed on the main thread.

If you think about it, your workflow is pretty sequential: 1 -> 3 -> 5; the parsing steps (2 and 4) are not presumably that expensive so that you want to execute them on a separate thread (I guess they are not expensive at all and you can disregard parsing time compared to waiting time for network communication).

Furthermore, if you use a communication framework like AFNetworking (or even NSURLConnection + blocks) your workflow will be pretty easy to implement:

1. retrieve track data
2. in “retrieve track data” response handler, get album id, then send new request for “album data”;
3. in “retrieve album data” response handler, get artist id, and so on…