Finish all asynchronous requests before loading data?

Question

I have run into an issue where I have multiple asynchronous requests occuring which grab images and information from the Facebook API and my Firebase database. I want to per

Answer 1

There is something wrong with this conceptually. It sounds like you want to wait until both of these functions complete before doing something else, but what you haven't explained is that getAttendeesPictures depends on the outcome of getAttendees. That means what you really want to do it execute one asynchronous block, then execute a second asynchronous block with the output of the first, and then execute your final completion block when both are finished.

GCD is not particularly suited for this; you're better of using NSOperationQueue with NSBlockOperations. There are two distinct advantages to this over GCD:

1. NSOperation uses familiar object-oriented syntax compared to GCD's c-type functions, so it's pretty easy to write and understand.
2. Operations in the queue can have explicit dependencies on one another, so you can make it clear that e.g. operation B will only be executed after operation A is complete.

There is a great writeup of this by NSHipster which I'd recommend you go read. It's talked about mostly in the abstract, but what you want to do is use NSBlockOperation to create two block operations, one for executing getAttendees and one for executing getAttendeesPictures, and then make it explicit that the second block depends on the first before adding them both to a queue. They will then both execute and you can use a completion block on the second operation to do something once both have completed.

Dave Roberts is right in his response though: an immediate problem with the code is that you don't use the output of the getAttendees function to actually create any attendees. Perhaps this part of the code is missing, but from what I can see the name and objectID are just printed out. If you want to pass something useful into the getAttendeesPictures function you will need to fix this part first.