How would you write fetching a collection the "Reactive Cocoa" way?

Question

The client I'm building is using Reactive Cocoa with Octokit and so far it has been going very well. However now I'm at a point where I want to fetch a collection of repositories and am having trouble wrapping my head around doing this the "RAC way"

// fire this when an authenticated client is set
[[RACAbleWithStart([GHDataStore sharedStore], client) 
  filter:^BOOL (OCTClient *client) {
      return client != nil && client.authenticated;
  }]
 subscribeNext:^(OCTClient *client) {
     [[[client fetchUserRepositories] deliverOn:RACScheduler.mainThreadScheduler]
      subscribeNext:^(OCTRepository *fetchedRepo) {
          NSLog(@" Received new repo: %@",fetchedRepo.name);
      }
      error:^(NSError *error) {
          NSLog(@"Error fetching repos: %@",error.localizedDescription);
      }];
 } completed:^{
     NSLog(@"Completed fetching repos");
 }];

I originally assumed that -subscribeNext: would pass an NSArray, but now understand that it sends the message every "next" object returned, which in this case is an OCTRepository.

Now I could do something like this:

NSMutableArray *repos = [NSMutableArray array];
// most of that code above
subscribeNext:^(OCTRepository *fetchedRepo) {
    [repos addObject:fetchedRepo];
}
// the rest of the code above

Sure, this works, but it doesn't seem to follow the functional principles that RAC enables. I'm really trying to stick to conventions here. Any light on capabilities of RAC/Octokit are greatly appreciated!

Simple as that, never knew that existed. Feel free to answer and I'll accept it! — rnystrom, Jun 24 '13 at 17:47

mdiep · Accepted Answer · 2013-06-24T20:48:09.853

It largely depends on what you want to do with the repositories afterward. It seems like you want to do something once you have all the repositories, so I'll set up an example that does that.

// Watch for the client to change
RAC(self.repositories) = [[[[[RACAbleWithStart([GHDataStore sharedStore], client) 
    // Ignore clients that aren't authenticated
    filter:^ BOOL (OCTClient *client) {
        return client != nil && client.authenticated;
    }]
    // For each client, execute the block. Returns a signal that sends a signal
    // to fetch the user repositories whenever a new client comes in. A signal of
    // of signals is often used to do some work in response to some other work.
    // Often times, you'd want to use `-flattenMap:`, but we're using `-map:` with
    // `-switchToLatest` so the resultant signal will only send repositories for
    // the most recent client.
    map:^(OCTClient *client) {
        // -collect will send a single value--an NSArray with all of the values
        // that were send on the original signal.
        return [[client fetchUserRepositories] collect];
    }]
    // Switch to the latest signal that was returned from the map block.
    switchToLatest]
    // Execute a block when an error occurs, but don't alter the values sent on
    // the original signal.
    doError:^(NSError *error) {
        NSLog(@"Error fetching repos: %@",error.localizedDescription);
    }]
    deliverOn:RACScheduler.mainThreadScheduler];

Now self.repositories will change (and fire a KVO notification) whenever the repositories are updated from the client.

A couple things to note about this:

It's best to avoid subscribeNext: whenever possible. Using it steps outside of the functional paradigm (as do doNext: and doError:, but they're also helpful tools at times). In general, you want to think about how you can transform the signal into something that does what you want.
If you want to chain one or more pieces of work together, you often want to use flattenMap:. More generally, you want to start thinking about signals of signals--signals that send other signals that represent the other work.
You often want to wait as long as possible to move work back to the main thread.
When thinking through a problem, it's sometimes valuable to start by writing out each individual signal to think about a) what you have, b) what you want, and c) how to get from one to the other.

EDIT: Updated to address @JustinSpahrSummers' comment below.

God I love learning from you guys. This is really great, appreciate the detailed reply! See you next meetup :) — rnystrom, Jun 24 '13 at 18:16
Great answer, though I would suggest using `-map:` and `-switchToLatest` here, so that only the results from the latest client are ever used. Otherwise, you could potentially get into a scenario where a previous client's results arrive _after_ a new client's, and then your `repositories` property is wrong. — Justin Spahr-Summers, Jun 24 '13 at 18:33
@mdiep why do you have a need to `// Switch to the latest signal that was returned from the map block.`? — Rui Peres, Feb 12 '14 at 17:34
This is so useful / common in my codebase now that I've made an extension, `collectLatest`, which maps collect and calls switchToLatest. — aehlke, Feb 16 '15 at 18:34

score 2 · Answer 2 · answered Jun 24 '13 at 17:53

There is a -collect operator that should do exactly what you're looking for.

// Collect all receiver's `next`s into a NSArray. nil values will be converted
// to NSNull.
//
// This corresponds to the `ToArray` method in Rx.
//
// Returns a signal which sends a single NSArray when the receiver completes
// successfully.
- (RACSignal *)collect;

How would you write fetching a collection the "Reactive Cocoa" way?

2 Answers2