Create a RACSignal which sends error if RACSignal sends next

Question

With ReactiveCocoa I'm sure there is a better way than doing this?

RACSignal *crashSignal = [cancelSignal tryMap:^id(id value, NSError **errorPtr) {
    *errorPtr = [self createError];
    return nil;
}];

score 11 · Accepted Answer · answered Feb 12 '14 at 13:57

11

More idiomatic would be:

RACSignal *crashSignal = [cancelSignal flattenMap:^(id value) {
    return [RACSignal error:[self createError]];
}];

answered Feb 12 '14 at 13:57

joshaber

I've never really understood `flattenMap`. Would you care to say a few lines about when it is useful and why it is used widely throughout RAC? – hfossli Feb 12 '14 at 15:29
4

@hfossli `-flattenMap:` allows you to map from a _value_ to a _signal_. The resulting signals (from mapping each value in the original) will be merged together automatically. For the case you're describing, injecting an `NSError` _value_ into the signal won't actually terminate it, so you actually want to map to an _error signal_ that will terminate the whole stream as soon as it gets merged in. – Justin Spahr-Summers Feb 12 '14 at 17:09
Makes perfect sense now!! Thanks a lot! – hfossli Feb 12 '14 at 21:35

1 Answers1