How to send error in signal chain - ios

Question

I have a signal A that contains integer values. A value of -1 indicates an invalid result, so I'd like to, instead of passing -1 along as a value, send an error. This way anything that subscribes to B will receive valid integers through subscribeNext: and the errors through subscribeError:.
I think I know how to do this with RACSubject:
RACSequence *A = [#[ #(2), #(6), #(5), #(-1), #(4) ] rac_sequence];
RACSubject *B = [RACSubject subject];
[A subscribeNext:^(NSNumber *val) {
if ( [val integerValue] == -1 ) {
[B sendError:[NSError errorWithDomain:#"MyDomain" code:0 userInfo:nil]];
} else {
[B sendNext:val];
}
} error:^(NSError *error) {
[B sendError:error];
} completed:^{
[B sendCompleted];
}];
I'm wondering if there's a more "inline" way to do this along the lines of:
RACSequence *A = [#[ #(2), #(6), #(5), #(-1), #(4) ] rac_sequence];
RACSignal *B = [A filter:^BOOL(id val) {
if ( [val integerValue] == -1 ) {
//FIXME: send an error to B's subscriber(s)
return NO;
} else {
return YES;
}
}

Answer 1

The primary method to do this is by using -flattenMap:, similar to how you've written the -filter: above. Using your example:
RACSignal *B = [A flattenMap:^(NSNumber *number) {
if (number.intValue == -1) {
return [RACSignal error:[NSError errorWithDomain:#"MyDomain" code:0 userInfo:nil]];
} else {
return [RACSignal return:number];
}
}];
Update
Alternatively, using the newer -try: operator:
RACSignal *B = [A try:^(NSNumber *number, NSError **error) {
if (number.intValue == -1) {
*error = [NSError errorWithDomain:#"MyDomain" code:0 userInfo:nil];
return NO;
}
return YES;
}];