block in ios: what is this method map: before a block?

Question

I don't understand what map: is, in the tutorial using blocks, I could not find a map method in RACSignal (link) or RACDisposable (link) .

- (RACSignal *)fetchJSONFromURL:(NSURL *)url {
    NSLog(@"Fetching: %@",url.absoluteString);

    // 1
    return [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        // 2
        NSURLSessionDataTask *dataTask = [self.session dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
            // TODO: Handle retrieved data
        }];

        // 3
        [dataTask resume];

        // 4
        return [RACDisposable disposableWithBlock:^{
            [dataTask cancel];
        }];
    }] doError:^(NSError *error) {
        // 5
        NSLog(@"%@",error);
    }];
}


- (RACSignal *)fetchCurrentConditionsForLocation:(CLLocationCoordinate2D)coordinate {
    // 1
    NSString *urlString = [NSString stringWithFormat:@"http://api.openweathermap.org/data/2.5/weather?lat=%f&lon=%f&units=imperial",coordinate.latitude, coordinate.longitude];
    NSURL *url = [NSURL URLWithString:urlString];

    // 2
    return [[self fetchJSONFromURL:url] map:^(NSDictionary *json) {
        // 3
        return [MTLJSONAdapter modelOfClass:[WXCondition class] fromJSONDictionary:json error:nil];
    }];
}

The link of the tutorial from raywenderlich website : link

Thanks

//...'map' callback

- (RACSignal *)fetchCurrentConditionsForLocation:(CLLocationCoordinate2D)coordinate {
    // 1
    NSString *urlString = [NSString stringWithFormat:@"http://api.openweathermap.org/data/2.5/weather?lat=%f&lon=%f&units=imperial",coordinate.latitude, coordinate.longitude];
    NSURL *url = [NSURL URLWithString:urlString];

    // 2
    return [[self fetchJSONFromURL:url] map:^(NSDictionary *json) {
        // 3
        return [MTLJSONAdapter modelOfClass:[WXCondition class] fromJSONDictionary:json error:nil];
    }];
}



//... 'map' method

- (instancetype)map:(id (^)(id value))block {
    NSCParameterAssert(block != nil);

    Class class = self.class;

    return [[self flattenMap:^(id value) {
        return [class return:block(value)];
    }] setNameWithFormat:@"[%@] -map:", self.name];
}

Answer 1

It's defined in RACSignal's superclass, RACStream:

/// Maps `block` across the values in the receiver.
///
/// This corresponds to the `Select` method in Rx.
///
/// Returns a new stream with the mapped values.
- (instancetype)map:(id (^)(id value))block;