gistfile1.m

// Re: https://twitter.com/iKenndac/status/430813574538092544

// `array` is an array of `SomeClass` objects, which have a `loaded` property
NSArray *loadedSignals = [[array.rac_sequence
    map:^RACSignal *(SomeClass *object) {
        return RACObserve(object, loaded);
    }] array];

[[[[RACSignal
    combineLatest:loadedSignals]
    and]
    filter:^BOOL(NSNumber *allLoaded) {
        return [allLoaded boolValue] == YES;
    }]
    subscribeNext:^(NSNumber *loaded) {
        // Called when all loaded
    }];