ReactivePersistence?

gistfile1.m

// The basics
[[self.connection fetch:@"the-answer"] subscribeNext:^(NSNumber *x) {
    NSLog(@"The answer is %@", x);
}];

[[self.connection writeObject:@42 forKey:@"the-answer"] subscribeCompleted:^{
    NSLog(@"Wrote it.");
}];

// Signal for changes to the object at a key
[[self.connection signalForKey:@"the-answer"] subscribeNext:^(NSNumber *x) {
    NSLog(@"The answer changed to %@", x);
}];

// Derived value
RACSignal *plusOne = [[self.connection
    signalForKey:@"the-answer"]
    map:^(NSNumber *answer) {
        return @(answer.unsignedIntegerValue + 1);
    }];
[self.connection rac_liftSelector:@selector(writeObject:forKey:) withObjects:plusOne, @"the-answer-plus-1"];

// Trimming
RACSignal *trimmed = [[[self.connection
    signalForKey:@"some-array"]
    filter:^ BOOL (NSArray *array) {
        return array.count > 50;
    }]
    map:^(NSArray *array) {
        return [array subarrayWithRange:NSMakeRange(array.count - 10, 10)];
    }];
[self.connection rac_liftSelector:@selector(writeObject:forKey:) withObjects:trimmed, @"some-array"];

// Validation
RACSignal *valid = [RACAble(self.someProperty) filter:^(id value) {
    return ![value isNotCrazy];
}]
[self.connection rac_liftSelector:@selector(writeObject:forKey:) withObjects:valid, @"some-stuff"];