transformingStreams.m

- (void) viewDidLoad {
    //...
    RACSignal *usernameIsValidSignal = RACObserve(self.viewModel, isUsernameValid);
    RAC(self.goButton, enabled) = usernameIsValidSignal;
    RAC(self.goButton, alpha) = [usernameIsValidSignal
        map:^id(NSNumber *usernameIsValid) {
            return usernameIsValid.boolValue ? @1.0 : @0.5;
        }];
}