Question about combining signals in ReactiveCocoa

gistfile1.m

@interface LoginViewController ()

@property (strong, nonatomic) RACSubject *textFieldReturnPressed;
@property (strong, nonatomic) UITextField *usernameTextField;
@property (strong, nonatomic) UITextField *passwordTextField;

@end

@implementation LoginViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        _textFieldReturnPressed = [RACSubject subject];
    }
    return self;
}

- (void)viewDidLoad {
    [super viewDidLoad];
    [self addSubview:self.usernameTextField];
    [self addSubview:self.passwordTextField];

    RACSignal *validUsername = [self.usernameTextField.rac_textSignal map:^(NSString *username) {
        return @(username.length > 0 && [username isAValidEmail]);
    }];
    RACSignal *validPassword = [self.passwordTextField.rac_textSignal map:^(NSString *password) {
        return @(password.length > 0);
    }];

    [[self.textFieldReturnPressed combineLatestWith:validUsername] subscribeNext:^(RACTuple *values) {
        RACTupleUnpack(UITextField *textField, NSNumber *validity) = values;
        if (validity.boolValue && [textField isEqual:self.usernameTextField]) {
            [self.passwordTextField becomeFirstResponder];
        }
    }];

    [[self.textFieldReturnPressed combineLatestWith:validPassword] subscribeNext:^(RACTuple *values) {
        RACTupleUnpack(UITextField *textField, NSNumber *validity) = values;
        if (validity.boolValue && [textField isEqual:self.passwordTextField]) {
            // start log in process
        }
    }];
}

- (BOOL)textFieldShouldReturn:(UITextField *)textField {
    [self.textFieldReturnPressed sendNext:textField];
    return YES;
}

// other implementation details skipped...

@end

Right now, I'm just trying to execute the code on line #34 - basically switch the focus from username text field to password text field when the user hits return (and if the input is correct, as determined by the validUsername signal).

The problem with my code is that because it's using combineWithLatest:, the beginFirstResponder is executed too often. I've tried to adapt the piece of code you suggested to this task, but it has the same effect.