Simple example with ReactiveCocoa framework.

ValidationView.m

#import "ValidationView.h"
#import "Validator.h"

@interface ValidationView ()

@property (nonatomic, weak) IBOutlet UITextField *nameTextField;
@property (nonatomic, weak) IBOutlet UITextField *passwordTextField;
@property (nonatomic, weak) IBOutlet UIActivityIndicatorView *activityView;
@property (nonatomic, weak) IBOutlet UILabel *errorLabel;
@property (nonatomic, weak) IBOutlet UIButton *validateButton;

@property (nonatomic) BOOL validating;

@end

@implementation ValidationView

- (void)awakeFromNib
{
    [super awakeFromNib];

    [self setupUI];
    [self setupUILogic];
}

- (void)setupUI
{
    self.layer.borderColor = [UIColor colorWithWhite:0.9 alpha:1.0].CGColor;
    self.layer.borderWidth = 1.0;
    self.layer.cornerRadius = 6.0;

    self.errorLabel.hidden = YES;

    self.validateButton.layer.borderWidth = 1.0;
    self.validateButton.layer.cornerRadius = 6.0;
    self.validateButton.backgroundColor = [UIColor whiteColor];
}

- (void)setupUILogic
{
    @weakify(self);

    RAC(self.validateButton, enabled, @NO) = [RACSignal combineLatest:@[
        self.nameTextField.rac_textSignal,
        self.passwordTextField.rac_textSignal,
        RACObserve(self, validating)
    ] reduce:^(NSString *name, NSString *password, NSNumber *validating) {
        return @(name.length > 0 && password.length > 0 && !validating.boolValue);
    }];

    [[RACObserve(self, validating) deliverOnMainThread] subscribeNext:^(NSNumber *validating) {
        @strongify(self)
        self.nameTextField.enabled = !validating.boolValue;
        self.passwordTextField.enabled = !validating.boolValue;
        self.activityView.hidden = !validating.boolValue;
        if (validating.boolValue) {
            self.errorLabel.hidden = YES;
        }
    }];

    [[RACObserve(self.validateButton, enabled) deliverOnMainThread] subscribeNext:^(NSNumber *enabled) {
        @strongify(self)
        if (enabled.boolValue) {
            self.validateButton.layer.borderColor = self.validateButton.tintColor.CGColor;
        } else {
            self.validateButton.layer.borderColor = [UIColor lightGrayColor].CGColor;
        }
    }];

    [[[self.validateButton rac_signalForControlEvents:UIControlEventTouchUpInside] deliverOnMainThread] subscribeNext:^(id _) {

        @strongify(self)

        self.validating = YES;

        // Emulation of an async task like a network request.
        RACSignal *validatorSignal = [Validator validatorSignalForName:self.nameTextField.text password:self.passwordTextField.text];
        [[validatorSignal deliverOnMainThread] subscribeError:^(NSError *error) {
            @strongify(self)
            [self finishValidationWithSuccess:NO error:error];
        } completed:^{
            @strongify(self)
            [self finishValidationWithSuccess:YES error:nil];
        }];
    }];
}

- (void)finishValidationWithSuccess:(BOOL)success error:(NSError *)error
{
    self.validating = NO;

    if (success) {
        if ([self.delegate respondsToSelector:@selector(validationViewDidFinishValidationWithSuccess:)]) {
            [self.delegate validationViewDidFinishValidationWithSuccess:self];
        }
    } else {
        self.errorLabel.text = error.localizedDescription;
    }

    self.errorLabel.hidden = success;
}

@end