Form Validation Example

gistfile1.m

@interface LoginForm : NSObject
@property (nonatomic, copy) NSString *username;
@property (nonatomic, copy) NSString *password;

@property (nonatomic, readonly) BOOL isValid;
@end

@implementation LoginForm

// Tell key-value observation that any time one of the keyPaths in the returned set changes, it affects the value
// of `isValid`. This lets us cascade validation state by changing simple property values.
+ (NSSet *)keyPathsForValuesAffectingIsValid
{
    return [NSSet setWithObjects:@"username", @"password"];
}

// Do your validation. We could get more fancy in here and maintain an array of errors to reflect the state if we want.
- (BOOL)isValid
{
    return [username length] > 5 && [password length] > 6;
}

@end

@interface MyController ()

@property (nonatomic, strong) LoginForm *loginForm;

// Controller is set as delegate for these fields in the Storyboard
@property (nonatomic, weak) IBOutlet UITextField *usernameTextField;
@property (nonatomic, weak) IBOutlet UITextField *passwordTextField;
@end

@implementation MyController

- (void)viewDidLoad
{
    self.loginForm = [LoginForm new];
    
    // NOTE: Using the initial option will ensure KVO fires to establish the baseline invalid state on initial view load
    [self.loginForm addObserver:self forKeyPath:@"isValid" options:NSKeyValueObservingOptionInitial context:nil]
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    // Every time `isValid` changes we want to update our UI to match.
    if ([keyPath isEqualToString:@"isValid"]) {
        // Enable/Disable the right bar button item as the `isValid` state changes, typically enabling/disabling the 'Done' button
        self.navigationItem.rightBarButtonItem.enabled = [self.loginForm isValid];
    }
}

#pragma mark - UITextFieldDelegate

// Bind changes to our text fields back to the form object modeling the Login
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSString *text = [textField.text stringByReplacingCharactersInRange:range withString:string];
    
    // Dispatch async so that that the text field value matches the text
    dispatch_async(dispatch_get_main_queue(), ^{
        if (textField == self.usernameTextField) {
            self.loginForm.username = text;
        } else if (textField == self.passwordTextField) {
            self.loginForm.password = text;
        }
    });
    return YES;
}

@end