Auto-tab between UITextFields with maximum numbers of chars (e.g. for a telephone number 000-222-3333)

example.m

// In IB, set up your UITextFields with tags 1,2,3
// Set the 'delegate' for each UITextField to be the main ViewController
// Set your ViewController to implement UITextFieldDelegate

// Add function below to your ViewController - use the values in the switch statement to limit the number of chars allowed in each UITextField

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    
    int maxSize;
    switch (textField.tag) {
        case 1:
            maxSize = 3;
            break;
        case 2:
            maxSize = 4;
            break;
        case 3:
            maxSize = 2;
            break;
    }
    
    // allow delete and tab to previous textfield if field is empty.
    if ([string length] == 0) {
            // tab to previous...
            int64_t delayInSeconds = 0.1;
            dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
            dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
                        if ([textField.text length] == 0) {
                            [[self.view viewWithTag:textField.tag-1] becomeFirstResponder];
                        }
            });
        return YES;
    }
    
    // if we're about to add last allowed char, tab to next
    if ( [textField.text length] == maxSize - 1) {
        int64_t delayInSeconds = 0.1;
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
        dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
            UIView* next = [self.view viewWithTag:textField.tag+1];
            
            // If there isn't a next one, close the keyboard.
            if (!next || ![next isKindOfClass:[UITextField class]]) {
                [textField resignFirstResponder];
            } else {
                [next becomeFirstResponder];
            }
        });
        
        return YES;
    }
    
    // add no more chars if at max size
    if ( ([textField.text length] >= maxSize) ) {
        return NO;
    }
    
    return YES;
}