Calculate UIKeyboard Height

KeyboardHeightViewController.m

@property CGFloat keyboardHeight;

- (void)viewDidLoad {
    [super viewDidLoad];
    _keyboardHeight = 0;
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillChange:)
                                                 name:UIKeyboardWillChangeFrameNotification
                                               object:nil];
}

- (void) keyboardWillChange:(NSNotification *)notification {
    NSDictionary* info = notification.userInfo;
    
    UIWindow *window = [UIApplication sharedApplication].keyWindow;
    CGRect kbRect = [info[UIKeyboardFrameEndUserInfoKey] CGRectValue];
    CGRect kbframe = [window convertRect:kbRect toWindow:window];
    
    CGFloat fromBottom = (CGRectGetHeight([UIScreen mainScreen].bounds)- CGRectGetMinY(kbframe));

    keyboardHeight = fromBottom;
}