UITextField，在辞退第一响应者时，导致奇怪的文本动画滚动

Question

I've got a couple of UITextFields implemented in a UITableView for a login form. When resigning first responder in both the very first time , a really strange animation jump is occurring. Since these are almost entirely build in Interface Builder with a .xib file, I've got virtually no code to add in. But here's a fun .gif that shows the behavior:

Update:

I've narrowed it down to the fact that I'm listening to keyboard events to adjust the view constraints. This is the code that's causing the problem:

func keyboardWillHide(notification: NSNotification) {
    // tried self.formContainer.layoutIfNeeded() here too to force pending layouts
    formContainerYConstraint.constant = 40
    UIView.animateWithDuration(0.4) { () -> Void in
        self.formContainer.layoutIfNeeded()
    }
}

... where the form container is a view that houses the table view and login button.

Answer 1

Feels like a total hack (and I'd love for someone to post a better answer) but in the mean time, I've resolved this by adding a slight delay to the animation action - I suspect this is related to the become- and resignFirstResponder events occurring when switching between two input fields.

let delay: dispatch_time_t = dispatch_time(DISPATCH_TIME_NOW, Int64(0.1 * Double(NSEC_PER_SEC)))
dispatch_after(delay, dispatch_get_main_queue()) { () -> Void in
    self.formContainerYConstraint.constant = 40
    UIView.animateWithDuration(0.4) { () -> Void in
        self.formContainer.layoutIfNeeded()
    }
}

Answer 2

Try this

- (void)textFieldDidEndEditing:(UITextField *)textField
{
  [textField layoutIfNeeded];
}