ScrollView with dynamic textview as subview - ios

Question

I want to fit a textView in a scrollView. As user types in the textView, the textView should expand in height. Everything seems to work fine until the textView reaches the height of the scrollView's height. The scrollView's contentSize is not updated. Here's my setup.
--scrollView
|--contentView
|--textView
The constraints are set as follow:
ScrollView - top, leading,trailing to the superview and bottom to bottomLayoutGuide
ContentView - top, leading, trailing, bottom to superview (ScrollView), align center x and align center y to superview (ScrollView)
TextView - top, leading, trailing to superview (ContentView), bottom is <=400 points to superview (ContentView)
In specific, I do not set a height for textView so it can resize dynamically as user types. I am using autolayout and textView is also set to scroll disabled.
I tried to force an update to the contentSize on TextViewDidChange delegate call.
func textViewDidChange(_ textView: UITextView) {
let size = CGSize(width: textView.frame.width, height: CGFloat(MAXFLOAT))
let value = textView.sizeThatFits(size)
let newHeight = value.height + textView.frame.minY
if newHeight > scrollView.contentSize.height {
contentView.frame = CGRect(x: 0, y: 0, width: textView.frame.width, height: newHeight)
scrollView.contentSize = CGSize(width: textView.frame.width, height: value.height + textView.frame.minY)
}
}
There's couple of issues with this approach. First will be the sizeToFit is called on every keystroke. And textView seems "slow down" the more text I typed to the textView.
Another issue is as soon a I rotate the screen, the scrolling of the scrollView stops working.
Can someone shed some light on this?
* update *
I was able to get it to a state where the scrollView recognize the dynamic height change of the textView.
My new constraints are like this:
ScrollView - top, leading,trailing to the superview and bottom to bottomLayoutGuide
ContentView - top, leading, trailing, bottom to superview (ScrollView), width to scrollView's superview
TextView - top, leading, trailing to superview (ContentView), bottom is =400 points to superview (ContentView)
Now this does update the contentSize accordingly and I don't need to calculate any frames. However this leads to another issue where the bottom constraint is ALWAYS 400. Ideally I only want the bottom constraint to be 400 on init. And as the textView expands, it should decrease to it reaches 0 (textView bottom reaches the bottom on the contentView)
Does anyone know how to go about this?