A UITextView subclass that grows with its text but allows scrolling according to AutoLayout constraints. Updates intrinsicContentSize. For an animatable version see https://gist.github.com/Bogidon/632e265b784ef978d5d8c0b86858c2ee

GrowingTextView.swift

//
//  GrowingTextView.swift
//  https://gist.github.com/Bogidon/cc0c9ae6f041413c39fb0ff146ad1b18
//
//  Created by Bogdan Vitoc on 02/22/2017.
//  Distributed under the MIT License: https://gist.github.com/Bogidon/cc0c9ae6f041413c39fb0ff146ad1b18#file-license
//

import UIKit

/**
 A subclass of `UITextView` that grows with its text without completely disabling scrolling. It does this by updating 
 `intrinsicContentSize`. This means you can set inequality constraints so the text is only scrollable under certain 
 conditions. (e.g. you can set a maximum with a ≤ constraint).

 - Note:
 See [this gist](https://gist.github.com/Bogidon/632e265b784ef978d5d8c0b86858c2ee) for an alternate version that can animate.
 */
@IBDesignable
class GrowingTextView: UITextView {
    override init(frame: CGRect, textContainer: NSTextContainer?) {
        super.init(frame: frame, textContainer: textContainer)
        commonInit()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }

    private func commonInit() {
        NotificationCenter.default.addObserver(self, selector: #selector(scrollToBottom), name: .UITextViewTextDidChange, object: nil)
    }

    override var contentSize: CGSize {
        didSet {
            invalidateIntrinsicContentSize()

            // This eliminates a weird UI glitch where inserting a new line sometimes causes there to be a 
            // content offset when self.bounds == self.contentSize causing the text at the top to be snipped
            // and a gap at the bottom.
            setNeedsLayout()
            layoutIfNeeded()
        }
    }

    override var intrinsicContentSize: CGSize {
        let width = super.intrinsicContentSize.width
        return CGSize(width: width, height: contentSize.height)
    }

    @objc override func scrollToBottom() {
        // This needs to happen so the superview updates the bounds of this text view from its new intrinsicContentSize
        // If not called, the bounds will be smaller than the contentSize at this moment, causing the guard to not be triggered.
        superview?.layoutIfNeeded()

        // Prevent scrolling if the textview is large enough to show all its content. Otherwise there is a jump.
        guard contentSize.height > bounds.size.height else {
            return
        }

        let offsetY = (contentSize.height + contentInset.top) - bounds.size.height
        UIView.animate(withDuration: 0.125) {
            self.setContentOffset(CGPoint(x: 0, y: offsetY), animated: false)
        }
    }
}

LICENSE

(The MIT License)

Copyright (c) 2017 Bogdan Vitoc

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
'Software'), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Lines 42 & 43 cause a stack overflow due to infinite recursion. If I remove those lines it kind of works, but with the content offset glitch.

Also, there's a superfluous override keyword on line 52.

scrollToBottom based on false assumption user types to the end of text
If user will scroll to top of TextView and will try to type scroll will work incorrect

Anyway, good work