Flexible UITextView with minimum and maximum number of lines using AutoLayout

FlexibleTextView.swift

import UIKit

class FlexibleTextView: UITextView {

  // MARK: - Properties
  var minNumberOfLines: Int = 5 {
    didSet {
      if minNumberOfLines > maxNumberOfLines {
        minNumberOfLines = oldValue
      }
    }
  }
  
  var maxNumberOfLines: Int = 10 {
    didSet {
      if maxNumberOfLines < minNumberOfLines {
        maxNumberOfLines = oldValue
      }
    }
  }
  
  private var heightConstraint: NSLayoutConstraint!
  private var minHeight: CGFloat {
    return font!.lineHeight * CGFloat(minNumberOfLines)
  }
  
  private var maxHeight: CGFloat {
    return font!.lineHeight * CGFloat(maxNumberOfLines)
  }
  
  // MARK: - Lifecycle and Setup
  override init(frame: CGRect, textContainer: NSTextContainer?) {
    super.init(frame: frame, textContainer: textContainer)
    setup()
  }
    
  required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
    setup()
  }
  
  deinit {
    NotificationCenter.default.removeObserver(self)
  }
  
  private func setup() {
    isScrollEnabled = false
    NotificationCenter.default.addObserver(self,
                                           selector: #selector(updateHeightConstraintIfNeeded),
                                           name: .UITextViewTextDidChange,
                                           object: nil)
    heightConstraint = heightAnchor.constraint(equalToConstant: minHeight)
    heightConstraint.isActive = true
  }
  
  @objc func updateHeightConstraintIfNeeded() {
    let width = frame.width
    let newSize = sizeThatFits(CGSize(width: width,
                                      height: CGFloat.greatestFiniteMagnitude))
    let height = min(max(newSize.height, minHeight), maxHeight)
    isScrollEnabled = height == maxHeight
    heightConstraint.constant = height
  }
}