A UITextView subclass with placeholder text support. Swift 4.2

PlaceholderTextView.swift

import UIKit

/// A UITextView subclass with placeholder text support.
/// It uses another UILabel to show the placeholder, shown when text is empty.
class PlaceholderTextView: UITextView {

  lazy var placeholderLabel: UILabel = {
    let label = UILabel()
    label.textColor = UIColor(white: 0.5, alpha: 0.85)
    label.backgroundColor = .clear
    return label
  }()

  override init(frame: CGRect, textContainer: NSTextContainer?) {
    super.init(frame: frame, textContainer: textContainer)
    setupNotificationObserver()
  }

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

  deinit {
    NotificationCenter.default.removeObserver(self)
  }

  private func setupNotificationObserver() {
    NotificationCenter.default.addObserver(self,
                                           selector: #selector(TextViewWithPlaceholder.textDidChangeHandler(notification:)),
                                           name: UITextView.textDidChangeNotification,
                                           object: self)
  }

  override func layoutSubviews() {
    super.layoutSubviews()

    if text.isEmpty {
      placeholderLabel.copyProperties(from: self)
      addSubview(placeholderLabel)
      bringSubviewToFront(placeholderLabel)
    } else {
      placeholderLabel.removeFromSuperview()
    }
  }

  @objc func textDidChangeHandler(notification: Notification) {
    setNeedsLayout()
  }
}

private extension UILabel {
  /// Copies common properties from UITextView. You can add more.
  func copyProperties(from textView: UITextView) {
    frame = textView.bounds.inset(by: textView.textContainerInset)
    lineBreakMode = textView.textContainer.lineBreakMode
    textAlignment = textView.textAlignment
    numberOfLines = textView.textContainer.maximumNumberOfLines
  }
}