Constraint that can be updated with safe area but only once

FixedSafeAreaLayoutConstraint.swift

import UIKit

/// Work around problem described here: https://stackoverflow.com/questions/47223680/safe-area-changes-in-parent-vc-when-presenting-modally-vc-in-landscape
/// When presenting screen with another orientation safe area changes on first screen
/// that affects constraints and all layout that depends on safe area.
/// To avoid this bug one should use UCFixedSafeAreaLayoutConstraint that can be updated with safe
/// area inset using `updateAllFixedSafeAreaConstraints(newSafeAreaInsets:)` in UIViewController
///
///
///
/// Custom constraint that preserves constant value when safe area changes.
/// One should attach it to corresponding view and it's superView, not safeAreaLayoutGuide!
///
/// To set initial safe Area inset `notifyOnSafeAreaChange` should be called in
/// `UIViewController` `viewSafeAreaInsetsDidChange()`. Take a look at protocol
/// `FixedSafeAreaConstraintsUpdating` designed to update all `UCFixedSafeAreaLayoutConstraint`
/// at once. The protocol has a default implementation for `UIViewController`
///
/// Example:
/// ```
/// override func viewSafeAreaInsetsDidChange() {
///   super.viewSafeAreaInsetsDidChange()
///   self.updateAllFixedSafeAreaConstraints(newSafeArea: view.safeAreaInsets)
/// }
/// ```
class FixedSafeAreaLayoutConstraint: NSLayoutConstraint {
  private var hasSetSafeArea: Bool = false

  /// Updates `constraint` with changed Safe Area Insets.
  /// If it `constraint` has not updated before with not zero edge insets, then
  /// safe area inset value would be added to `self.constant`, depending on
  /// first and second `attribute`.
  ///
  /// If constraint connects left and left or trailing and trailing then `safeAreaInsets.left`
  /// will be used as a additive value. Same applies to other attribute types.
  ///
  ///
  ///
  /// - Parameter safeAreaInsets: new safe area insets value
  func notifyOnSafeAreaChange(_ safeAreaInsets: UIEdgeInsets) {
    guard hasSetSafeArea == false else {
      return
    }

    guard safeAreaInsets != UIEdgeInsets.zero else {
      return
    }

    hasSetSafeArea = true
    switch (firstAttribute, secondAttribute){
    case (.left, .left), (.leading, .leading):
      self.constant += direction * safeAreaInsets.left
    case (.right, .right), (.trailing, .trailing):
      self.constant += -direction * safeAreaInsets.right
    case (.top, .top):
      self.constant += direction * safeAreaInsets.top
    case (.bottom, .bottom):
      self.constant += -direction * safeAreaInsets.bottom
    case (.width, .width):
      let alignedMultiplier: CGFloat = direction == 1.0 ? multiplier : 1.0
      let left = safeAreaInsets.left * alignedMultiplier
      let right = safeAreaInsets.right * alignedMultiplier
      self.constant += -direction * (left + right)
    case (.height, .height):
      let alignedMultiplier: CGFloat = direction == 1.0 ? multiplier : 1.0
      let top = safeAreaInsets.top * alignedMultiplier
      let bottom = safeAreaInsets.bottom * alignedMultiplier
      self.constant += -direction*(top + bottom)
    default:
      break
    }
  }

  private var direction: CGFloat {
    if let viewFirst = self.firstItem as? UIView,
      let viewSecond = self.secondItem as? UIView {

      for superView in sequence(first: viewFirst, next: { $0.superview }) {
        if superView == viewSecond {
          return 1.0
        }
      }
      return -1.0
    } else {
      return 1.0
    }
  }
}


extension UIViewController {
  /// Implementation for UIViewController which updates all UCFixedSafeAreaLayoutConstraint in self.view.
  /// Example:
  /// ```
  /// override func viewSafeAreaInsetsDidChange() {
  ///   super.viewSafeAreaInsetsDidChange()
  ///   self.updateAllFixedSafeAreaConstraints(newSafeArea: view.safeAreaInsets)
  /// }
  /// ```
  ///
  /// Work around problem described here: https://stackoverflow.com/questions/47223680/safe-area-changes-in-parent-vc-when-presenting-modally-vc-in-landscape
  /// When presenting screen with another orientation safe area changes on first screen
  /// that affects constraints and all layout that depends on safe area.
  /// To avoid this bug one should use UCFixedSafeAreaLayoutConstraint that can be updated with safe
  /// area inset using `updateAllFixedSafeAreaConstraints(newSafeAreaInsets:)` in UIViewController
  func updateAllFixedSafeAreaConstraints(newSafeArea: UIEdgeInsets) {
    for case let fixedSafeAreaConstraint as UCFixedSafeAreaLayoutConstraint in self.view.constraints {
      fixedSafeAreaConstraint.notifyOnSafeAreaChange(newSafeArea)
    }
  }