Find the least common ancestor of 2 views.

LowestCommonAncestor.swift

import Foundation
import UIKit

// From an example view hierarchy...
// 1 -> 2 -> 3 -> 4 -> 5
//      |
//      ---> 6
// Find the least common ancestor of 2 views.

// Input

let view1 = UIView()
let view2 = UIView()
let view3 = UIView()
let view4 = UIView()
let view5 = UIView()
let view6 = UIView()

view1.addSubview(view2)
view2.addSubview(view3)
view3.addSubview(view4)
view4.addSubview(view5)
view2.addSubview(view6)

// Output

commonSuperview(view5, view6) == view2

// Helper functions

extension UIView {
  var lineage: [UIView] {
    var candidateView: UIView? = self
    var output: [UIView] = []
    while let c = candidateView {
      output.append(c)
      candidateView = c.superview
    }
    return output
  }
}

//view5.lineage == [view5, view4, view3, view2, view1]
//view6.lineage == [view6, view2, view1]

func commonSuperview(_ v1: UIView, _ v2: UIView) -> UIView? {
  // First we create an array of lineage of both views.
  // - We use reversed() because ending parts (heighest ancestor) of both arrays are likely to be identical.
  //   So it is easier to compare them.
  // - We use makeIterator() to allow moving element by element easily.
  var lineage1 = v1.lineage.reversed().makeIterator()
  var lineage2 = v2.lineage.reversed().makeIterator()

  // Then find the last matching views between both arrays.
  var lastMatch: UIView? = nil
  while let c1 = lineage1.next(), let c2 = lineage2.next(), c1 == c2 {
    lastMatch = c1
  }

  return lastMatch
}