ExpandingViewController.swift

import Foundation
import UIKit
import SwiftUI

extension UIScrollView {
  var visibleContentHeight: CGFloat {
    frame.height - (adjustedContentInset.top + adjustedContentInset.bottom)
  }
}

extension UIView {
  var firstScrollView: UIScrollView? {
    var subviews = subviews
    while !subviews.isEmpty {
      let subview = subviews.removeFirst()
      subviews.append(contentsOf: subview.subviews)
      if let scrollView = subview as? UIScrollView {
        return scrollView
      }
    }
    return nil
  }
}

final class ExpandingViewController<Content: View>: UIHostingController<Content> {
  
  private var hasModifiedHeight = false
  private var didCall = false
  
  var expansionSettled: (() -> Void)?
  
  private func runCallback() {
    guard !didCall else { return }

    didCall = true
    expansionSettled?()
  }

  override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    self.updateScrollViewHeight()
  }

  func updateScrollViewHeight() {
    let scrollView = view.firstScrollView
    if let scrollView {
      let diff = scrollView.contentSize.height - scrollView.visibleContentHeight
      if abs(diff) > 0.001 {
        if hasModifiedHeight || diff > 0 {
          hasModifiedHeight = true
          view.frame = CGRect(x: view.frame.minX, y: view.frame.minY, width: view.frame.width, height: view.frame.height + diff)
        } else {
          runCallback()
        }
      } else {
        runCallback()
      }
    } else {
      runCallback()
    }
  }

}