UIScrollViewShenanigans.swift

import Foundation
import UIKit

// emulates table view sticky headers and demonstrates content inset usage
public class FakeTableViewController: UIViewController, UIScrollViewDelegate {
    // set up views neatly using anonymous closures
    let scrollView: UIScrollView = {
        let v = UIScrollView()
        v.backgroundColor = UIColor.lightGray
        return v
    }()

    // this will be the header that gets 'scrolled over', kind of like a refresh control
    let headerView: UIView = {
        let v = UIView()
        v.backgroundColor = UIColor.blue.withAlphaComponent(0.5)
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()

    // this will be a sticky section header ala UITableView plain
    let sectionHeaderView: UIView = {
        let v = UIView()
        v.backgroundColor = UIColor.red.withAlphaComponent(0.5)
        v.translatesAutoresizingMaskIntoConstraints = false;
        return v
    }()

    // some content to take up the screen
    let fakeContentView: UIView = {
        let v = UIView()
        v.backgroundColor = UIColor.white
        v.translatesAutoresizingMaskIntoConstraints = false;
        return v
    }()

    // the constraints we need to modify later
    var headerTopConstraint: NSLayoutConstraint!
    var contentWidthConstraint: NSLayoutConstraint!

    // we don't want no nibs
    public override func loadView() {
        // these lines happen automatically in UITableViewController/UICollectionViewController subclasses
        self.scrollView.delegate = self;
        self.scrollView.alwaysBounceVertical = true

        // cause it's a fake table
        self.scrollView.isDirectionalLockEnabled = true

        // add views and fix to edges
        // order matters for layering
        [self.headerView, self.fakeContentView, self.sectionHeaderView].forEach {
            self.scrollView.addSubview($0)
            $0.leftAnchor.constraint(equalTo: self.scrollView.leftAnchor).isActive = true
            $0.rightAnchor.constraint(equalTo: self.scrollView.rightAnchor).isActive = true
        }

        // fixed constraints - heights
        self.headerView.heightAnchor.constraint(equalToConstant: 50).isActive = true
        self.sectionHeaderView.heightAnchor.constraint(equalToConstant: 20).isActive = true
        self.fakeContentView.heightAnchor.constraint(equalToConstant: 1000).isActive = true

        // fix content to top and bottom
        self.fakeContentView.topAnchor.constraint(equalTo: self.scrollView.topAnchor).isActive = true
        self.fakeContentView.bottomAnchor.constraint(equalTo: self.scrollView.bottomAnchor).isActive = true

        // keep the section bottom fixed to the top of the content *if it can* be
        do {
            let c = self.sectionHeaderView.bottomAnchor.constraint(equalTo: self.fakeContentView.topAnchor)
            c.priority = UILayoutPriorityDefaultLow
            c.isActive = true
        }
        
        // require that it's no higher than the top of the background header (which is the top of the content window)
        self.sectionHeaderView.topAnchor.constraint(greaterThanOrEqualTo: self.headerView.topAnchor).isActive = true

        // edge insets
        self.scrollView.contentInset = UIEdgeInsets(top: 70, left: 0, bottom: 0, right: 0)
        self.scrollView.scrollIndicatorInsets = UIEdgeInsets(top: 70, left: 0, bottom: 0, right: 0)

        // mutable constraints
        
        // make sure width of content exists, otherwise size = 0,0 - table views do not have this problem
        self.contentWidthConstraint = {
            let c = self.headerView.widthAnchor.constraint(equalToConstant: 0)
            c.isActive = true
            return c
        }()

        // keep the 'behind' header fixed
        self.headerTopConstraint = {
            let c = self.headerView.topAnchor.constraint(equalTo: self.scrollView.topAnchor)
            c.isActive = true
            return c
        }()

        // always set self.view last and don't use it until then
        self.view = self.scrollView
    }

    public override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()

        // this forces the scrolling content to fill the space (width) given to the view controller
        self.contentWidthConstraint.constant = self.view.frame.width
        
        // TODO: take into account the edge insets of the VC w/r/t status bar and UINavigationBar
    }

    public func scrollViewDidScroll(_ scrollView: UIScrollView) {
        self.headerTopConstraint.constant = scrollView.contentOffset.y

        // adjust scroll indicator offsets here to match content bouncing
    }
}