PagerController.swift

//
//  PagerController.swift
//

import UIKit

open class PagerController: BaseViewController {
    
    let scrollView = UIScrollView()
    let viewControllers: [UIViewController]
    let startingPage: Int
    
    public var onShake: (() -> Void)?
    
    public init(viewControllers: [UIViewController], startAt page: Int = 1) {
        self.viewControllers = viewControllers
        self.startingPage = page
        super.init(nibName: nil, bundle: nil)
    }
    
    public required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    open override func viewDidLoad() {
        super.viewDidLoad()
        
        automaticallyAdjustsScrollViewInsets = false
        
        scrollView.delegate = self
        scrollView.frame = view.bounds
        scrollView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        scrollView.isPagingEnabled = true
        scrollView.showsVerticalScrollIndicator = false
        scrollView.showsHorizontalScrollIndicator = false
        scrollView.isDirectionalLockEnabled = true
        view.addSubview(scrollView)
        
        for (i, vc) in viewControllers.enumerated() {
            self.addChildViewController(vc)
            scrollView.addSubview(vc.view)
            vc.didMove(toParentViewController: self)
            
            var frame = vc.view.frame
            frame.origin.x = CGFloat(i) * frame.width
            vc.view.frame = frame
            vc.view.autoresizingMask = [.flexibleWidth, .flexibleHeight]
            vc.automaticallyAdjustsScrollViewInsets = false
            
            scrollView.contentSize = CGSize(width: vc.view.frame.maxX, height: scrollView.frame.height)
        }
        
        scrollTo(page: startingPage, animated: false)
    }
    
    open override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        let position = CGPoint(x: 0, y: topLayoutGuide.length)
        let size = CGSize(width: view.bounds.width, height: view.bounds.height - position.y)

        scrollView.frame = CGRect(origin: position, size: size)
        scrollView.contentSize = CGSize(width: scrollView.contentSize.width, height: size.height)
    }
    
    public func scrollTo(page: Int, animated: Bool) {
        let index = page - 1
        guard index < viewControllers.count else { return }
        let vc = viewControllers[index]
        scrollView.scrollRectToVisible(vc.view.frame, animated: animated)
    }
    
    open override func motionEnded(_ motion: UIEventSubtype, with event: UIEvent?) {
        guard motion == .motionShake else { return }
        onShake?()
    }
}

extension PagerController: UIScrollViewDelegate {
    
    public func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        let offset = scrollView.contentOffset
        
        for vc in viewControllers {
            if vc.view.frame.minX == offset.x {
                vc.viewDidDisappear(true)
            }
        }
    }
    
    public func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
        let offset = scrollView.contentOffset
        
        for vc in viewControllers {
            if vc.view.frame.minX == offset.x {
                vc.viewDidAppear(true)
            }
        }
    }
}