ScrollPageController.swift

//
//  ScrollPageController.swift
//  ScrollPageController
//
//  Created by Chiharu Nameki on 2015/06/21.
//  Copyright (c) 2015 Chiharu Nameki. All rights reserved.
//

import UIKit

class ScrollPageController: UIViewController {
    private var pages: [UIView?] = []  // viewControllersのview, 貼り付けていない場合はnil
    var viewControllers: [UIViewController] = [] {
        willSet {
            for (index, _) in enumerate(viewControllers) {
                hideViewControllerAtIndex(index)
            }
        }
        didSet {
            pages = []
            for _ in viewControllers {
                pages.append(nil)
            }
            selectedIndex = 0
            setupScrollView()
        }
    }
    
    private(set) var selectedIndex: Int = 0
    var selectedViewController: UIViewController {
        return viewControllers[selectedIndex]
    }
    
    func selectViewControllerAtIndex(index: Int) {
        selectedIndex = index
        scrollView.bounds = frameOfPageAtIndex(selectedIndex)
    }
    
    var scrollView: UIScrollView! {
        return view as! UIScrollView
    }
    
    override func loadView() {
        let scrollView = UIScrollView(frame: UIScreen.mainScreen().bounds)
        scrollView.pagingEnabled = true
        view = scrollView
    }
    
    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)
        scrollView.addObserver(self, forKeyPath: "frame", options: .Initial | .New, context: nil)
        scrollView.addObserver(self, forKeyPath: "bounds", options: .Initial | .New, context: nil)
    }
    
    override func viewDidDisappear(animated: Bool) {
        super.viewDidDisappear(animated)
        scrollView.removeObserver(self, forKeyPath: "frame")
        scrollView.removeObserver(self, forKeyPath: "bounds")
    }
    
    override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
        if keyPath == "frame" {
            setupScrollView()
        } else if keyPath == "bounds" {
            layoutPages()
            selectedIndex = Int(scrollView.bounds.midX / scrollView.bounds.size.width)
        }
    }
    
    private func setupScrollView() {
        let pageSize = scrollView.bounds.size
        scrollView.contentSize.height = pageSize.height
        scrollView.contentSize.width = pageSize.width * CGFloat(viewControllers.count)
        scrollView.bounds = frameOfPageAtIndex(selectedIndex)
    }
    
    private func layoutPages() {
        let pageSize = scrollView.bounds.size
        let startIndex = Int(scrollView.bounds.minX / pageSize.width)
        let endIndex = Int(scrollView.bounds.maxX / pageSize.width)
        
        for (index, page) in enumerate(pages) {
            if startIndex ... endIndex ~= index {
                showViewControllerAtIndex(index)
            } else {
                hideViewControllerAtIndex(index)
            }
        }
    }
    
    private func hideViewControllerAtIndex(index: Int) {
        if let page = pages[index] {
            let controller = viewControllers[index]
            controller.willMoveToParentViewController(nil)
            controller.view.removeFromSuperview()
            controller.removeFromParentViewController()
            pages[index] = nil
        }
    }
    
    private func showViewControllerAtIndex(index: Int) {
        if let page = pages[index] {
            page.frame = frameOfPageAtIndex(index)
            
        } else {
            let controller = viewControllers[index]
            addChildViewController(controller)
            controller.view.frame = frameOfPageAtIndex(index)
            view.addSubview(controller.view)
            controller.didMoveToParentViewController(self)
            pages[index] = controller.view
        }
    }
    
    private func frameOfPageAtIndex(index: Int) -> CGRect {
        var pageFrame = scrollView.bounds
        pageFrame.origin.x = scrollView.bounds.size.width * CGFloat(index)
        return pageFrame
    }
}