//
//  PageViewController.swift
//  PaiAi
//
//  Created by ffib on 2018/12/6.
//  Copyright © 2018 yb. All rights reserved.
//

import UIKit

struct PageOption {
    var font: UIFont
    var normalColor: UIColor
    var selectedColor: UIColor
    var spacing: CGFloat
    
    init(font: UIFont = UIFont.systemFont(ofSize: 15),
         normalColor: UIColor = UIColor(white: 1, alpha: 0.7),
         selectedColor: UIColor = UIColor.white,
         spacing: CGFloat = 10) {
        self.font = font
        self.normalColor = normalColor
        self.selectedColor = selectedColor
        self.spacing = spacing
    }
}

struct PageItem {
    var title: String
    var viewController: UIViewController
    init(title: String, viewController: UIViewController) {
        self.title = title
        self.viewController = viewController
    }
}

fileprivate let baseTag = 55161750

class PageViewController: UIViewController {
    
    private enum ScrollDirection {
        case left
        case right
    }

    private var scrollView = UIScrollView()
    private var menuView = UIView()
    private var sliderView = UIView()
    private var contentRect = CGRect.zero
    
    //animation auxiliary
    private var currentIndex: Int = 0
    private var distance: CGFloat = 0
    private var currentItem: UILabel?
    private var startOffset: CGFloat = 0
    private var isBeginScroll: Bool = false
    private var menuItemWidths: [CGFloat] = []
    private var direction: ScrollDirection = .right
    private var sliderConstraint: NSLayoutConstraint?
   
    var option = PageOption()
    var pageItems = [PageItem]() {
        didSet {
            configurationPageItems()
            configurationMenuItems()
        }
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        contentRect = CGRect(x: 0, y: 0, width: view.frame.width, height: view.frame.height)
        configurationScrollView()
        configurationMenuView()
        configurationSliderView()
    }
    
    override func viewDidLayoutSubviews() {
        cacheMenuItemWidth()
    }
    
    fileprivate func cacheMenuItemWidth() {
        guard menuItemWidths.isEmpty else { return }
        menuItemWidths = Array(repeating: 0, count: menuView.subviews.count - 1)
        for view in menuView.subviews {
            guard let label = view as? UILabel else { continue }
            menuItemWidths[label.tag - baseTag] = label.frame.width
        }
    }
    
    fileprivate func configurationScrollView() {
        scrollView.bounces = false
        scrollView.delegate = self
        scrollView.frame = contentRect
        scrollView.isPagingEnabled = true
        scrollView.showsVerticalScrollIndicator = false
        scrollView.showsHorizontalScrollIndicator = false
        
        view.addSubview(scrollView)
        
        if #available(iOS 11.0, *) {
            scrollView.contentInsetAdjustmentBehavior = .never
        } else {
            automaticallyAdjustsScrollViewInsets = false
        }
    }
    
    fileprivate func configurationMenuView() {
        
        guard let barContentView = navigationController?.navigationBar else { return }
        
        barContentView.addSubview(menuView)
        menuView.translatesAutoresizingMaskIntoConstraints = false
        
        let top = menuView.topAnchor
            .constraint(equalTo: barContentView.topAnchor)
        let bottom = menuView.bottomAnchor
            .constraint(equalTo: barContentView.bottomAnchor)
        let centerX = menuView.centerXAnchor
            .constraint(equalTo: barContentView.centerXAnchor)
        
        NSLayoutConstraint.activate([top, bottom, centerX])
        
        let tap = UITapGestureRecognizer(target: self, action: #selector(tapMenu(tap:)))
        menuView.addGestureRecognizer(tap)
    }
    
    fileprivate func configurationSliderView() {
        sliderView.backgroundColor = option.selectedColor
        sliderView.translatesAutoresizingMaskIntoConstraints = false
        sliderView.layer.cornerRadius = 2.5
        menuView.addSubview(sliderView)
        
        let width = sliderView.widthAnchor.constraint(equalToConstant: 5)
        let height = sliderView.heightAnchor.constraint(equalToConstant: 5)
        let bottom = sliderView.bottomAnchor
            .constraint(equalTo: menuView.bottomAnchor, constant: -2)
        
        NSLayoutConstraint.activate([width, height, bottom])
    }
    
    fileprivate func configurationPageItems() {
        scrollView.contentSize = CGSize(width: contentRect.width * CGFloat(pageItems.count),
                                        height: contentRect.height)
        
        for (i, item) in pageItems.enumerated() {
            addChildViewController(item.viewController)
            scrollView.addSubview(item.viewController.view)
            
            item.viewController.view.translatesAutoresizingMaskIntoConstraints = false
            
            let width = item.viewController.view
                .widthAnchor
                .constraint(equalToConstant: contentRect.width)
            let heigth = item.viewController.view
                .heightAnchor
                .constraint(equalToConstant: contentRect.height)
            let top = item.viewController.view
                .topAnchor
                .constraint(equalTo: scrollView.topAnchor, constant: 0)
            let leading = item.viewController.view
                .leadingAnchor
                .constraint(equalTo: scrollView.leadingAnchor, constant: CGFloat(i) * contentRect.width)
            
            NSLayoutConstraint.activate([width, heigth, top, leading])
        }
    }
    
    fileprivate func configurationMenuItems() {
        var last: UILabel?
        for (i, item) in pageItems.enumerated() {
            let label = UILabel()
            label.text = item.title
            label.tag = baseTag + i
            label.font = option.font
            label.textAlignment = .center
            label.textColor = option.normalColor
            label.translatesAutoresizingMaskIntoConstraints = false
            
            menuView.addSubview(label)
            
            let left: NSLayoutConstraint
            if let lastLabel = last {
                left = label.leftAnchor
                    .constraint(equalTo: lastLabel.rightAnchor, constant: option.spacing)
            } else {
                left = label.leadingAnchor
                    .constraint(equalTo: menuView.leadingAnchor)
            }
            
            let centerY = label.centerYAnchor
                .constraint(equalTo: menuView.centerYAnchor)
            
            if i == 0 {
                label.textColor = option.selectedColor
                label.font = UIFont.systemFont(ofSize: option.font.pointSize + 1)
                currentItem = label
            } else if i == pageItems.count - 1 {
                NSLayoutConstraint.activate([label.trailingAnchor
                    .constraint(equalTo: menuView.trailingAnchor)])
            }
            
            NSLayoutConstraint.activate([left, centerY])
            last = label
        }
        
        configurationSliderViewDetail()
    }
    
    fileprivate func configurationSliderViewDetail() {
        guard let label = menuView.viewWithTag(baseTag) else { return }
        sliderConstraint = sliderView.centerXAnchor
            .constraint(equalTo: label.centerXAnchor)
        
        NSLayoutConstraint.activate([sliderConstraint!])
    }
}

//action
extension PageViewController {
    @objc fileprivate func tapMenu(tap: UITapGestureRecognizer) {
        var x = tap.location(in: menuView).x
        for (i, width) in menuItemWidths.enumerated() {
            x -= (width + option.spacing / 2)
            guard x <= 0 else { continue }
            if i != currentIndex {
                didSelect(i)
            }
            return
        }
    }
}

extension PageViewController: UIScrollViewDelegate {
    
    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        startOffset = self.scrollView.contentOffset.x
        isBeginScroll = true
    }
    
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        guard scrollView.isDragging || scrollView.isDecelerating || scrollView.isTracking else { return }
        
        initializeScrollParameter()
        moveSlider(percentage: (self.scrollView.contentOffset.x - startOffset ) / contentRect.width)
    }
    
    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        guard !decelerate else { return }
        pageViewDidEndScroll()
    }

    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        pageViewDidEndScroll()
    }
    
    func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
        pageViewDidEndScroll()
    }
}

//linkage
extension PageViewController {
    fileprivate func didSelect(_ index: Int) {
        startOffset = self.scrollView.contentOffset.x
        sliderAnimation(index)
        didSelectPageItem(index)
    }
    
    fileprivate func initializeScrollParameter() {
        if isBeginScroll {
            direction = self.scrollView.contentOffset.x - startOffset > 0 ? .right : .left
            guard let label = currentItem else { return }
            switch direction {
            case .left:
                distance = menuItemWidths[label.tag - baseTag] / 2 + menuItemWidths[label.tag - baseTag - 1] / 2 + option.spacing
            case .right:
                distance = menuItemWidths[label.tag - baseTag] / 2 + menuItemWidths[label.tag - baseTag + 1] / 2 + option.spacing
            }
            isBeginScroll = false
        }
    }
}

//menu
extension PageViewController {
    fileprivate func moveSlider(percentage: CGFloat) {
        sliderConstraint?.constant = percentage * distance
    }
    
    fileprivate func didSelectMenuItem(_ index: Int) {
        guard let currentLabel = currentItem,
            let label = menuView.viewWithTag(baseTag + index) as? UILabel else { return }
        
        currentItem = label
        
        currentLabel.font = option.font
        currentLabel.textColor = option.normalColor
        
        label.textColor = option.selectedColor
        label.font = UIFont.systemFont(ofSize: option.font.pointSize + 1)
        
        currentIndex = index
        
        updateSliderConstraint()
    }
    
    fileprivate func sliderAnimation(_ index: Int) {
        let isLeft = currentIndex - index > 0
        var animationDistance: CGFloat = 0
        for i in isLeft ? (index..<currentIndex) : (currentIndex..<index) {
            animationDistance += menuItemWidths[i] / 2 + menuItemWidths[i + 1] / 2 + option.spacing
        }
        
        UIView.animate(withDuration: 0.25) {
            self.sliderConstraint?.constant = isLeft ? -animationDistance : animationDistance
            self.menuView.layoutIfNeeded()
        }
    }
    
    fileprivate func updateSliderConstraint() {
        NSLayoutConstraint.deactivate([sliderConstraint!])
        sliderConstraint = sliderView.centerXAnchor.constraint(equalTo: currentItem!.centerXAnchor)
        NSLayoutConstraint.activate([sliderConstraint!])
    }
}

//page
extension PageViewController {
    fileprivate func didSelectPageItem(_ index: Int) {
        scrollView.setContentOffset(CGPoint.init(x: CGFloat(index) * contentRect.width, y: 0), animated: true)
    }
    
    fileprivate func pageViewDidEndScroll() {
        guard self.scrollView.contentOffset.x != startOffset else { return }
        let index = Int(self.scrollView.contentOffset.x / contentRect.width)
        
        switch direction {
        case .left:
            didSelectMenuItem(index)
        case .right:
            didSelectMenuItem(index)
        }
    }
}