InteractivePushableNavigationController.swift

//
//  InteractivePushableNavigationController.swift
//  InteractivePushGestureRecognizerExample
//
//  Created by Li-Heng Hsu on 2020/3/11.
//  Copyright © 2020 Li-Heng Hsu. All rights reserved.
//

import UIKit

class InteractivePushableNavigationController: UINavigationController {
    
    // 因為官方沒有統合用來追蹤pop、push事件的API，所以只好自己實作。
    enum TransitionState {
        case popping([UIViewController]), pushing, setting, none
    }
    private(set) var transitionState = TransitionState.none
    
    // 將被pop掉的VC儲存到一個stack裡。
    var poppedViewControllerStack = [UIViewController]()
    
    // 負責interactive push轉場的物件。
    var interactivePushTransition: InteractivePushTransition?
    
    // Interactive push手勢。
    lazy var interactivePushGestureRecognizer: UIGestureRecognizer = {
        let gesture = UIScreenEdgePanGestureRecognizer(target: self, action: #selector(startPushGesture(_:)))
        gesture.edges = .right
        return gesture
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.delegate = self
        
        // 設定手勢。
        self.view.addGestureRecognizer(interactivePushGestureRecognizer)
        
        self.interactivePopGestureRecognizer?.delegate = self
        self.interactivePushGestureRecognizer.delegate = self
    }
    
    // 負責處理interactive push手勢，轉給self.parallaxTransitionController。
    @objc func startPushGesture(_ sender: UIScreenEdgePanGestureRecognizer) {
        
        let percentage: CGFloat = (view.frame.width - sender.location(in: view).x) / view.frame.width
        switch sender.state {
        case .began:
            guard let vc = poppedViewControllerStack.last else { return }
            interactivePushTransition = .init()
            super.pushViewController(vc, animated: true)
        case .changed:
            interactivePushTransition?.update(percentage)
        case .ended:
            if sender.velocity(in: view).x > 10 {
                interactivePushTransition?.cancel()
            } else if sender.velocity(in: view).x < -10 {
                interactivePushTransition?.finish()
                poppedViewControllerStack.removeLast()
            } else {
                if percentage >= 0.5 {
                    interactivePushTransition?.finish()
                    poppedViewControllerStack.removeLast()
                } else {
                    interactivePushTransition?.cancel()
                }
            }
            interactivePushTransition = nil
        default:
            break
        }
    }
    
    // 將pop、push事件儲存成self.transitionState。
    
    override func setViewControllers(_ viewControllers: [UIViewController], animated: Bool) {
        super.setViewControllers(viewControllers, animated: animated)
        self.transitionState = .setting
    }
    
    override func pushViewController(_ viewController: UIViewController, animated: Bool) {
        super.pushViewController(viewController, animated: animated)
        self.transitionState = .pushing
    }
    
    override func popViewController(animated: Bool) -> UIViewController? {
        guard let poppedVC = super.popViewController(animated: animated) else { return nil }
        self.transitionState = .popping([poppedVC])
        return poppedVC
    }
    
    override func popToViewController(_ viewController: UIViewController, animated: Bool) -> [UIViewController]? {
        guard let poppedVCs = super.popToViewController(viewController, animated: animated) else { return nil }
        self.transitionState = .popping(poppedVCs)
        return poppedVCs
    }
    
    override func popToRootViewController(animated: Bool) -> [UIViewController]? {
        guard let poppedVCs = super.popToRootViewController(animated: animated) else { return nil }
        self.transitionState = .popping(poppedVCs)
        return poppedVCs
    }

}

extension InteractivePushableNavigationController: UIGestureRecognizerDelegate {
    
    func gestureRecognizerShouldBegin(_ gestureRecognizer: UIGestureRecognizer) -> Bool {
        switch gestureRecognizer {
        case interactivePopGestureRecognizer:
            return viewControllers.count > 1
        case interactivePushGestureRecognizer:
            return !poppedViewControllerStack.isEmpty
        default:
            return true
        }
        
    }
}

extension InteractivePushableNavigationController: UINavigationControllerDelegate {
    
    // 只有在interactive push的時候，self.parallaxTransitionController才會存在，才會被調用。其它時候都是回傳nil用內建的轉場。
    
    func navigationController(_ navigationController: UINavigationController, animationControllerFor operation: UINavigationController.Operation, from fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        interactivePushTransition
    }
    
    func navigationController(_ navigationController: UINavigationController, interactionControllerFor animationController: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning? {
        interactivePushTransition
    }
    
    // 獲取push與pop事件。
    func navigationController(_ navigationController: UINavigationController, didShow viewController: UIViewController, animated: Bool) {
        switch self.transitionState {
        case .popping(let poppedVCs):
            self.poppedViewControllerStack.append(contentsOf: poppedVCs.reversed())
        case .pushing, .setting:
            self.poppedViewControllerStack.removeAll()
        case .none:
            break
        }
        
        self.transitionState = .none
    }
}

class InteractivePushTransition: UIPercentDrivenInteractiveTransition, UIViewControllerAnimatedTransitioning {
    
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        0.5
    }
    
    // 模仿內建的parallax效果。
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        guard let fromView = transitionContext.view(forKey: .from), let toView = transitionContext.view(forKey: .to) else { return }
        let containerView = transitionContext.containerView
        let dimmingView = UIView()
        dimmingView.frame = fromView.frame
        
        containerView.insertSubview(toView, aboveSubview: fromView)
        containerView.insertSubview(dimmingView, belowSubview: toView)
        dimmingView.addSubview(fromView)
        fromView.frame = dimmingView.bounds
        toView.frame = dimmingView.frame
        toView.frame.origin.x += toView.frame.width
        
        UIView.animate(withDuration: transitionDuration(using: transitionContext), delay: 0, options: .curveLinear, animations: {
            toView.frame = dimmingView.frame
            dimmingView.alpha = 0.9
            dimmingView.frame.origin.x -= dimmingView.frame.width * 0.3
        }) { (finished) in
            dimmingView.removeFromSuperview()
            if transitionContext.transitionWasCancelled {
                containerView.addSubview(fromView)
                transitionContext.completeTransition(false)
            } else {
                transitionContext.completeTransition(true)
            }
        }
    }
    
    
    
}