Gemini/Gemini-iOS/BrowserViewController.swift

//
//  BrowserViewController.swift
//  Gemini-iOS
//
//  Created by Shadowfacts on 9/28/20.
//

import UIKit
import SwiftUI
import BrowserCore
import Combine

class BrowserViewController: UIViewController, UIScrollViewDelegate {
    
    let navigator: NavigationManager
    
    private var scrollView: UIScrollView!
    
    private var browserHost: UIHostingController<BrowserView>!
    private var navBarHost: UIHostingController<NavigationBar>!
    private var toolBarHost: UIHostingController<ToolBar>!
    
    private var prevScrollViewContentOffset: CGPoint?
    
    private var barAnimator: UIViewPropertyAnimator?
    
    private var cancellables = [AnyCancellable]()
    
    init(navigator: NavigationManager) {
        self.navigator = navigator
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .systemBackground
        
        scrollView = UIScrollView()
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.keyboardDismissMode = .interactive
        view.addSubview(scrollView)
        scrollView.delegate = self
        NSLayoutConstraint.activate([
            scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            scrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            scrollView.topAnchor.constraint(equalTo: view.topAnchor),
            scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])
        
        browserHost = UIHostingController(rootView: BrowserView(navigator: navigator, scrollingEnabled: false))
        browserHost.view.translatesAutoresizingMaskIntoConstraints = false
        scrollView.addSubview(browserHost.view)
        addChild(browserHost)
        browserHost.didMove(toParent: self)
        NSLayoutConstraint.activate([
            scrollView.contentLayoutGuide.leadingAnchor.constraint(equalTo: browserHost.view.leadingAnchor),
            scrollView.contentLayoutGuide.trailingAnchor.constraint(equalTo: browserHost.view.trailingAnchor),
            scrollView.contentLayoutGuide.topAnchor.constraint(equalTo: browserHost.view.topAnchor),
            scrollView.contentLayoutGuide.bottomAnchor.constraint(equalTo: browserHost.view.bottomAnchor),
            browserHost.view.widthAnchor.constraint(equalTo: view.widthAnchor),
            
            // make sure the browser host view is at least the screen height so the loading indicator appears centered
            browserHost.view.heightAnchor.constraint(greaterThanOrEqualTo: view.heightAnchor),
        ])
        
        navBarHost = UIHostingController(rootView: NavigationBar(navigator: navigator))
        navBarHost.view.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(navBarHost.view)
        addChild(navBarHost)
        navBarHost.didMove(toParent: self)
        
        NSLayoutConstraint.activate([
            navBarHost.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            navBarHost.view.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            navBarHost.view.topAnchor.constraint(equalTo: view.topAnchor),
        ])
        
        toolBarHost = UIHostingController(rootView: ToolBar(navigator: navigator))
        toolBarHost.view.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(toolBarHost.view)
        addChild(toolBarHost)
        toolBarHost.didMove(toParent: self)
        NSLayoutConstraint.activate([
            toolBarHost.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            toolBarHost.view.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            toolBarHost.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])
        
        navigator.$currentURL
            .sink { (_) in
                self.scrollView.contentOffset = .zero
                self.navBarHost.view.transform = .identity
                self.toolBarHost.view.transform = .identity
            }
            .store(in: &cancellables)
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        let insets = UIEdgeInsets(
            top: navBarHost.view.bounds.height - view.safeAreaInsets.top,
            left: 0,
            bottom: toolBarHost.view.bounds.height - view.safeAreaInsets.bottom,
            right: 0
        )
        scrollView.contentInset = insets
        scrollView.scrollIndicatorInsets = insets
    }
    
    // MARK: - UIScrollViewDelegate
    
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        var scrollViewDelta: CGFloat = 0
        if let prev = prevScrollViewContentOffset {
            scrollViewDelta = scrollView.contentOffset.y - prev.y
        }
        prevScrollViewContentOffset = scrollView.contentOffset
        
        // When certain state changes happen, the scroll view seems to "scroll" by top the safe area inset.
        // It's not actually user scrolling, and this screws up our animation, so we ignore it.
        guard abs(scrollViewDelta) != view.safeAreaInsets.top,
              scrollViewDelta != 0,
              scrollView.contentOffset.y > 0 else { return }
        
        let barAnimator: UIViewPropertyAnimator
        if let animator = self.barAnimator {
            barAnimator = animator
        } else {
            navBarHost.view.transform = .identity
            toolBarHost.view.transform = .identity
            barAnimator = UIViewPropertyAnimator(duration: 0.2, curve: .linear) {
                self.navBarHost.view.transform = CGAffineTransform(translationX: 0, y: -self.navBarHost.view.frame.height)
                self.toolBarHost.view.transform = CGAffineTransform(translationX: 0, y: self.toolBarHost.view.frame.height)
            }
            if scrollViewDelta < 0 {
                barAnimator.fractionComplete = 1
            }
            barAnimator.addCompletion { (_) in
                self.barAnimator = nil
            }
            self.barAnimator = barAnimator
        }
        
        let progressDelta = scrollViewDelta / navBarHost.view.bounds.height
        barAnimator.fractionComplete = max(0, min(1, barAnimator.fractionComplete + progressDelta))
    }
    
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        if let barAnimator = barAnimator {
            if barAnimator.fractionComplete < 0.5 {
                barAnimator.isReversed = true
            }
            barAnimator.startAnimation()
        }
    }
    
}