Gemini/Gemini-iOS/ContentView.swift

//
//  ContentView.swift
//  Gemini-iOS
//
//  Created by Shadowfacts on 7/15/20.
//

import SwiftUI
import BrowserCore

// This is not currently used as SwiftUI's ScrollView has no mechanism for detecting when it stops deceleraing,
// which is necessary to preven tthe bars from being left in a partially visible state.
struct ContentView: View {
    @ObservedObject private var navigator: NavigationManager
    @State private var urlFieldContents: String
    @State private var showPreferencesSheet = false
    private let shareCurrentURL: () -> Void
    @State private var prevScrollOffset: CGFloat = 0
    @State private var scrollOffset: CGFloat = 0 {
        didSet {
            prevScrollOffset = oldValue
        }
    }
    @State private var barOffset: CGFloat = 0
    @State private var navBarHeight: CGFloat = 0
    @State private var toolBarHeight: CGFloat = 0

    init(navigator: NavigationManager, shareCurrentURL: @escaping () -> Void) {
        self.navigator = navigator
        self._urlFieldContents = State(initialValue: navigator.currentURL.absoluteString)
        self.shareCurrentURL = shareCurrentURL
    }
    
    var body: some View {
        ZStack {
            GeometryReader { (outer: GeometryProxy) in
                ScrollView(.vertical) {
                    Color.clear.frame(height: navBarHeight)
                    
                    BrowserView(navigator: navigator, scrollingEnabled: false)
                        .background(GeometryReader { (inner: GeometryProxy) in
                            Color.clear.preference(key: ScrollOffsetPrefKey.self, value: -inner.frame(in: .global).minY + outer.frame(in: .global).minY)
                        })
                    
                    Color.clear.frame(height: toolBarHeight)
                }
                .onPreferenceChange(ScrollOffsetPrefKey.self) {
                    scrollOffset = $0
                    let delta = scrollOffset - prevScrollOffset
                    
                    // When certain state changes happen, the scroll view seems to "scroll" by the top safe area inset.
                    // It's not actually user scrolling, and this screws up our animation, so we ignore it.
                    guard abs(delta) != outer.safeAreaInsets.top else { return }
                    
                    if delta != 0 {
                        barOffset += delta
                    }
                    
                    barOffset = max(0, min(navBarHeight + outer.safeAreaInsets.top, barOffset))
                }
            }
            
            VStack(spacing: 0) {
                NavigationBar(navigator: navigator)
                    .background(GeometryReader { (geom: GeometryProxy) in
                        Color.clear.preference(key: NavBarHeightPrefKey.self, value: geom.frame(in: .global).height)
                    })
                    .offset(y: -barOffset)
                
                Spacer()
                
                ToolBar(navigator: navigator, shareCurrentURL: shareCurrentURL)
                    .background(GeometryReader { (geom: GeometryProxy) in
                        Color.clear.preference(key: ToolBarHeightPrefKey.self, value: geom.frame(in: .global).height)
                    })
                    .offset(y: barOffset)
            }
            .onPreferenceChange(NavBarHeightPrefKey.self) {
                navBarHeight = $0
                print("nav bar height: \($0)")
            }
            .onPreferenceChange(ToolBarHeightPrefKey.self) {
                toolBarHeight = $0
                print("tool bar height: \($0)")
            }
        }
        .onAppear(perform: tweakAppearance)
        .onReceive(navigator.$currentURL, perform: { (new) in
            urlFieldContents = new.absoluteString
        })
        .sheet(isPresented: $showPreferencesSheet, content: {
            PreferencesView(presented: $showPreferencesSheet)
        })
    }
    
    private func tweakAppearance() {
        UIScrollView.appearance().keyboardDismissMode = .interactive
    }
    
    private func commitURL() {
        guard let url = URL(string: urlFieldContents) else { return }
        navigator.changeURL(url)
    }
}

fileprivate struct ScrollOffsetPrefKey: PreferenceKey {
    typealias Value = CGFloat
    static var defaultValue: CGFloat = 0
    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
        value += nextValue()
    }
}

fileprivate struct NavBarHeightPrefKey: PreferenceKey {
    typealias Value = CGFloat
    static var defaultValue: CGFloat = 0
    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
        value += nextValue()
    }
}

fileprivate struct ToolBarHeightPrefKey: PreferenceKey {
    typealias Value = CGFloat
    static var defaultValue: CGFloat = 0
    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
        value += nextValue()
    }
}

fileprivate enum ScrollDirection {
    case up, down, none
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView(navigator: NavigationManager(url: URL(string: "gemini://localhost/overview.gmi")!), shareCurrentURL: {})
    }
}