cs193p-emojiart/EmojiArt/EmojiArtDocumentView.swift

//
//  EmojiArtDocumentView.swift
//  EmojiArt
//
//  Created by ching on 2023/2/19.
//

import SwiftUI

struct EmojiArtDocumentView: View {
    @ObservedObject var document: EmojiArtDocument

    var body: some View {
        VStack(spacing: 0) {
            documentBody
            PaletteChooser(emojiFontSize: const.defaultEmojiFontSize)
        }
    }

    var documentBody: some View {
        GeometryReader { geometry in
            ZStack {
                Color.yellow.overlay(
                    OptionalImage(uiImage: document.backgroundImage)
                        .scaleEffect(zoomScale)
                        .position(convertFromEmojiCoordinates((0, 0), in: geometry))
                )
                .gesture(doubleTapToZoom(in: geometry.size))
                if document.backgroundImageFetchStatus == .fetching {
                    ProgressView().scaleEffect(2)
                } else {
                    ForEach(document.emojis) { emoji in
                        Text(emoji.text)
                            .font(.system(size: fontSize(for: emoji)))
                            .scaleEffect(zoomScale)
                            .position(position(for: emoji, in: geometry))
                    }
                }
            }
            .clipped()
            .onDrop(of: [.plainText, .url, .image], isTargeted: nil) { providers, location in
                drop(providers: providers, at: location, in: geometry)
            }
            .gesture(panGesture().simultaneously(with: zoomGesture()))
            .alert(item: $alertToShow) { alertToShow in
                alertToShow.alert()
            }
            .onChange(of: document.backgroundImageFetchStatus) { status in
                switch status {
                case .failed(let url):
                    showBackgroundImageFetchFailedAlert(url)
                default:
                    break
                }
            }
        }
    }

    @State private var alertToShow: IdentifiableAlert?

    private func showBackgroundImageFetchFailedAlert(_ url: URL) {
        alertToShow = IdentifiableAlert(id: "Fetch failed: " + url.absoluteString, alert: {
            Alert(
                title: Text("Background Image Fetch"),
                message: Text("Couldn't load image from \(url)."),
                dismissButton: .default(Text("OK")))
        })
    }

    private func drop(providers: [NSItemProvider], at location: CGPoint, in geometry: GeometryProxy) -> Bool {
        var found = providers.loadObjects(ofType: URL.self) { url in
            document.setBackground(EmojiArtModel.Background.url(url.imageURL))
        }
        if !found {
            found = providers.loadObjects(ofType: UIImage.self) { image in
                if let data = image.jpegData(compressionQuality: 1.0) {
                    document.setBackground(.imageData(data))
                }
            }
        }
        if !found {
            found = providers.loadObjects(ofType: String.self) { string in
                if let emoji = string.first, emoji.isEmoji {
                    document.addEmoji(
                        String(emoji),
                        at: convertToEmojiCoordinates(location, in: geometry),
                        size: const.defaultEmojiFontSize / zoomScale)
                }
            }
        }
        return found
    }

    private func fontSize(for emoji: EmojiArtModel.Emoji) -> CGFloat {
        CGFloat(emoji.size)
    }

    private func position(for emoji: EmojiArtModel.Emoji, in geometry: GeometryProxy) -> CGPoint {
        convertFromEmojiCoordinates((emoji.x, emoji.y), in: geometry)
    }

    private func convertToEmojiCoordinates(_ location: CGPoint, in geometry: GeometryProxy) -> (x: Int, y: Int) {
        let center = geometry.frame(in: .local).center
        let location = CGPoint(
            x: (location.x - center.x - panOffset.width) / zoomScale,
            y: (location.y - center.y - panOffset.height) / zoomScale)
        return (Int(location.x), Int(location.y))
    }

    private func convertFromEmojiCoordinates(_ location: (x: Int, y: Int), in geometry: GeometryProxy) -> CGPoint {
        let center = geometry.frame(in: .local).center
        return CGPoint(
            x: center.x + CGFloat(location.x) * zoomScale + panOffset.width,
            y: center.y + CGFloat(location.y) * zoomScale + panOffset.height)
    }

    @State private var steadyStatePanOffset: CGSize = .zero
    @GestureState private var gesturePanOffset: CGSize = .zero
    private var panOffset: CGSize {
        (steadyStatePanOffset + gesturePanOffset) * zoomScale
    }

    @State private var steadyStateZoomScale: CGFloat = 1
    @GestureState private var gestureZoomScale: CGFloat = 1
    private var zoomScale: CGFloat {
        steadyStateZoomScale * gestureZoomScale
    }

    private func panGesture() -> some Gesture {
        DragGesture()
            .updating($gesturePanOffset) { latestDragGestureValue, gesturePanOffset, _ in
                gesturePanOffset = latestDragGestureValue.translation / zoomScale
            }
            .onEnded { finalDragGestureValue in
                steadyStatePanOffset = steadyStatePanOffset + (finalDragGestureValue.translation / zoomScale)
            }
    }

    private func zoomGesture() -> some Gesture {
        MagnificationGesture()
            .updating($gestureZoomScale) { latestGestureScale, gestureZoomScale, _ in
                gestureZoomScale = latestGestureScale
            }
            .onEnded { gestureScaleAtEnd in
                steadyStateZoomScale *= gestureScaleAtEnd
            }
    }

    private func doubleTapToZoom(in size: CGSize) -> some Gesture {
        TapGesture(count: 2)
            .onEnded {
                withAnimation {
                    zoomToFit(document.backgroundImage, in: size)
                }
            }
    }

    private func zoomToFit(_ image: UIImage?, in size: CGSize) {
        if let image = image, image.size.width > 0, image.size.height > 0, size.width > 0, size.height > 0 {
            let hZoom = size.width / image.size.width
            let vZoom = size.height / image.size.height
            steadyStatePanOffset = .zero
            steadyStateZoomScale = min(hZoom, vZoom)
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        EmojiArtDocumentView(document: EmojiArtDocument())
    }
}