Gemini/Gemini-iOS/BrowserWebViewController.swift

//
//  BrowserWebViewController.swift
//  Gemini-iOS
//
//  Created by Shadowfacts on 12/16/20.
//

import UIKit
import BrowserCore
import WebKit
import GeminiProtocol
import GeminiFormat
import GeminiRenderer

class BrowserWebViewController: UIViewController {

    let navigator: NavigationManager
    let url: URL
    
    weak var scrollViewDelegate: UIScrollViewDelegate? {
        didSet {
            if isViewLoaded {
                webView.scrollView.delegate = scrollViewDelegate
            }
        }
    }
    
    private var task: GeminiDataTask?
    private let renderer = GeminiHTMLRenderer()
    private(set) var document: Document?
    private var loaded = false
    
    private var errorStack: UIStackView!
    private var errorMessageLabel: UILabel!
    private var activityIndicator: UIActivityIndicatorView!
    private var webView: WKWebView!
    
    init(navigator: NavigationManager, url: URL) {
        self.navigator = navigator
        self.url = url
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let documentURL = self.url
        renderer.linkPrefix = { (url: URL) -> String in
            let symbolClass: String
            if url.scheme == "gemini" {
                if url.host == documentURL.host {
                    symbolClass = "arrow-right"
                } else {
                    symbolClass = "link"
                }
            } else if url.scheme == "http" || url.scheme == "https" {
                symbolClass = "safari"
            } else if url.scheme == "mailto" {
                symbolClass = "envelope"
            } else {
                symbolClass = "arrow-up-left-square"
            }
            return "<span class=\"symbol \(symbolClass)\" aria-hidden=\"true\"></span>"

        }
        
        view.backgroundColor = .systemBackground        
        
        webView = WKWebView()
        webView.backgroundColor = .systemBackground
        webView.isOpaque = false
        webView.navigationDelegate = self
        // it is safe to set the delegate of the web view's internal scroll view becuase WebKit takes care of forwarding between its internal delegate and our own
        webView.scrollView.delegate = scrollViewDelegate
        // this doesn't default to .default :S
        webView.scrollView.indicatorStyle = .default
        webView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(webView)
        NSLayoutConstraint.activate([
            webView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            webView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            webView.topAnchor.constraint(equalTo: view.topAnchor),
            webView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])
        
        let errorTitle = UILabel()
        errorTitle.text = "An error occurred"
        errorTitle.font = UIFont(descriptor: UIFontDescriptor.preferredFontDescriptor(withTextStyle: .headline).withDesign(.serif)!, size: 0)
        errorTitle.numberOfLines = 0
        errorMessageLabel = UILabel()
        errorMessageLabel.font = UIFont(descriptor: UIFontDescriptor.preferredFontDescriptor(withTextStyle: .body).withDesign(.serif)!, size: 0)
        errorMessageLabel.numberOfLines = 0
        errorStack = UIStackView(arrangedSubviews: [
            errorTitle,
            errorMessageLabel,
        ])
        errorStack.translatesAutoresizingMaskIntoConstraints = false
        errorStack.axis = .vertical
        errorStack.alignment = .center
        errorStack.isHidden = true
        view.addSubview(errorStack)
        NSLayoutConstraint.activate([
            errorStack.centerYAnchor.constraint(equalTo: view.centerYAnchor),
            errorStack.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            errorStack.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        ])
        
        activityIndicator = UIActivityIndicatorView(style: .large)
        activityIndicator.translatesAutoresizingMaskIntoConstraints = false
        activityIndicator.isHidden = true
        view.addSubview(activityIndicator)
        NSLayoutConstraint.activate([
            activityIndicator.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            activityIndicator.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        loadDocument()
    }
    
    private func loadDocument() {
        guard !loaded else { return }
        
        webView.isHidden = true
        errorStack.isHidden = true
        activityIndicator.isHidden = false
        activityIndicator.startAnimating()
        
        let url = self.url
        task = try! GeminiDataTask(url: url) { (response) in
            self.task = nil
            self.loaded = true
            
            switch response {
            case let .failure(error):
                DispatchQueue.main.async {
                    self.showError(message: error.localizedDescription)
                }
            case let .success(response):
                if response.status.isRedirect {
                    if let redirect = URL(string: response.meta) {
                        self.navigator.changeURL(redirect)
                    } else {
                        DispatchQueue.main.async {
                            self.showError(message: "Invalid redirect URL: '\(response.meta)'")
                        }
                    }
                } else if response.status.isSuccess,
                          let text = response.bodyText {
                    self.renderDocument(GeminiParser.parse(text: text, baseURL: url))
                } else {
                    DispatchQueue.main.async {
                        self.showError(message: "Unknown error: \(response.header)")
                    }
                }
            }
        }
        task!.resume()
    }
    
    private func showError(message: String) {
        webView.isHidden = true
        errorStack.isHidden = false
        activityIndicator.isHidden = true
        activityIndicator.stopAnimating()
        
        errorMessageLabel.text = message
    }
    
    private func renderDocument(_ doc: Document) {
        self.document = doc
        
        let html = BrowserWebViewController.preamble + renderer.renderDocumentToHTML(doc) + BrowserWebViewController.postamble
        DispatchQueue.main.async {
            self.webView.isHidden = false
            self.errorStack.isHidden = true
            self.activityIndicator.isHidden = true
            self.activityIndicator.stopAnimating()
            
            if let title = doc.title {
                self.navigationItem.title = title
            }
            
            self.webView.loadHTMLString(html, baseURL: Bundle.main.bundleURL)
        }
    }
    
    func scrollToLine(index: Int, animated: Bool) {
        if animated {
            webView.evaluateJavaScript("document.getElementById('l\(index)').getBoundingClientRect().top + window.scrollY") { (result, error) in
                guard let result = result as? CGFloat else {
                    return
                }
                let scrollView = self.webView.scrollView
                let y = result * scrollView.zoomScale - scrollView.safeAreaInsets.top
                let maxY = scrollView.contentSize.height - scrollView.bounds.height + scrollView.safeAreaInsets.bottom
                let finalOffsetY = min(y, maxY)
                self.webView.scrollView.setContentOffset(CGPoint(x: 0, y: finalOffsetY), animated: true)
            }
        } else {
            webView.evaluateJavaScript("document.getElementById('l\(index)').scrollIntoView();")
        }
    }
    
    private static let preamble = """
    <!doctype html>
    <html>
    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no" />
        <link rel="stylesheet" href="\(Bundle.main.url(forResource: "browser", withExtension: "css")!.absoluteString)">
        <style>
        \(symbolStyles)
        </style>
    </head>
    <body>
    """
    
    private static let symbolStyles = SymbolCache.symbols.map { (k, v) in
        ".symbol.\(k.replacingOccurrences(of: ".", with: "-")) { background-image: url(\"data:image/png;base64,\(v)\"); }"
    }.joined(separator: "\n")
    
    private static let postamble = """
    </body>
    </html>
    """

}

extension BrowserWebViewController: WKNavigationDelegate {
    func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, decisionHandler: @escaping (WKNavigationActionPolicy) -> Void) {
        let url = navigationAction.request.url!
        if url.scheme == "file" {
            decisionHandler(.allow)
        } else {
            decisionHandler(.cancel)
            navigator.changeURL(url)
            loadDocument()
        }
    }
}