NetNewsWire/iOS/Article/WebViewProvider.swift

//
//  WebViewProvider.swift
//  NetNewsWire-iOS
//
//  Created by Maurice Parker on 9/21/19.
//  Copyright © 2019 Ranchero Software. All rights reserved.
//

import Foundation
import WebKit

/// WKWebView has an awful behavior of a flash to white on first load when in dark mode.
/// Keep a queue of WebViews where we've already done a trivial load so that by the time we need them in the UI, they're past the flash-to-shite part of their lifecycle.
class WebViewProvider: NSObject, WKNavigationDelegate {
	
	private struct MessageName {
		static let domContentLoaded = "domContentLoaded"
	}

	let articleIconSchemeHandler: ArticleIconSchemeHandler
	
	private let minimumQueueDepth = 3
	private let maximumQueueDepth = 6
	private var queue = UIView()
	
	private var waitingForFirstLoad = true
	private var waitingCompletionHandler: ((WKWebView) -> ())?

	init(coordinator: SceneCoordinator, viewController: UIViewController) {
		articleIconSchemeHandler = ArticleIconSchemeHandler(coordinator: coordinator)
		super.init()
		viewController.view.insertSubview(queue, at: 0)
		
		replenishQueueIfNeeded()
		
		NotificationCenter.default.addObserver(self, selector: #selector(didEnterBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)
		NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)
	}
	
	@objc func didEnterBackground() {
		flushQueue()
	}

	@objc func willEnterForeground() {
		replenishQueueIfNeeded()
	}

	func flushQueue() {
		queue.subviews.forEach { $0.removeFromSuperview() }
		waitingForFirstLoad = true
	}

	func replenishQueueIfNeeded() {
		while queue.subviews.count < minimumQueueDepth {
			let webView = WKWebView(frame: .zero, configuration: buildConfiguration())
			enqueueWebView(webView)
		}
	}
	
	func dequeueWebView(completion: @escaping (WKWebView) -> ()) {
		if waitingForFirstLoad {
			waitingCompletionHandler = completion
		} else {
			completeRequest(completion: completion)
		}
	}
	
	func enqueueWebView(_ webView: WKWebView) {
		guard queue.subviews.count < maximumQueueDepth else {
			return
		}

		webView.configuration.userContentController.add(WrapperScriptMessageHandler(self), name: MessageName.domContentLoaded)
		queue.insertSubview(webView, at: 0)

		webView.loadFileURL(ArticleRenderer.page.url, allowingReadAccessTo: ArticleRenderer.page.baseURL)
	}

	// MARK: WKNavigationDelegate
	
	func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
		if waitingForFirstLoad {
			waitingForFirstLoad = false
			if let completion = waitingCompletionHandler {
				DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
					self.completeRequest(completion: completion)
					self.waitingCompletionHandler = nil
				}
			}
		}
	}
	
}

// MARK: WKScriptMessageHandler

extension WebViewProvider: WKScriptMessageHandler {

	func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
		switch message.name {
		case MessageName.domContentLoaded:
			if waitingForFirstLoad {
				waitingForFirstLoad = false
				if let completion = waitingCompletionHandler {
					DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
						self.completeRequest(completion: completion)
						self.waitingCompletionHandler = nil
					}
				}
			}
		default:
			return
		}
	}
	
}

// MARK: Private

private extension WebViewProvider {
	
	func completeRequest(completion: @escaping (WKWebView) -> ()) {
		if let webView = queue.subviews.last as? WKWebView {
			webView.removeFromSuperview()
			webView.configuration.userContentController.removeScriptMessageHandler(forName: MessageName.domContentLoaded)
			replenishQueueIfNeeded()
			completion(webView)
			return
		}

		assertionFailure("Creating WKWebView in \(#function); queue has run dry.")
		let webView = WKWebView(frame: .zero)
		completion(webView)
	}

	func buildConfiguration() -> WKWebViewConfiguration {
		let preferences = WKPreferences()
		preferences.javaScriptCanOpenWindowsAutomatically = false
		preferences.javaScriptEnabled = true

		let configuration = WKWebViewConfiguration()
		configuration.preferences = preferences
		configuration.setValue(true, forKey: "allowUniversalAccessFromFileURLs")
		configuration.allowsInlineMediaPlayback = true
		configuration.mediaTypesRequiringUserActionForPlayback = .video
		configuration.setURLSchemeHandler(articleIconSchemeHandler, forURLScheme: ArticleRenderer.imageIconScheme)
		
		return configuration
	}
}