From f7b53283d6602d1a34f1e3b6a0ca7a12bf863242 Mon Sep 17 00:00:00 2001
From: Jim Correia <correia@omnigroup.com>
Date: Sat, 31 Aug 2019 12:53:03 -0700
Subject: [PATCH] Maintain a queue of "prepared" web views for use in
 DetailWebViewController.

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.
---
 iOS/AppCoordinator.swift              |  3 ++
 iOS/Detail/DetailViewController.swift | 53 ++++++++++++++++++++++++++-
 2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/iOS/AppCoordinator.swift b/iOS/AppCoordinator.swift
index 1eaf7f4e5..415048466 100644
--- a/iOS/AppCoordinator.swift
+++ b/iOS/AppCoordinator.swift
@@ -237,6 +237,9 @@ class AppCoordinator: NSObject, UndoableCommandRunner, UnreadCountProvider {
 		
 		NotificationCenter.default.addObserver(self, selector: #selector(userDefaultsDidChange(_:)), name: UserDefaults.didChangeNotification, object: nil)
 		NotificationCenter.default.addObserver(self, selector: #selector(accountDidDownloadArticles(_:)), name: .AccountDidDownloadArticles, object: nil)
+		
+		// Force lazy initialization of the web view provider so that it can warm up the queue of prepared web views
+		let _ = DetailViewControllerWebViewProvider.shared
 	}
 	
 	func start() -> UIViewController {
diff --git a/iOS/Detail/DetailViewController.swift b/iOS/Detail/DetailViewController.swift
index 632a5134f..214e3dc8e 100644
--- a/iOS/Detail/DetailViewController.swift
+++ b/iOS/Detail/DetailViewController.swift
@@ -26,10 +26,16 @@ class DetailViewController: UIViewController {
 
 	weak var coordinator: AppCoordinator!
 	
+	deinit {
+		webView.removeFromSuperview()
+		DetailViewControllerWebViewProvider.shared.enqueueWebView(webView)
+		webView = nil
+	}
+	
 	override func viewDidLoad() {
 		super.viewDidLoad()
 
-		webView = WKWebView(frame: webViewContainer.bounds)
+		webView = DetailViewControllerWebViewProvider.shared.dequeueWebView()
 		webView.translatesAutoresizingMaskIntoConstraints = false
 		webView.navigationDelegate = self
 		
@@ -237,3 +243,48 @@ private extension DetailViewController {
 	}
 	
 }
+
+// MARK: -
+
+/// 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 DetailViewControllerWebViewProvider {
+	static var shared = DetailViewControllerWebViewProvider()
+	
+	func dequeueWebView() -> WKWebView {
+		if let webView = queue.popLast() {
+			replenishQueueIfNeeded()
+			return webView
+		}
+		
+		assertionFailure("Creating WKWebView in \(#function); queue has run dry.")
+		let webView = WKWebView(frame: .zero)
+		return webView
+	}
+	
+	func enqueueWebView(_ webView: WKWebView) {
+		webView.uiDelegate = nil
+		webView.navigationDelegate = nil
+
+		let html = ArticleRenderer.noSelectionHTML(style: .defaultStyle)
+		webView.loadHTMLString(html, baseURL: nil)
+
+		queue.insert(webView, at: 0)
+	}
+
+	// MARK: Private
+
+	private let minimumQueueDepth = 3
+	private var queue: [WKWebView] = []
+	
+	private init() {
+		replenishQueueIfNeeded()
+	}
+	
+	private func replenishQueueIfNeeded() {
+		while queue.count < minimumQueueDepth {
+			let webView = WKWebView(frame: .zero)
+			enqueueWebView(webView)
+		}
+	}
+}