Serialize access to the web view provider. Issue #2043

iOS/Article/WebViewProvider.swift

@@ -7,42 +7,116 @@
 //
 
 import Foundation
+import RSCore
 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 {
 	
-	let articleIconSchemeHandler: ArticleIconSchemeHandler
-	
-	private let minimumQueueDepth = 3
-	private let maximumQueueDepth = 6
+	private let articleIconSchemeHandler: ArticleIconSchemeHandler
+	private let operationQueue = MainThreadOperationQueue()
 	private var queue = UIView()
 	
 	init(coordinator: SceneCoordinator, viewController: UIViewController) {
 		articleIconSchemeHandler = ArticleIconSchemeHandler(coordinator: coordinator)
 		super.init()
 		viewController.view.insertSubview(queue, at: 0)
-		
 		replenishQueueIfNeeded()
 	}
 	
 	func flushQueue() {
-		queue.subviews.forEach { $0.removeFromSuperview() }
+		operationQueue.add(WebViewProviderFlushQueueOperation(queue: queue))
 	}
 
 	func replenishQueueIfNeeded() {
-		while queue.subviews.count < minimumQueueDepth {
-			enqueueWebView(PreloadedWebView(articleIconSchemeHandler: articleIconSchemeHandler))
-		}
+		operationQueue.add(WebViewProviderReplenishQueueOperation(queue: queue, articleIconSchemeHandler: articleIconSchemeHandler))
 	}
 	
 	func dequeueWebView(completion: @escaping (PreloadedWebView) -> ()) {
+		operationQueue.add(WebViewProviderDequeueOperation(queue: queue, articleIconSchemeHandler: articleIconSchemeHandler, completion: completion))
+		operationQueue.add(WebViewProviderReplenishQueueOperation(queue: queue, articleIconSchemeHandler: articleIconSchemeHandler))
+	}
+	
+}
+
+class WebViewProviderFlushQueueOperation: MainThreadOperation {
+	
+	// MainThreadOperation
+	public var isCanceled = false
+	public var id: Int?
+	public weak var operationDelegate: MainThreadOperationDelegate?
+	public var name: String? = "WebViewProviderFlushQueueOperation"
+	public var completionBlock: MainThreadOperation.MainThreadOperationCompletionBlock?
+
+	private var queue: UIView
+	
+	init(queue: UIView) {
+		self.queue = queue
+	}
+	
+	func run() {
+		queue.subviews.forEach { $0.removeFromSuperview() }
+		self.operationDelegate?.operationDidComplete(self)
+	}
+	
+}
+
+class WebViewProviderReplenishQueueOperation: MainThreadOperation {
+	
+	// MainThreadOperation
+	public var isCanceled = false
+	public var id: Int?
+	public weak var operationDelegate: MainThreadOperationDelegate?
+	public var name: String? = "WebViewProviderReplenishQueueOperation"
+	public var completionBlock: MainThreadOperation.MainThreadOperationCompletionBlock?
+
+	private let minimumQueueDepth = 3
+
+	private var queue: UIView
+	private var articleIconSchemeHandler: ArticleIconSchemeHandler
+	
+	init(queue: UIView, articleIconSchemeHandler: ArticleIconSchemeHandler) {
+		self.queue = queue
+		self.articleIconSchemeHandler = articleIconSchemeHandler
+	}
+	
+	func run() {
+		while queue.subviews.count < minimumQueueDepth {
+			let webView = PreloadedWebView(articleIconSchemeHandler: articleIconSchemeHandler)
+			queue.insertSubview(webView, at: 0)
+			webView.preload()
+		}
+		self.operationDelegate?.operationDidComplete(self)
+	}
+	
+}
+
+class WebViewProviderDequeueOperation: MainThreadOperation {
+	
+	// MainThreadOperation
+	public var isCanceled = false
+	public var id: Int?
+	public weak var operationDelegate: MainThreadOperationDelegate?
+	public var name: String? = "WebViewProviderFlushQueueOperation"
+	public var completionBlock: MainThreadOperation.MainThreadOperationCompletionBlock?
+
+	private var queue: UIView
+	private var articleIconSchemeHandler: ArticleIconSchemeHandler
+	private var completion: (PreloadedWebView) -> ()
+	
+	init(queue: UIView, articleIconSchemeHandler: ArticleIconSchemeHandler, completion: @escaping (PreloadedWebView) -> ()) {
+		self.queue = queue
+		self.articleIconSchemeHandler = articleIconSchemeHandler
+		self.completion = completion
+	}
+	
+	func run() {
 		if let webView = queue.subviews.last as? PreloadedWebView {
 			webView.ready { preloadedWebView in
 				preloadedWebView.removeFromSuperview()
-				self.replenishQueueIfNeeded()
-				completion(preloadedWebView)
+				self.completion(preloadedWebView)
+				self.operationDelegate?.operationDidComplete(self)
 			}
 			return
 		}
@@ -50,18 +124,11 @@ class WebViewProvider: NSObject {
 		assertionFailure("Creating PreloadedWebView in \(#function); queue has run dry.")
 		
 		let webView = PreloadedWebView(articleIconSchemeHandler: articleIconSchemeHandler)
-		webView.ready { preloadedWebView in
-			self.replenishQueueIfNeeded()
-			completion(preloadedWebView)
-		}
-	}
-	
-	func enqueueWebView(_ webView: PreloadedWebView) {
-		guard queue.subviews.count < maximumQueueDepth else {
-			return
-		}
-		queue.insertSubview(webView, at: 0)
 		webView.preload()
+		webView.ready { preloadedWebView in
+			self.completion(preloadedWebView)
+			self.operationDelegate?.operationDidComplete(self)
+		}
 	}
 	
 }