Adds "Find in Article" activity to the share sheet

addresses #1750

iOS/Article/WebViewController.swift

@@ -223,7 +223,7 @@ class WebViewController: UIViewController {
 			return
 		}
 
-		let activityViewController = UIActivityViewController(url: url, title: article?.title, applicationActivities: [OpenInSafariActivity()])
+		let activityViewController = UIActivityViewController(url: url, title: article?.title, applicationActivities: [FindInArticleActivity(), OpenInSafariActivity()])
 		activityViewController.popoverPresentationController?.barButtonItem = popOverBarButtonItem
 		present(activityViewController, animated: true)
 	}
@@ -678,3 +678,64 @@ private extension WebViewController {
 	}
 	
 }
+
+// MARK: Find in Article
+
+private struct FindInArticleOptions: Codable {
+	var text: String
+	var caseSensitive = false
+}
+
+
+internal struct FindInArticleState: Codable {
+	struct WebViewClientRect: Codable {
+		let x: Double
+		let y: Double
+		let width: Double
+		let height: Double
+	}
+	
+	struct FindInArticleResult: Codable {
+		let rects: [WebViewClientRect]
+		let bounds: WebViewClientRect
+		let index: UInt
+		let matchGroups: [String]
+	}
+	
+	let index: UInt?
+	let results: [FindInArticleResult]
+	let count: UInt
+}
+
+extension WebViewController {
+	func searchText(_ searchText: String, completionHandler: @escaping (FindInArticleState) -> Void) {
+		guard let json = try? JSONEncoder().encode(FindInArticleOptions(text: searchText)) else {
+			return
+		}
+		let encoded = json.base64EncodedString()
+		
+		webView?.evaluateJavaScript("updateFind(\"\(encoded)\")") {
+			(result, error) in
+			guard error == nil,
+				let b64 = result as? String,
+				let rawData = Data(base64Encoded: b64),
+				let findState = try? JSONDecoder().decode(FindInArticleState.self, from: rawData) else {
+					return
+			}
+			
+			completionHandler(findState)
+		}
+	}
+	
+	func endSearch() {
+		webView?.evaluateJavaScript("endFind()")
+	}
+	
+	func selectNextSearchResult() {
+		webView?.evaluateJavaScript("selectNextResult()")
+	}
+	
+	func selectPreviousSearchResult() {
+		webView?.evaluateJavaScript("selectPreviousResult()")
+	}
+}