Implemented simple pagination approach to retrieving questions

Author: wibosco

NetworkingInOperations-Example/Data/Managers/Questions/Operations/QuestionsRetrievalOperation.swift

@@ -8,23 +8,25 @@
 
 import Foundation
 
-class QuestionsRetrievalOperation: ConcurrentOperation<[Question]> {
+class QuestionsRetrievalOperation: ConcurrentOperation<QuestionPage> {
 
     private let session: URLSession
     private let urlRequestFactory: QuestionsURLRequestFactory
     private var task: URLSessionTask?
+    private let pageIndex: Int
 
     // MARK: - Init
 
-    init(session: URLSession = URLSession.shared, urlRequestFactory: QuestionsURLRequestFactory = QuestionsURLRequestFactory()) {
+    init(pageIndex: Int, session: URLSession = URLSession.shared, urlRequestFactory: QuestionsURLRequestFactory = QuestionsURLRequestFactory()) {
+        self.pageIndex = pageIndex
         self.session = session
         self.urlRequestFactory = urlRequestFactory
     }
 
     // MARK: - Start
 
     override func start() {
-        let urlRequest = urlRequestFactory.requestToRetrieveQuestions()
+        let urlRequest = urlRequestFactory.requestToRetrieveQuestions(pageIndex: pageIndex)
 
         task = session.dataTask(with: urlRequest) { (data, response, error) in
             guard let data = data else {
@@ -42,7 +44,7 @@ class QuestionsRetrievalOperation: ConcurrentOperation<[Question]> {
                 let page = try JSONDecoder().decode(QuestionPage.self, from: data)
 
                 DispatchQueue.main.async {
-                    self.complete(result: .success(page.questions))
+                    self.complete(result: .success(page))
                 }
             } catch let error {
                 DispatchQueue.main.async {

NetworkingInOperations-Example/Data/Managers/Questions/QuestionsDataManager.swift

@@ -20,8 +20,8 @@ class QuestionsDataManager {
 
     // MARK: - Retrieval
 
-    func retrievalQuestions(completionHandler: @escaping (_ result: DataRequestResult<[Question]>) -> Void) {
-        let operation = QuestionsRetrievalOperation()
+    func retrievalQuestions(pageIndex: Int, completionHandler: @escaping (_ result: DataRequestResult<QuestionPage>) -> Void) {
+        let operation = QuestionsRetrievalOperation(pageIndex: pageIndex)
         operation.completionHandler = completionHandler
         queueManager.enqueue(operation)
     }

NetworkingInOperations-Example/Data/Models/Question.swift

@@ -13,18 +13,21 @@ private struct DummyCodable: Codable {}
 struct QuestionPage: Codable {
 
     let questions: [Question]
+    let hasMore: Bool
 
     // MARK: - CodingKeys
 
     enum CodingKeys: String, CodingKey {
         case questions = "items"
+        case hasMore = "has_more"
     }
 
     // MARK: - Init
 
     init(from decoder: Decoder) throws {        
         let container = try decoder.container(keyedBy: CodingKeys.self)
         questions = try container.decodeArrayByIgnoringInvalidElements(Question.self, forKey: .questions)
+        hasMore = try container.decode(Bool.self, forKey: .hasMore)
     }
 }
 

NetworkingInOperations-Example/Data/Requests/QuestionsURLRequestFactory.swift

@@ -12,8 +12,14 @@ class QuestionsURLRequestFactory: URLRequestFactory {
 
     // MARK: - Retrieval
 
-    func requestToRetrieveQuestions() -> URLRequest {
-        var request = jsonRequest(endPoint: "questions?order=desc&sort=activity&tagged=ios&pagesize=100&site=stackoverflow")
+    func requestToRetrieveQuestions(pageIndex: Int) -> URLRequest {
+        var urlString = "questions?order=desc&sort=activity&tagged=ios&pagesize=30&site=stackoverflow"
+        
+        if pageIndex != 0 {
+            urlString += "&page=\(pageIndex)"
+        }
+        
+        var request = jsonRequest(endPoint: urlString)
         request.httpMethod = HTTPRequestMethod.get.rawValue
 
         return request

NetworkingInOperations-Example/ViewControllers/QuestionsViewController.swift

@@ -17,6 +17,10 @@ class QuestionsViewController: UIViewController {
     fileprivate let questionDataManager = QuestionsDataManager()
     fileprivate var questions = [Question]()
 
+    fileprivate var pageIndex: Int = 0
+    fileprivate var retrievingQuestions: Bool = false
+    fileprivate var hasMoreQuestionsToBeRetrieved: Bool = true
+    
     // MARK: - ViewLifecycle
 
     override func viewDidLoad() {
@@ -27,21 +31,29 @@ class QuestionsViewController: UIViewController {
         tableView.rowHeight = UITableViewAutomaticDimension
         tableView.estimatedRowHeight = 120
 
-        retrieveQuestions()
+        retrieveQuestions(pageIndex: pageIndex)
     }
 
     // MARK: - Questions
 
-    private func retrieveQuestions() {
+    private func retrieveQuestions(pageIndex: Int) {
+        guard retrievingQuestions == false else {
+            return
+        }
+        
+        retrievingQuestions = true
         tableView.tableFooterView = loadingFooterView
-        questionDataManager.retrievalQuestions { (result) in
+        questionDataManager.retrievalQuestions(pageIndex: pageIndex) { (result) in
             self.tableView.tableFooterView = nil
+            self.retrievingQuestions = false
             switch result {
             case .failure(let error):
-                print("Error: \(error)")
+                print("ERROR: \(error)")
                 self.tableView.tableFooterView = self.errorFooterView
-            case .success(let questions):
-                self.questions = questions
+            case .success(let page):
+                self.hasMoreQuestionsToBeRetrieved = page.hasMore
+                self.pageIndex += 1
+                self.questions.append(contentsOf: page.questions)
                 self.tableView.reloadData()
             }
         }
@@ -67,6 +79,15 @@ extension QuestionsViewController: UITableViewDataSource {
 
         return cell
     }
+    
+    func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
+        let row = indexPath.row
+        let triggerRetrievalFromLimit = 15
+        
+        if hasMoreQuestionsToBeRetrieved && (triggerRetrievalFromLimit > (questions.count - row)) {
+            retrieveQuestions(pageIndex: pageIndex)
+        }
+    }
 }
 
 // MARK: - UITableViewDelegate