Making Cache data displayed responsive while New Data is Loaded - ios

Question

I am using CoreData to display cached data while new data is loaded and then updated onto a tableView
The cached data is loaded fine but the problem is as soon as API is called to load the new data , the tableView becomes unresponsive i.e user can't scroll on the table or click anything and when the new data has completely loaded , it updates the tableView and it becomes responsive again
What I want to achieve is the app displays the cache data right away and the api is called and data loaded in the background.The tableView shouldn't be unresponsive while the data is being loaded and when the data loading from API is complete User can click a refresh button or swipe up to update the data
// DID LOAD
override func viewDidLoad() {
super.viewDidLoad()
print("did load")
getAvatar()
tableView.separatorStyle = .none
updateTableContents()
self.tableView.delegate = self
self.tableView.dataSource = self
}
My function where I display cache Data and call the API
func updateTableContents()
{
do {
try self.fetchedhResultController.performFetch()
print("COUNT FETCHED FIRST: \(self.fetchedhResultController.sections?[0].numberOfObjects)")
} catch let error {
print("ERROR: \(error)")
}
print("function called")
let retrievedToken: String? = KeychainWrapper.standard.string(forKey: "acessTokenKey")
let headers = [
"Authorization" : "Bearer "+retrievedToken!,
"Content-Type" : "application/json"
]
let url = "someURL"
Alamofire.request(url, method: .get , headers: headers).responseJSON { response in
switch response.result {
case .success:
let json = response.result.value as! [String:Any]
let data = json["data"] as! [[String : Any]]
self.clearData()
self.saveInCoreDataWith(array: data)
self.nextToken = json["nextPageToken"] as? String ?? "empty"
print("Token = "+self.nextToken!)
for dic in data{
self.news.append(News(dictionary: dic))
print(self.news.count)
}
DispatchQueue.main.async {
self.loadingIndicator.stopAnimating()
self.tableView.reloadData()
}
case .failure: break
}
}
}
My tableView Code
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
if let count = fetchedhResultController.sections?.first?.numberOfObjects {
return count
}
return 0
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "NewsCell") as! NewsCell
if let fetchedNews = fetchedhResultController.object(at: indexPath) as? NewsObject {
cell.test(object : fetchedNews)
print(self.count)
self.count+=1;
}
and all the Core Data storing and fetching functionality
// Creating an Object
private func createNewsEntityFrom(dictionary: [String: Any]) -> NewsObject {
let context = CoreDataStack.sharedInstance.managedObjectContext
let newsEntity = NewsObject(context: context)
newsEntity.newsAuthor = dictionary["author"] as? String ?? "default"
newsEntity.newsTitle = dictionary["title"] as? String ?? "default"
let images = dictionary["image"] as? [String: Any]
newsEntity.newsImageURL = images?["link"] as? String ?? "default"
newsEntity.newsID = dictionary["_id"] as? String ?? "default"
newsEntity.newsPublisher = dictionary["publisher"] as? String ?? "default"
newsEntity.newsPublishorIconURL = dictionary["shortenedLogo"] as? String ?? "default"
newsEntity.liked = dictionary["liked"] as? Bool ?? false
newsEntity.bookmarked = dictionary["bookmarked"] as? Bool ?? false
return newsEntity
}
// Saving Data in Core Data
private func saveInCoreDataWith(array: [[String: Any]]) {
for dict in array {
_ = self.createNewsEntityFrom(dictionary: dict)
}
do {
try CoreDataStack.sharedInstance.persistentContainer.viewContext.save()
} catch let error {
print(error)
}
}
// Fetching Data from Core Data
lazy var fetchedhResultController: NSFetchedResultsController<NSFetchRequestResult> = {
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "NewsObject")
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "newsID", ascending: true)]
let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: CoreDataStack.sharedInstance.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
frc.delegate = self
return frc
}()
// Function used to Clear Data from Core Data
private func clearData() {
do {
let context = CoreDataStack.sharedInstance.persistentContainer.viewContext
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "NewsObject")
do {
let objects = try context.fetch(fetchRequest) as? [NSManagedObject]
_ = objects.map{$0.map{context.delete($0)}}
CoreDataStack.sharedInstance.saveContext()
} catch let error {
print("ERROR DELETING : \(error)")
}
}
}
I was following this tutorial to know how to implement CoreData if that helps https://medium.com/#jamesrochabrun/parsing-json-response-and-save-it-in-coredata-step-by-step-fb58fc6ce16f
EDIT :
Tried calling Alamofire completion handler in background thread
func updateTableContents()
{
do {
try self.fetchedhResultController.performFetch()
print("COUNT FETCHED FIRST: \(self.fetchedhResultController.sections?[0].numberOfObjects)")
} catch let error {
print("ERROR: \(error)")
}
print("function called")
let retrievedToken: String? = KeychainWrapper.standard.string(forKey: "acessTokenKey")
let headers = [
"Authorization" : "Bearer "+retrievedToken!,
"Content-Type" : "application/json"
]
let url = "https://api.tapin.news/v1/posts/home"
Alamofire.request(url, method: .get , headers: headers).responseJSON { response in
DispatchQueue.global(qos: .background).async {
switch response.result {
case .success:
let json = response.result.value as! [String:Any]
let data = json["data"] as! [[String : Any]]
self.nextToken = json["nextPageToken"] as? String ?? "empty"
print("Token = "+self.nextToken!)
self.clearData()
self.saveInCoreDataWith(array: data)
case .failure: break
}
}
self.loadingIndicator.stopAnimating()
self.tableView.reloadData()
}
}
EDIT 2 : So I was testing around a bit and I don't think its an issue of background Tasks , since I moved the fetching code from updateTableContents to viewDidLoad and removed the function from viewDidLoad. So background API calling and saving to coreData is not even being performed
Still the UI takes a couple seconds to become responsive during which time I see no images , and then as soon as images load it becomes responsive
override func viewDidLoad() {
super.viewDidLoad()
print("did load")
getAvatar()
tableView.separatorStyle = .none
self.tableView.delegate = self
self.tableView.dataSource = self
do {
try self.fetchedhResultController.performFetch()
print("COUNT FETCHED FIRST: \(self.fetchedhResultController.sections?[0].numberOfObjects)")
} catch let error {
print("ERROR: \(error)")
}
}
// Fetching Data from Core Data
lazy var fetchedhResultController: NSFetchedResultsController<NSFetchRequestResult> = {
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "NewsObject")
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "newsID", ascending: true)]
let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: CoreDataStack.sharedInstance.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
frc.delegate = self
return frc
}()
EDIT 3 : Here is my CoreDataStack
import UIKit
import CoreData
class CoreDataStack: NSObject {
static let sharedInstance = CoreDataStack()
private override init() {}
lazy var managedObjectContext : NSManagedObjectContext = {
return self.persistentContainer.viewContext
}()
lazy var persistentContainer: NSPersistentContainer = {
let container = NSPersistentContainer(name: "MyAppName")
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
return container
}()
// MARK: - Core Data Saving support
func saveContext () {
let context = persistentContainer.viewContext
if context.hasChanges {
do {
try context.save()
} catch {
// Replace this implementation with code to handle the error appropriately.
// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
let nserror = error as NSError
fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
}
}
}
func applicationDocumentsDirectory() {
if let url = FileManager.default.urls(for: .libraryDirectory, in: .userDomainMask).last {
print(url.absoluteString)
}
}
}

Answer 1

In your Alamofire's completion handler, you assume it is being run in background thread, and then you call the main thread and update the tableView.
The reality is: Alamofire's completions handlers already run in the main thread.
This is causing your UI blockings, because you have expensive methods there, like these ones:
self.clearData()
self.saveInCoreDataWith(array: data)
Also this one might be expensive:
for dic in data {
self.news.append(News(dictionary: dic))
print(self.news.count)
}
Try wrapping your Alamofire completion handler in a background thread:
DispatchQueue.global(qos: .background).async {
<#code#>
}