I have a table view that is made up of three parts. One part is passed in from the previous view controller, but the other two parts need to be loaded asynchronously. I am displaying "placeholder loading spinners" in the areas that are waiting for HTTP responses. When one section returns, I try updating the table data, but I'm finding that I can get into a situation where both responses return around the same time and try to update the table at the same time, resulting in a crash. It seems like I need to apply some sort of lock and queue so that it does not crash from multiple asynchronous requests trying to update the table at the same time.
I would like to know, what is the iOS best practice for safely loading/updating partial sections of a UITableView asynchronously. I'm not looking for a code sample. Rather, I'm looking for the terminology and method calls that are used to achieve this.
If you're using different sections(and a static number of sections), try reloading them instead of reloading the table view. When an API returns, update its respective section:
[self.tableView reloadSections: withRowAnimation:]
Short answer: main thread. More specifically:
Update your data model on the main thread
Reload table view data on the main thread (in fact, do all UI stuff on the main thread, always)
If you do the above, you should have no issue.
If you're using something like NSURLConnection, you can specify the queue to which the completion proc should be dispatched when data is received (that'd be NSOperationQueue.mainQueue()). If you're doing something else that ends up executing on a different thread, you can dispatch back to the main thread with something like performSelectorOnMainThread or dispatch_async to dispatch_get_main_queue.
You can reload just particular sections (via reloadSections:withRowAnimation:) or even just certain rows (reloadRowsAtIndexPaths:withRowAnimation:), but I wouldn't bother with any of that unless/until there's an issue (e.g., slow performance or flicker due to excessive redraw). Start off just reloading the whole table until you've observed that you need to do otherwise.
I know you said you're not looking for a code sample, but I just can't help myself; I communicate better in code than in words.
Main thing is tableView:cellForRowAtIndexPath:, which makes a URL request (via NSURLConnection). The completion proc (which is dispatched to the main queue) parses some JSON, updates the model, and reloads the table. That's it.
class ViewController: UIViewController, UITableViewDataSource {
#IBOutlet weak var tableView: UITableView!
private var appIds = [ "391439366", "549762657", "568903335", "327630330", "281796108", "506003812" ]
private var ratings = [String : Int]() // AppID : RatingCount
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return self.appIds.count;
}
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let aCell = UITableViewCell(style: .Value2, reuseIdentifier: "RatingCell")
let appId = appIds[indexPath.row]
aCell.textLabel?.text = appId
if let count = self.ratings[appId] {
// Already got rating count for this app - display it.
aCell.detailTextLabel!.text = String(count)
aCell.accessoryView = nil
}
else {
// Don't have rating count: go get it.
self.getNumberOfRatingsForAppID(appId) {
success, number in
if success {
// Update model and reload table.
self.ratings[appId] = number
self.tableView.reloadData()
}
}
// Progress indicator while we wait for data.
let spinner = UIActivityIndicatorView(activityIndicatorStyle: .Gray)
spinner.startAnimating()
aCell.accessoryView = spinner
}
return aCell
}
typealias GetRatingsCompletion = (Bool, Int) -> ()
func getNumberOfRatingsForAppID( appID: String, completion: GetRatingsCompletion ) {
let appStoreURL = NSURL(string: "https://itunes.apple.com/lookup?id=\(appID)")
let request = NSURLRequest(URL: appStoreURL!)
NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.mainQueue() ) {
response, data, error in
guard data != nil else {
completion( false, 0 )
return
}
if let
jsonResult = (try? NSJSONSerialization.JSONObjectWithData(data!, options:[])) as? NSDictionary,
results = jsonResult["results"] as? NSArray,
result = results[0] as? NSDictionary,
numberOfRatings = result["userRatingCountForCurrentVersion"] as? Int
{
completion( true, numberOfRatings )
return
}
completion( false, 0 )
}
}
}
Related
My app fetches data via FileManager for each cell in UITableView. The cells need data from a file which requires open UIDocument objects. However, it seems like code inside open completion handler get executed non predictably, so the cells don't get displayed in the order I wish.
How do I solve this problem? I appreciate if anyone gives any clue.
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(true)
fetchCellsData()
}
func fetchCellsData() {
do {
let files = getUrlsOfFiles() //files in a certain order
for file in files {
let document = MyDocument(fileURL: file) //MyDocument subclassing UIDocument
//open to get data for cell
document.open { (success) in
if !success {
print("File not open at %#", file)
return
}
let data = populateCellData(document.fileInfo)
self.cellsData.append(data)
self.tableView.reloadData()
/// tried below also
// DispatchQueue.main.async {
// self.cellsData.append(data)
// self.tableView.reloadData()
// }
}
document.close(completionHandler: nil)
}
} catch {
print(error)
}
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: cellId, for: indexPath) as! MyCell
let fileInfo = cellsData[indexPath.row].fileInfo
//assign fileInfo into MyCell property
return cell
}
I expect cells get rendered in the order of 'files', however, it seems like the order is a bit unpredictable presuming that it's due to the fact that 'cellForRowAt' gets called before it knows about the full set of 'cellsData'.
From Apple documentation on UIdocument . Apple doc:
open(completionHandler:)
Opens a document asynchronously.
Which means that even if you trigger document.open in the right order, nothing guarantees that the completionHandler sequence will be in the same order, this is why the order is unpredictible.
However, you know that they will eventually all get done.
What you could do is :
1 - place all your datas from opened document into another list
2 - order this list in accordance to your need
3 - place this list into cellsData (which I assume is bound to your tableViesDatasource)
var allDatas: [/*your data type*/] = []
...
do {
// reset allDatas
allDatas = []
let files = getUrlsOfFiles()
for file in files {
let document = MyDocument(fileURL: file)
document.open { (success) in
if !success {
print("File not open at %#", file)
return
}
let data = populateCellData(document.fileInfo)
self.allDatas.append(data) // "buffer" list
self.tableView.reloadData()
}
document.close(completionHandler: nil)
}
} catch { ...
Then you change cellsData to a computed property like so :
var cellsData: [/*your data type*/] {
get {
return allDatas.order( /* order in accordance to your needs */ )
}
}
this way, each time a new data is added, the list is orderer before being redisplayed.
Discussion
this is the easier solution regarding the state of your code, however this may not be the overall prefered solution. For instance in your code, you reload your tableview each time you add a new value, knowing that there will be more data added after, which is not optimised.
I suggest you to read on Dispatch Group, this is a way to wait until all asynchronous operation your triggered are finished before executing certain actions (such as reloading your tableview in this case) (Readings: Raywenderlich tuts)
My Tableview does not respond to any touches when it starts loading data from firebase. (It already shows the cells, but does not react) After a while it does the scrolling you tried to do when the tableview wasn't reacting.
var filteredData = [archivCellStruct]()
func firData() {
filteredData.removeAll()
var databaseRef : DatabaseReference!
databaseRef = Database.database().reference()
databaseRef.child("Aufträge").child("Archiv").queryOrderedByKey().observe(.childAdded, with:{
snapshot in
let snap = snapshot.value as? NSDictionary
//extracting the data and appending it to an Array
self.filteredData.append(//myData)
self.tableView.reloadData()
})
}
So it is working for smaller amounts of data (tableview rows) but with big delay(and I am already filtering the data to limit the data displayed in the tableView).
I think it has something to do with the tableView.reloadData() (maybe userinteraction is disabled while reloading?)
everytime you have to reload your tableview asynchronously -
var filteredData = [archivCellStruct]()
func firData() {
filteredData.removeAll()
var databaseRef : DatabaseReference!
databaseRef = Database.database().reference()
databaseRef.child("Aufträge").child("Archiv").queryOrderedByKey().observe(.childAdded, with:{
snapshot in
let snap = snapshot.value as? NSDictionary
//extracting the data and appending it to an Array
self.filteredData.append(//myData)
DispatchQueue.main.async { //change this in you code
self.tableView.reloadData()
}
})
}
With the firebase you can go manual way:
UseCase:
struct UseCaseName {
struct Request {}
struct Response {
let firebaseCallbackData: [FirebaseModelType]
}
struct ViewModel {
let data: [DisplayType]
}
}
ViewController:
var filteredData: [DisplayType]! = []
override func viewWillAppear() {
// this one can make you trouble, adjust the observation logic to whatever you need. `WillAppear` can fire multiple times during the view lifecycle
super.viewWillAppear()
interractor?.observe()
}
func showData(viewModel: Scene.Usecase.ViewModel) {
// this is where the different approach begins
tableView.beginUpdates()
var indexPaths = [NSIndexPath]()
for row in (filteredData.count..<(FilteredData.count + viewModel.data.count)) {
indexPaths.append(NSIndexPath(forRow: row, inSection: 0))
}
filteredData += viewModel.data
tableView.insertRowsAtIndexPaths(indexPaths, withRowAnimation: .Automatic)
tableView.endUpdates()
}
Interractor:
func observe(request: Scene.UseCase.Request) { /* signup for updates with observe(_ eventType: DataEventType, with block: #escaping (DataSnapshot) -> Void) -> UInt */
callback is something like { DataSnapshot in presenter.presentData(response: Scene.Usecase.Response())
}
Presenter:
func presentData(response: Scene.UseCase.Response) {
/* format for representation */
DispatchQueue.main.async {
controller.present(viewModel: Scene.UseCase.ViewModel())
}
}
Sorry for the separation of the flow, I've got addicted to this way.
Also I'm assuming, that the data in the firebase is not modified, but added (because of observe(.childAdded, part. If I'm wrong, please edit your question to reflect that. Another assumption is that you have the single section. Don't forget to change the inSection: 0 to the proper section. I'm to lazy and SO isn't that friendly for mobile devices
This way only appends new values sent by Firebase and works faster
EDIT: on another answer.
DispatchQueue.main.async { //change this in you code
self.tableView.reloadData()
}
Sometimes it's not good to reload all items. It depends on the count however. If my assumptions are correct, it'll be better to update separate cells without reloading the whole table on change. Quick example: something like a chat with the Firebase "backend". explanation: adding a single row will work faster then reloadData() anyway, but the difference is heavy only when you call those methods often. With the chat example, the difference may be huge enough in case the chat is spammy to optimize the UITableView reload behaviour in your controller.
Also it'll be nice to see the code, which affects the methods. It maybe a threading issue, like the John Ayers told in the comments. Where do you call func firData()?
I have a tabBar controller with 2 tabs: tabA which contains ClassA and tabB which contains ClassB. I send data to Firebase Database in tabA/ClassA and I observe the Database in tabB/ClassB where I retrieve it and add it to a tableView. Inside the tableView's cell I show the number of sneakers that are currently inside the database.
I know the difference between .observeSingleEvent( .value) vs .observe( .childAdded). I need live updates because while the data is getting sent in tabA, if I switch to tabB, I want to to see the new data get added to the tableView once tabA/ClassA is finished.
In ClassB I have my observer in viewWillAppear. I put it inside a pullDataFromFirebase() function and every time the view appears the function runs. I also have Notification observer that listens for the data to be sent in tabA/ClassA so that it will update the tableView. The notification event runs pullDataFromFirebase() again
In ClassA, inside the callback of the call to Firebase I have the Notification post to run the pullDataFromFirebase() function in ClassB.
The issue I'm running into is if I'm in tabB while the new data is updating, when it completes, the cell that displays the data has a count and the count is thrown off. I debugged it and the the sneakerModels array that holds the data is sometimes duplicating and triplicating the newly added data.
For example if I am in Class B and there are 2 pairs of sneakers in the database, the pullDataFromFirebase() func will run, and the tableView cell will show "You have 2 pairs of sneakers"
What was happening was if I switched to tabA/ClassA, then added 1 pair of sneakers, while it's updating I switched to tabB/ClassB, the cell would still say "You have 2 pairs of sneakers" but then once it updated the cell would say "You have 5 pairs of sneakers" and 5 cells would appear? If I switched tabs and came back it would correctly show "You have 3 pairs of sneakers" and the correct amount of cells.
That's where the Notification came in. Once I added that if I went through the same process and started with 2 sneakers the cell would say "You have 2 pairs of sneakers", I go to tabA, add another pair, switch back to tabB and still see "You have 2 pairs of sneakers". Once the data was sent the cell would briefly show "You have 5 pairs of sneakers" and show 5 cells, then it would correctly update to "You have 3 pairs of sneakers" and the correct amount of cells (I didn't have to switch tabs).
The Notification seemed to work but there was that brief incorrect moment.
I did some research and the most I could find were some posts that said I need to use a semaphore but apparently from several ppl who left comments below they said semaphores aren't meant to be used asynchronously. I had to update my question to exclude the semaphore reference.
Right now I'm running tableView.reloadData() in the completion handler of pullDataFromFirebase().
How do I reload the tableView outside of the observer once it's finished to prevent the duplicate values?
Model:
class SneakerModel{
var sneakerName:String?
}
tabB/ClassB:
ClassB: UIViewController, UITableViewDataSource, UITableViewDelegate{
var sneakerModels[SneakerModel]
override func viewDidLoad() {
super.viewDidLoad()
NotificationCenter.default.addObserver(self, selector: #selector(pullDataFromFirebase), name: NSNotification.Name(rawValue: "pullFirebaseData"), object: nil)
}
override func viewWillAppear(_ animated: Bool){
super.viewWillAppear(animated)
pullDataFromFirebase()
}
func pullDataFromFirebase(){
sneakerRef?.observe( .childAdded, with: {
(snapshot) in
if let dict = snapshot.value as? [String:Any]{
let sneakerName = dict["sneakerName"] as? String
let sneakerModel = SneakerModel()
sneakerModel.sneakerName = sneakerName
self.sneakerModels.append(sneakerModel)
//firebase runs on main queue
self.tableView.reloadData()
}
})
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return sneakerModels.count
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "SneakerCell", for: indexPath) as! SneakerCell
let name = sneakerModels[indePath.row]
//I do something else with the sneakerName and how pairs of each I have
cell.sneakerCount = "You have \(sneakerModels.count) pairs of sneakers"
return cell
}
}
}
tabA/ClassA:
ClassA : UIViewController{
#IBAction fileprivate func postTapped(_ sender: UIButton) {
dict = [String:Any]()
dict.updateValue("Adidas", forKey: "sneakerName")
sneakerRef.?.updateChildValues(dict, withCompletionBlock: {
(error, ref) in
//1. show alert everything was successful
//2. post notification to ClassB to update tableView
NotificationCenter.default.post(name: Notification.Name(rawValue: "pullFirebaseData"), object: nil)
}
}
}
In other parts of my app I use a filterDuplicates method that I added as an extension to an Array to filter out duplicate elements. I got it from filter array duplicates:
extension Array {
func filterDuplicates(_ includeElement: #escaping (_ lhs:Element, _ rhs:Element) -> Bool) -> [Element]{
var results = [Element]()
forEach { (element) in
let existingElements = results.filter {
return includeElement(element, $0)
}
if existingElements.count == 0 {
results.append(element)
}
}
return results
}
}
I couldn't find anything particular on SO to my situation so I used the filterDuplicates method which was very convenient.
In my original code I have a date property that I should've added to the question. Any way I'm adding it here and that date property is what I need to use inside the filterDuplicates method to solve my problem:
Model:
class SneakerModel{
var sneakerName:String?
var dateInSecs: NSNumber?
}
Inside tabA/ClassA there is no need to use the Notification inside the Firebase callback however add the dateInSecs to the dict.
tabA/ClassA:
ClassA : UIViewController{
#IBAction fileprivate func postTapped(_ sender: UIButton) {
//you must add this or whichever date formatter your using
let dateInSecs:NSNumber? = Date().timeIntervalSince1970 as NSNumber?
dict = [String:Any]()
dict.updateValue("Adidas", forKey: "sneakerName")
dict.updateValue(dateInSecs!, forKey: "dateInSecs")//you must add this
sneakerRef.?.updateChildValues(dict, withCompletionBlock: {
(error, ref) in
// 1. show alert everything was successful
// 2. no need to use the Notification so I removed it
}
}
}
And in tabB/ClassB inside the completion handler of the Firebase observer in the pullDataFromFirebase() function I used the filterDuplicates method to filter out the duplicate elements that were showing up.
tabB/ClassB:
func pullDataFromFirebase(){
sneakerRef?.observe( .childAdded, with: {
(snapshot) in
if let dict = snapshot.value as? [String:Any]{
let sneakerName = dict["sneakerName"] as? String
let sneakerModel = SneakerModel()
sneakerModel.sneakerName = sneakerName
self.sneakerModels.append(sneakerModel)
// use the filterDuplicates method here
self.sneakerModels = self.sneakerModels.filterDuplicates{$0.dateInSecs == $1.dateInSecs}
self.tableView.reloadData()
}
})
}
Basically the filterDuplicates method loops through the sneakerModels array comparing each element to the dateInSecs and when it finds them it excludes the copies. I then reinitialize the sneakerModels with the results and everything is well.
Also take note that there isn't any need for the Notification observer inside ClassB's viewDidLoad so I removed it.
I have an application that pulls information from a Parse database, and displays it in a UITableView. I pull the information from parse in the viewWillAppear function, and i display it in the tableView(cellForRowAtIndexPath) function. Sometimes i receive an error because the array that stores the Parse information has a length of 0, and i try to access information at an index outside of the bounds of the array. I believe this is because the cellForRowAtIndexPath is getting called before the viewWillAppear is finished running. Is this possible or is my error definitely coming from somewhere else?
EDIT: The error does not occur every time, and i cannot find a way to reproduce it
override func viewWillAppear(animated: Bool) {
//begin ignoring events until the information is finished being pulled
UIApplication.sharedApplication().beginIgnoringInteractionEvents()
resultsArray.removeAll(keepCapacity: false)
//run query
let query = PFQuery(className: "Answers")
query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
if let objects = objects {
//append information to the resultsArray
}
}
self.tableView.reloadData()
}
}
//information is now pulled, so allow interaction
UIApplication.sharedApplication().endIgnoringInteractionEvents()
}
override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier("cell", forIndexPath: indexPath) as! answerCell
// THIS IS WHERE THE ERROR OCCURS
resultsArray[indexPath.row].imageFile.getDataInBackgroundWithBlock { (data, error) -> Void in
//set image within cell
}
return cell
}
I would suggest that you load your data from Parse into a temporary array and then assign this to your property array right before you call reloadData - this will avoid any potential race conditions and remove the need for the removeAll which is potentially a big part of your problem;
override func viewWillAppear(animated: Bool) {
//begin ignoring events until the information is finished being pulled
UIApplication.sharedApplication().beginIgnoringInteractionEvents()
//run query
let query = PFQuery(className: "Answers")
query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
var localArray=[SomeType]()
if let objects = objects {
//append information to the localArray
}
}
self.resultsArray=localArray
self.tableView.reloadData()
}
}
//information is now pulled, so allow interaction
UIApplication.sharedApplication().endIgnoringInteractionEvents()
}
Looks like in viewWillAppear you have a background block findObjectsInBackgroundWithBlock that has some work to do in a different thread (AKA off the main thread), that means that viewWillAppear will finish while the block will get a callback.
This explains why cellForRowAtIndexPath is being called after viewWillAppear finishes, because of the callback block.
That means that everything is alright and viewWillAppear actually do finish a legit "run".
You can actually put a breaking point inside the callback method (in viewWillAppear) and a breaking point inside cellForRowAtIndexPath and see when the callback happens while cellForRowAtIndexPath is being called.
If you need a different method from Parse perhaps you should look in their documentation.
Actually if your callback not access to self.tableView, everything will go on as you think as usual. You can have a try.
It happened to me when I access to the view on the screen in init method viewDidLoad method called before init ends.
Anyway, you should know that fact. And you access to your tableView in callback (called before viewWillAppear finishing) which needs cellForRowAtIndexPath.
SOMETIMES THE REFRESH WORKS SOMETIMES IT DOESN'T
I have a UITableViewController which is basically a news feed. I have also implemented a pull to refresh feature. However sometimes when I pull to refresh it gives me the error
'Array index out of range'.
I know this means an item it is trying to get does not exist but can you tell me why? Here is my code:
override func viewDidLoad() {
super.viewDidLoad()
refresher = UIRefreshControl()
refresher.attributedTitle = NSAttributedString(string: "Pull to refresh")
refresher.addTarget(self, action: "refresh", forControlEvents: UIControlEvents.ValueChanged)
self.tableView.addSubview(refresher)
refresh()
tableView.delegate = self
tableView.dataSource = self
}
and the refresh() function:
func refresh() {
//disable app while it does stuff
UIApplication.sharedApplication().beginIgnoringInteractionEvents()
//get username and match with userId
let getUser = PFUser.query()
getUser?.findObjectsInBackgroundWithBlock({ (objects, error) -> Void in
if let users = objects {
//clean arrays and dictionaries so we dont get indexing error???
self.messages.removeAll(keepCapacity: true)
self.users.removeAll(keepCapacity: true)
self.usernames.removeAll(keepCapacity: true)
for object in users {
if let user = object as? PFUser {
//make userId = username
self.users[user.objectId!] = user.username!
}
}
}
})
let getPost = PFQuery(className: "Posts")
getPost.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
if error == nil {
if let objects = objects {
self.messages.removeAll(keepCapacity: true)
self.usernames.removeAll(keepCapacity: true)
for object in objects {
self.messages.append(object["message"] as! String)
self.usernames.append(self.users[object["userId"] as! String]!)
self.tableView.reloadData()
}
}
}
}
self.refresher.endRefreshing()
UIApplication.sharedApplication().endIgnoringInteractionEvents()
}
and:
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let myCell = tableView.dequeueReusableCellWithIdentifier("SinglePostCell", forIndexPath: indexPath) as! PostCell
//ERROR GETS REPORTED ON THE LINE BELOW
myCell.usernamePosted.text = usernames[indexPath.row]
myCell.messagePosted.text = messages[indexPath.row]
return myCell
}
You have a race condition given you are doing two background tasks, where the second depends on values returned from the first. getUser?.findObjectsInBackgroundWithBlockwill return immediately, and getPost.findObjectsInBackgroundWithBlock will start executing. The getPost should be inside the block for getUser, to ensure the sequence is correct.
Similarly, the following two lines should be inside the second block:
self.refresher.endRefreshing()
UIApplication.sharedApplication().endIgnoringInteractionEvents()
Given the error line, you probably also have a race condition between the two background tasks and displaying the tableView. I would be inclined to try:
func tableView(tableView:UITableView!, numberOfRowsInSection section:Int) {
return self.refresher.refreshing ? 0 : self.usernames.count
}
This way you won't touch self.usernames until the background refresh is finished (as long as you remember to put endRefreshing inside the second block, which is also put inside the first block).
I Believe that in self.users[user.objectId!] = user.username! the user.ObjectId is some random value assigned by parse which looks like this: "34xcf4". This is why you might be getting 'Array index out of range'.
There are two required methods for configuring a UITableView:
tableView(_:cellForRowAtIndexPath:)
and
tableView(_:numberOfRowsInSection:)
In your code you are presenting only one required method, if you don't implement the second method then it that may cause errors.
Check the documentation at:
https://developer.apple.com/library/ios/documentation/UIKit/Reference/UITableViewDataSource_Protocol/#//apple_ref/occ/intfm/UITableViewDataSource/tableView:cellForRowAtIndexPath:
You are calling self.tableView.reloadData() on every addition to your array and doing so in a background thread.
As a general rule, you should not do UI updates in a background thread. When you clear self.messages and self.usernames, because you are in background thread, nothing prevents the tableview from trying to get a cell at an index that no longer has any data in the array.
If you want to keep your code in the background thread (risky as it may be), you should at least call .beginUpdates before reloading your arrays and wait until they're all done before calling reload and endUpdates.