When I try to run the following code on a Simulator (also on a Device, the result is the same) I have a problem: the function executes first the part 2 of the following code (the comments are there just to have a reference, the code itself is entirely in the viewDidLoad. Just to be clear) and then it executes the part 1 (the firebase part). I need the ref.child.... function to be performed before the other function because the "createAlbums" func needs the albums taken from my database (So a completion block would be perfect, but that function doesn't allow me to put any).
I thought about a pair of solutions:
A way to add a completion block (remember that the ref.child... func is like a cycle so it will print the entire "albums" array for every single value that it finds on the database.
Create a Int var and make the 2 part of the code run only if the value of the var is the same as the number of values of "Albums" in the database, so it will fill up the local "Albums" array.
Use two different functions: the the viewDidLoad and the viewDidAppear. As I know, the second one will be performed after the first one.
Which one is the best solution? Is there any better solution?
I also ask you to make suggests on how I can create my own solutions, because I can logically arrive to the point, but I'm not sure to be able to develop them in Xcode.
Just to summarize the entire thread: how can I make the 2 part of the following code run after the 1 part? And why does it run before, even if the order of writing is correct and the compiler runs from the line 1 to the last line?
Thank you for your help and sorry for my bad English, I hope that you can understand everything, unfortunately trying to explain a "complex" problem in English is not always easy.
But I think that the moderators will be great as always to make some corrections!
// 1
let ref = Database.database().reference()
var albums: [String] = [String]()
override func viewDidLoad() {
super.viewDidLoad()
ref.child("Albums").observe(.childAdded) { (snapshot) in
albums.append(snapshot.key)
print("\(albums)")
print("Test print")
}
// 2
createAlbums()
setupConstraints()
}
You'll need to move createAlbums() and setupConstraints() into where you are doing Test Print.
Although it looks like your code is all happening in one place, it's really not.
These lines, between the curly braces...
albums.append(snapshot.key)
print("\(albums)")
print("Test print")
... are happening asynchronously, when firebase calls you back because it has data.
So you need something more like this:
// 1
let ref = Database.database().reference()
var albums: [String] = [String]()
override func viewDidLoad() {
super.viewDidLoad()
var createdAlbums = false
ref.child("Albums").observe(.childAdded) { (snapshot) in
albums.append(snapshot.key)
print("\(albums)")
print("Test print")
if !createdAlbums {
// Only run this part once, when we
// get our first callback from firebase
self.createAlbums()
self.setupConstraints()
createdAlbums = true
}
}
// 2
}
OTHER WORKING SOLUTION
In that case, observe the .value event on the same reference/query. In your closure, loop over the children as shown here stackoverflow.com/a/27342233, and at the end of the loop call createAlbums().
From a Frank van Puffelen comment
Related
I am trying to fetch initial value of EventDetailsModel and subscribe to all future updates.
When I call eventDetails(..), all the publishers already have some current value in them (i.e. chatModels and userModels have 10+ items); the problem is that because there are no new updates, the resulting pipe never returns EventDetailModels since .map(...) never gets called.
How can I make the combine pipe do at least one pass through the existing values when I am constructing it so my sink has some initial value?
var chatModels: Publishers.Share<CurrentValueSubject<[ChatModel], Never>> = CurrentValueSubject([]).share()
var userModels: CurrentValueSubject<[String: UserModel], Never> = CurrentValueSubject([:])
func eventDetails(forChatId chatId: String) -> AnyPublisher<EventDetailsModel?, Never> {
return chatModels
.combineLatest(userModels)
.map({ (chatList, userModels) -> EventDetailsModel? in
// Never gets called, even if chatModels and userModels has some existing data 😢
if let chatModel = (chatList.first { $0.id == chatId}) {
return EventDetailsModel(chatModel, userModels)
}
return nil
})
.eraseToAnyPublisher()
}
With combineLatest, you won't get any events until there is a latest for both publishers. That is what combineLatest means. The problem here is not chatModels, which does have a latest. The problem is userModels. Until it publishes for the first time, you won't get any events in this pipeline.
EDIT Okay, so now you've updated your code to reveal that both your publishers are CurrentValueSubjects. Well, in that case, you do get an initial event, as this toy example proves:
var storage = Set<AnyCancellable>()
let sub1 = CurrentValueSubject<Int,Never>(1)
let sub2 = CurrentValueSubject<String,Never>("howdy")
override func viewDidLoad() {
super.viewDidLoad()
sub1.combineLatest(sub2)
}
So if that isn't happening for you, the problem lies elsewhere. For example, maybe you forgot to store your pipeline, so you can't get any events at all. (But who knows? You have concealed the relevant code.)
I have a controller, that allows the user to type in a TextField.
Every time the user types a character, the string in that textfield is compared to an array of strings. If there is a match, the resulting array is displayed in a uitableview.
Here's the code:
func searchAutocompleteEntriesWithSubstring(substring:String){
let SUBSSTRING = substring.uppercased()
autocompleteStrings.removeAll()
for thisSchool in schoolArray{
if(thisSchool.name?.uppercased() .contains(SUBSSTRING))!{
autocompleteStrings.append(thisSchool)
}
}
autocompleteTableView.reloadData()
}
Basically, this works fine. BUT!
If the user types rather fast, the autocompleteTableView displays one or more (empty) rows than there actually are strings in the autocompleteStrings array.
I tried encapsulating the above code in DispatchQueue.main.async {}, but that made things even worse.
I guess it has something to do with NeedsLayout or NeedsDisplay, but I've never really understood the mechanism behind it, and how/where to apply these.
I hope you can advise me
Try this code
func searchAutocompleteEntriesWithSubstring(substring: String) {
let filtered = schoolArray.filter() { ($0.name ?? "").uppercased().range(of: substring.uppercased()) != nil }
autocompleteStrings = filtered.map() { $0.name! }
autocompleteTableView.reloadData()
}
Maybe you need a lock?
1.in a async queue.
2.lock locks.
3.matching and array appending
4.lock unlocks.
5.reload in mainqueue
This question already has answers here:
Waiting for Asynchronous function call to complete
(2 answers)
Closed 5 years ago.
I have a callback function that returns an NSDictionary:
override func viewDidLoad()
{
super.viewDidLoad()
var nd: NSDictionary = [:]
parseJSON(callback: {dict in
nd = dict
//print(nd) This one prints the filled dictionary correctly
})
//print(nd) This one prints an empty dictionary
}
I want to store the values returned from the callback function in "nd", but the print statement outside the callback is still printing an empty NSDictionary.
What am I doing wrong here, and how can I fix it?
EDIT:
var nd: NSDictionary! = nil
override func viewDidLoad()
{
super.viewDidLoad()
loadData()
print(self.nd)
}
func loadData()
{
parseJSON(callback: {dict in
self.nd = dict
print(self.nd)
})
}
I changed it to this, so the function completes its loading, and then prints. However, the print statement in viewDidLoad() prints before the one in loadData(), and the print statement in viewDidLoad() still prints nil. Why does this happen, and what do I need to change specifically?
Looks like the parseJSON runs in the background and might only complete after your viewDidLoad method ends. As such, nd will stay empty immediately after the call.
My suggestion: add your dictionary processing logic inside the aforementioned callback (and then copy the results to a variable outside your viewDidLoadmethod).
I read some questions about that. But I still have issues with asynchronous functions.
For example: I have a viewController1 where a button perform a segue to a viewController2. In the viewController2 class, I initialize some values in another class file named exampleClass. These values are retrieved from Firebase database or location values. These values need a little moment to be retrieved. I return thes values from the exampleClass into my viewController2. I print these values in the viewController2 viewDidLoad().
My issue: The device doesn't wait that the values are retrieved and execute following functions. Result: When I touch the button, printed values are nil values. It can also make the app crash if I don't secure the code.
What I've found so far: I learned that I only have to call a func at the end of a Firebase snapshot (for example) like this:
userRef.observeSingleEvent(of: .value, with: { (snapshot) -> Void in
self.name = snapshot.value as! String!
print(self.name)
self.forceEnd()
}) { (error) in
print(error.localizedDescription)
}
I named this function forceEnd to be clear. This is not working for me. I also tried to create handlers but no positive results.
My question: How can I force the device to wait for the values to be retrieved before performing the following question?
How can I force the device to wait for the values to be retrieved before performing the following question?
You don't want to force the device to wait, only need to perform some operations once these values are retrieved from Firebase database.
Performing an operation asynchronously can be done in multiple ways like blocks, protocols, notifications, etc.
Generally, blocks are the more elegant approach.
Some sample code can be like:
func myFirebaseNetworkDataRequest(finished: () -> Void) { // the function thats going to take a little moment
...
print("Doing something!") // firebase network request
finished()
}
// usage of above function can be as-
override func viewDidLoad() {
myFirebaseNetworkDataRequest {
// perform further operations here after data is fetched
print("Finally! It took a lot of moments to end but now I can do something else.")
}
}
My question is very similar to several others here but I just can't get it to work. I'm making an API call via a helper class that I wrote.
First I tried a standard function with a return value and the result was as expected. The background task completed after I tired to assign the result.
Now I'm using a closure and I can get the value back into my view controller but its still stuck in the closure, I have the same problem. I know I need to use GCD to get the assignment to happen in the main queue.
this is what I have in my view controller
var artists = [String]()
let api = APIController()
api.getArtistList("foo fighters") { (thelist) -> Void in
if let names = thelist {
dispatch_async(dispatch_get_main_queue()) {
artists = names
print("in the closure: \(artists)")
}
}
}
print ("method 1 results: \(artists)")
as the results are:
method 1 results: []
in the closure: [Foo Fighters & Brian May, UK Foo Fighters, John Fogerty with Foo Fighters, Foo Fighters, Foo Fighters feat. Norah Jones, Foo Fighters feat. Brian May, Foo Fighters vs. Beastie Boys]
I know why this is happening, I just don't know how to fix it :( The API calls need to be async, so what is the best practice for capturing these results? Based on what the user selects in the table view I'll be making subsequent api calls so its not like I can handle everything inside the closure
I completely agree with the #Craig proposal of the use of the GCD, but as your question involves the request of the API call every time you select a row, you can do the following:
Let's suppose you use the tableView:didSelectRowAtIndexPath: method to handle the selection, then you can do the following inside it:
func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
// it is just a form to get the item
let selectedItem = items.objectAtIndex(indexPath.row) as String
api.getArtistList(selectedItem) { (thelist) -> Void in
if let names = thelist {
dispatch_async(dispatch_get_main_queue()) {
artists = names
}
}
}
}
And then you can observe the property and handle do you want inside it :
var artists: [String] = [] {
didSet {
self.tableView.reloadData() // or anything you need to handle.
}
}
It just another way to see it. I hope this help you.
The easy solution is to do whatever you're doing at your print(), inside the closure.
Since you're already dispatch_asyncing to the main queue (the main/GUI thread), you can complete any processing there. Push a new view controller, present some modal data, update your current view controller, etc.
Just make sure that you don't have multiple threads modifying/accessing your local/cached data that is being displayed. Especially if it's being used by UITableViewDelegate / UITableViewDataSource implementations, which will throw fits if you start getting wishy-washy or inconsistent with your return values.
As long as you can retrieve the data in the background, and the only processing that needs to occur on the main thread is an instance variable reassignment, or some kind of array appending, just do that on the main thread, using the data you retrieved on the back end. It's not heavy. If it is heavy, then you're going to need more sophisticated synchronization methods to protect your data.
Normally the pattern looks like:
dispatch_async(getBackgroundQueue(), {
var theData = getTheDataFromNetwork();
dispatch_async(dispatch_get_main_queue() {
self.data = theData // Update the instance variable of your ViewController
self.tableView.reloadData() // Or some other 'reload' method
});
})
So where you'd normally refresh a table view or notify your ViewController that the operation has completed (or that local data has been updated), you should continue your main-thread processing.