As follows, what expression should I write to access let learningList? In the code, query is a class, find is a function. Many thanks!
_ = query.find { result in
switch result {
case .success(objects: let learningList):
break
case .failure(error: let error):
print(error)
}
}
That is a local variable, so you can access it only in the decelerated context, i.e. inside case. If you need to use it later you have to create a property in caller class to hold it, like
_ = query.find { [weak self] result in // << ref to caller
switch result {
case .success(objects: let learningList):
self?.learningList = learningList // << safe in caller's property
break
case .failure(error: let error):
print(error)
}
}
Related
Following this tutorial, I've written the following class:
import RealmSwift
import Darwin
import SwiftUI
let app = App(id: "my-app-id")
class AccessManager: Object {
#objc dynamic var isInTime: Bool = false
func foo2() -> Bool {
return true
}
func foo1() {
app.login(credentials: Credentials.anonymous) { (result) in
DispatchQueue.main.async {
switch result {
case .failure(let error):
print("Login failed: \(error)")
case .success(let user):
print("Login as \(user) succeeded!")
let client = app.currentUser!.mongoClient("mongodb-atlas")
let database = client.database(named: "my-database")
let collection = database.collection(withName: "my-collection")
let identity = "my-identity"
collection.find(filter: ["_partition": AnyBSON(identity)], { (result) in
switch result {
case .failure(let error):
print("Call to MongoDB failed: \(error.localizedDescription)")
case .success(let documents):
self.bar = self.foo2()
print(self.bar) // prints true
}
})
print(self.bar) // prints false
}
}
}
}
}
When I change the value of self.bar inside of collection.find's scope (using the self.foo2 function), its value doesn't change outside of that scope - i.e in the first print(self.bar) - true is being printed, but in the second one - false is printed.
How can I change self.bar's value so the change will also take effect outside of collection.find's scope?
As #Jay commented:
closures are asynchronous and the code following the closure will
(may) execute before the code in the closure. So that code will print
false before the value is set to true. Code is faster than the
internet so data is only valid in the closure.
This is why in my case, the print(self.bar) outside of the closure was executed before the collection.find closure. Therefore it's result was false instead of true.
I am studying and trying out a few stuff with Combine to apply on my own and came into the following situation with this contrived example..
let sequencePublisher = [70, 5, 17].publisher
var cancellables = [AnyCancellable]()
sequencePublisher
// .spellOut()
.flatMap { query -> URLSession.DataTaskPublisher in
return URLSession.shared.dataTaskPublisher(for: URL(string: "http://localhost:3000?q=\(query)")!)
}
.compactMap { String(data: $0.data, encoding: .utf8) }
.sink(receiveCompletion: { completion in
switch completion {
case .failure(let error):
print(error.localizedDescription)
default: print("finish")
}
}) { value in
print(value)
}
.store(in: &cancellables)
I have a sequence publisher that emits 3 Integers and I pass it through flatMap and send a Get request request to my local API that simply returns back the same value it got embedded in a string.
It all works fine, I get all 3 API responses in sink, as long as I don't uncomment the spellOut() custom operator, this operator is supposed to fail if the number is smaller than 6, here is what it does:
enum ConversionError: LocalizedError {
case lessThanSix(Int)
var errorDescription: String? {
switch self {
case .lessThanSix(let n):
return "could not convert number -> \(n)"
}
}
}
extension Publisher where Output == Int {
func spellOut() -> Publishers.TryMap<Self, String> {
tryMap { n -> String in
let formatter = NumberFormatter()
formatter.numberStyle = .spellOut
guard n > 6, let spelledOut = formatter.string(from: n as NSNumber) else { throw ConversionError.lessThanSix(n) }
return spelledOut
}
}
}
The code doesn't even compile if I add another map operator before flatMap it works, but with a tryMap it just says
No exact matches in call to instance method 'flatMap'
Is there any way of achieving this or why is it not allowed?
Thank you in advance for the answers
The problem here is that FlatMap requires the returned publisher created in its closure to have the same Failure type as its upstream (unless upstream has a Never failure).
So, a Sequence publisher, like:
let sequencePublisher = [70, 5, 17].publisher
has a failure type of Never and all works.
But TryMap, which is what .spellOut operator returns, has a failure type of Error, and so it fails, because DataTaskPublisher has a URLError failure type.
A way to fix is to match the error type inside the flatMap:
sequencePublisher
.spellOut()
.flatMap { query in
URLSession.shared.dataTaskPublisher(for: URL(...))
.mapError { $0 as Error }
}
// etc...
You have to map the error after the tryMap.
publisher
.tryMap({ id in
if let id = id { return id } else { throw MyError.unknown("noId") }
})
.mapError { $0 as? MyError ?? MyError.unknown("noId") }
.flatMap { id -> AnyPublisher<Model, MyError> in
fetchDataUseCase.execute(id: id)
}
.eraseToAnyPublisher()
In case the error types already match, another point of failure can be, when you work with any Publisher as return values. Then you need to call eraseToAnyPublisher() on the publishers - the first one and the one returned from the flatMap closure.
anyPublisher.eraseToAnyPublisher()
.flatMap { value in
anotherPublisher.eraseToAnyPublisher()
}
I am trying to Apollo framework and a graphql api to obtain the data then return it. Once I have the data in another swift file, I want to call on certain parts of the data and assign it to a variable. The errors I get is variable used before it is initialized. and if try to return the variable from within the closure I get "Unexpected Non-Void Return Value In Void Function ". I heard of ways to get around that error but I don't completely understand it and how it works with my code. If you need more code or context you can message me and I can share my GitHub repo. Sorry if the code is bad, please don't roast me. I am still a beginner.
import Foundation
import Apollo
struct AniListAPI {
let aniListUrl = "https://graphql.anilist.co"
func ObtainData(AnimeID: Int)-> QueryQuery.Data{
var theData: QueryQuery.Data
let theInfo = QueryQuery(id: AnimeID)
GraphClient.fetch(query: theInfo) { result in
switch result {
case .failure(let error):
print("A big No no happened \(error)")
case .success(let GraphQLResult):
guard let Info = GraphQLResult.data else {return}
theData = Info
}
}
return theData
}
}
Unexpected Non-Void Return Value In Void Function.
The reason you're getting this warning is because you can't return value from inside the closure. Use closure instead of returning value.
func ObtainData(AnimeID: Int, completion: #escaping (Data) -> Void) {
var TheData: QueryQuery.Data
let TheInfo = QueryQuery(id: AnimeID)
GraphClient.fetch(query: TheInfo) { result in
switch result {
case .failure(let error):
print("A big no no happened retard \(error)")
case .success(let GraphQLResult):
guard let Info = GraphQLResult.data else {return}
TheData = Info
completion(TheData)
}
}
}
and call it like..
ObtainData(AnimeID: 123) { (anyData) in
print (anyData)
// continue your logic
}
How can I use the dispatchQueue or something like "await" in Javascript to return a value in self.arrayData (because the end of my loop is ran before the previous content). I am used to R and Python where the code runs line by line, What is the best behavior to adopt in Swift ?
Here is the function :
func fetch2(){
var i:Int = 0
repeat {
AF.request(itemLookUp[i]).validate().responseJSON { response in
switch response.result {
case .failure(let error):
print("\(error) in fetch2")
case .success(let value):
let json = JSON(value)
//Extract the Matiere for ML Extraction
self.matiereInput = json["ResultSet"]["0"]["Result"]["0"]["SpAdditional"].string ?? "none"
let energyCheck:Bool = self.matiereInput.contains("エネルギー") //energy-kcal
if energyCheck==true && self.arrayData[0]==0.0{
//regular expression
var patEnergy = #"(エネルギー)(([^\d]+)(\d+)(\.)(\d+)|([^\d]+)(\d+))"# //avoid the repetition of the pattern within the same matiereinput
let patEnergy2 = self.matches(for: patEnergy, in: self.matiereInput)
patEnergy = patEnergy2.joined(separator:"")
let valueEnergy = self.matches(for: self.regex2, in: patEnergy)
self.arrayData[0] = Double(valueEnergy.joined(separator: "")) ?? 0.0
}
}
}
i = i+1
print(self.arrayData[0])
} while i <= (self.returned-1)
}
Thank you in advance !
The standard pattern is notify with a DispatchGroup, and then use a completion handler to asynchronously notify the caller of the result:
func fetchAll(completion: #escaping (Result<[Double], Error>) -> Void) {
let group = DispatchGroup()
var results: [Double] = []
var errors: [Error] = []
for item in lookupItems {
group.enter() // enter before request
AF.request(item).validate().responseJSON { response in
defer { group.leave() } // leave when this closure is done
switch response.result {
case .failure(let error):
errors.append(error)
case .success(let value):
let result = ...
results.append(result)
}
}
}
group.notify(queue: .main) {
if let error = errors.first { // I don’t know what you want to do if there were multiple errors, so for now I’ll just grab the first one
completion(.failure(error))
} else {
completion(.success(results))
}
}
}
And then you’d use it like so:
fetchAll { result in
switch result {
case .failure(let error):
print(error)
case .success(let values):
print(values)
}
}
Now, I wasn’t able to reverse engineer what you were trying to do (you appear to be updating self.arrayData[0] in every iteration!), so I just returned an array of Double. But you can obviously change the type of results and the parameter of the completion closure to match whatever is relevant in your case.
But don’t get lost in the details of the above example, but rather just focus on a few key observations:
Supply completion handler closure to call when all the requests are done.
Use DispatchGroup to keep track of when all the requests are done.
Supply a notify closure to your DispatchGroup which will be called when all the group.enter() calls are offset by their respective group.leave() calls.
A more subtle observation is that you should refrain from updating properties from within the responseJSON block. Within your asynchronous code, you really want to limit your interaction to local variables if at all possible. Pass the result back in the completion closure (and the caller can update the model and the UI as it sees fit).
I am trying to update the value stored in an array property of a class via the use of KeyPaths. Here is my code:
func listenAndUpdateDocuments<T: JSONDecodable>(
_ property: ReferenceWritableKeyPath<MyModel, [T]?>,
from model: MyModel) {
guard let reference = reference else {
return
}
guard listener == nil else {
return
}
listener = backendClient.listenToDocuments(reference) { [weak model] (result: Result<[T], RequestError>) in
switch result {
case .success(let value):
model?[keyPath: property] = value
case .failure:
model?[keyPath: property] = []
}
}
}
The problem is when I call this function like this:
myListener.listenAndUpdateDocuments(\.viewers, from: self)
where viewers is of type [ViewersModel], it always comes back with the following error:
Type of expression is ambiguous without more context
How do I solve this? I have a similar version of the code but where the property parameter isn't an array, and that works.
I struggled with something similar:
_ = Token.query(on: req).filter(\.expiry < Date()).delete()
The solution I found was to use a more up-to-date api to handle my request parameters.
_ = Token.query(on: req).filter(\.expiry, .lessThan, Date()).delete()
It had less to do with the keypath itself than I thought!