Trying out RxSwift and trying to convert my network calls. I can't seem to display my data in the view because im not sure how I convert my observable to something my views can use. Here's an example of my request:
class SomeService {
let provider = Provider()
func getData() -> Observable<[Object]?> { // Returns json
return provider
.request(.getSomething())
.debug()
.mapArrayOptional(type: Object.self)
// Using Moya_Modelmapper map each item in the array
}
}
In my view controller I get the data:
let data = Service.getData()
print(data) ... <Swift.Optional<Swift.Array<MyApp.Item>>>
I have tried to subscribe to the response to the sequence but I don't know how I actually convert it to something like an array I can use in my view.
UPDATE: With answer implemented:
func itemsObserver() {
print("Time to print step 1") // This gets printed
data
.filter { $0 != nil }.map { $0! }
.subscribe(
onNext: { objects in
print(objects as Any)
print("Step 2") // This does not get executed at all
},
onCompleted:{ objects in
print(objects as Any) // This is ()
print("Complete") // This gets printed
}
).addDisposableTo(disposeBag)
}
itemsObserver()
Console output:
Time to print step 1
Service.swift:21 (getData()) -> subscribed
Service.swift:21 (getData()) -> Event next(Status Code: 200, Data Length: 141)
Service.swift:21 (getData()) -> Event completed
Service.swift:21 (getData()) -> isDisposed
()
Complete
Update:
If your onNext block isn't getting called at all, it's because data never produced anything. Either your producer isn't producing any objects or mapArrayOptional is not transforming them.
The onCompleted block doesn't accept any arguments so the objects var you have in it is meaningless/Void.
Try this:
let data = service.getData()
data
.filter { $0 != nil }.map { $0! } // this removes the optionality of the result.
.subscribe(onNext: { objects in
// in here `objects` will be an array of the objects that came through.
}).disposed(by: bag)
Related
// In the code below I am trying to return an array from data in firestore, the array always returned empty when I put the handler outside the for loop so I had to use an if statement inside the for loop to get the array containing the data. after using the print statement you see in the code i found out that the compiler is going over the entire function before entering the for loop, (print("5") & (print("6") are the first to run and when I put the handler outside the for it will also be triggered and return an empty array
**
func getMyGames(joinedGamesIDs: [String], handler: #escaping(_ games: [GameViewModal]) -> ()) {
var games = [GameViewModal]()
if !joinedGamesIDs.isEmpty{
for id in joinedGamesIDs {
db.collection("games").document(id).getDocument { (document, error) in
if let document = document, document.exists {
if let game = self.getGameViewModal(document: document){
games.append(game)
print("1")
print(games.count)
}
print("2")
print(games.count)
}
print("3")
print(games.count)
if games.count == (joinedGamesIDs.count){
handler(games)
}
print("4")
print(games.count)
}
}
print("5")
print(games.count)
}
print("6")
print(games.count)
}
**
I've embedded my explanations in the code commentary for easier reading. But the problem you have is that you aren't coordinating these async tasks (the getting of each document). You must coordinate them so when the last one finishes, you can "return" the array from the function. This function doesn't technically "return" anything (except Void) but the completion handler, in a way, "returns" the array which is why I put it in quotes. These semantic details matter and it helps to understand everything better.
func getMyGames(joinedGamesIDs: [String], handler: #escaping (_ games: [GameViewModel]) -> ()) {
guard !joinedGamesIDs.isEmpty else {
// If there is nothing to do, always consider
// calling the handler anyway, with an empty
// array, so the caller isn't left hanging.
return handler([])
}
// Set up a Dispatch Group to coordinate the multiple
// async tasks. Instatiate outside of the loop.
let group = DispatchGroup()
var games: [GameViewModel] = []
for id in joinedGamesIDs {
// Enter the group on each iteration of async work
// to be performed.
group.enter()
db.collection("games").document(id).getDocument { (document, error) in
if let doc = document,
doc.exists,
let game = self.getGameViewModal(document: doc) {
games.append(game)
} else if let error = error {
// Always print errors when in development.
print(error)
}
// No matter what happens inside the iteration,
// whether there was a success in getting the
// document or a failure, always leave the group.
group.leave()
}
}
// Once all of the calls to enter the group are equalled
// by the calls to leave the group, this block is called,
// which is the group's own completion handler. Here is
// where you ultimately call the function's handler and
// return the array.
group.notify(queue: .main) {
handler(games)
}
}
I have the following pipeline setup, and for some reason I can't understand, the second flatMap is skipped:
func letsDoThis() -> SignalProducer<(), MyError> {
let logError: (MyError) -> Void = { error in
print("Error: \(error); \((error as NSError).userInfo)")
}
return upload(uploads) // returns: SignalProducer<Signal<(), MyError>.Event, Never>
.collect() // SignalProducer<[Signal<(), MyError>.Event], Never>
.flatMap(.merge, { [uploadContext] values -> SignalProducer<[Signal<(), MyError>.Event], MyError> in
return context.saveSignal() // SignalProducer<(), NSError>
.map { values } // SignalProducer<[Signal<(), MyError>.Event], NSError>
.mapError { MyError.saveFailed(error: $0) } // SignalProducer<[Signal<(), MyError>.Event], MyError>
})
.flatMap(.merge, { values -> SignalProducer<(), MyError> in
if let error = values.first(where: { $0.error != nil })?.error {
return SignalProducer(error: error)
} else {
return SignalProducer(value: ())
}
})
.on(failed: logError)
}
See the transformations/signatures starting with the upload method.
When I say skipped I mean even if I add breakpoints or log statements, they are not executed.
Any idea how to debug this or how to fix?
Thanks.
EDIT: it is most likely has something to do with the map withing the first flatMap, but not sure how to fix it yet.
See this link.
EDIT 2: versions
- ReactiveCocoa (10.1.0):
- ReactiveObjC (3.1.1)
- ReactiveObjCBridge (6.0.0):
- ReactiveSwift (6.1.0)
EDIT 3: I found the problem which was due to my method saveSignal sending sendCompleted.
extension NSManagedObjectContext {
func saveSignal() -> SignalProducer<(), NSError> {
return SignalProducer { observer, disposable in
self.perform {
do {
try self.save()
observer.sendCompleted()
}
catch {
observer.send(error: error as NSError)
}
}
}
}
Sending completed make sense, so I can't change that. Any way to change the flatMap to still do what I intended to do?
I think the reason your second flatMap is never executed is that saveSignal never sends a value; it just finishes with a completed event or an error event. That means map will never be called, and no values will ever be passed to your second flatMap. You can fix it by doing something like this:
context.saveSignal()
.mapError { MyError.saveFailed(error: $0) }
.then(SignalProducer(value: values))
Instead of using map (which does nothing because there are no values to map), you just create a new producer that sends the values after saveSignal completes successfully.
Imagine the following chain where a user wants to save a list of some sort:
var saveChain = userTappedSaveListSubject
.doOnNext { list -> Void in // create pdf version
let pdfFactory = ArticleListPDFFactory()
list.pdf = try pdfFactory.buildPDF(list)
try database.save(list)
}
.flatMap { list in
AuthorizedNetworking.shared.request(.createList(try ListRequestModel(list)))
.filter(statusCode: 201)
.map { _ in list }
}
.doOnNext { list in
list.uploaded = true
try database.save(list)
try Printer().print(list)
}
.materialize()
.share()
On every operator in the chain errors can occur, which would terminate the stream and the user would be unable to retry saving and printing the list (the whole chain gets disposed).
In the end the user should see either a "success" or "failure" screen by binding the observable to a textField:
Observable.of(
saveChain.elements().map { _ in
("List saved!", subtitle: "Saving successfull")
},
saveChain.errors().map { error in
("Error!", subtitle: error.localizedDescription)
})
.merge()
How should the error be handled?
Here's the obvious fix:
let saveChain = userTappedSaveListSubject
.flatMap { list in
Observable.just(list)
.do(onNext: { list -> Void in // create pdf version
let pdfFactory = ArticleListPDFFactory()
list.pdf = try pdfFactory.buildPDF(list)
try database.save(list)
})
.flatMap { list in
AuthorizedNetworking.shared.request(.createList(try ListRequestModel(list)))
.filter(statusCode: 201)
.map { _ in list }
}
.do(onNext: { list in
list.uploaded = true
try database.save(list)
try Printer().print(list)
})
.materialize()
}
.share()
However, there are a host of problems with this code because of the mixed paradigms.
You are passing around a mutable class inside your Observables. This is problematic because it's a functional paradigm so the system expects the contained type to be either a struct/enum or an immutable class.
Your reliance on side effects to load up said mutable class object again is quite odd and against the paradigm.
I can't understand the difference between map and flatMap In RxSwift. In the RxSwift playground examples and the books, flatMap is used as converting Observables which has inner Observable property.
However I see flatMap being used directly on Observable of basic types. For example for below code, both of them produces the same output. Can someone help me to understand the difference between map and flatMap
struct Student {
let score:Int
}
let ryan = Student(score:80)
let student = PublishSubject<Student>()
let deneme = student.map({ val in
return Student(score: val.score+10)
})
deneme.subscribe(onNext: {
print("StudentMAP: \($0.score)")
})
let deneme2 = student.flatMap({ val -> Observable<Student> in
return Observable.of(Student(score: val.score + 10))
})
deneme2.subscribe(onNext: {
print("StudentFlatMAP: \($0.score)")
})
student.onNext(ryan)
map get value from stream and return another value of whatever type, result is Observable< whatever type >.
flatMap get value from stream and return an Observable of whatever type.
This means you can use flatMap when:
you already have a function declared which returns Observable< ? >, so you may want to use it in flatMap
func foo(_ number: Int) -> Observable<String> {
return Observable.just(String(number))
}
Observable.just(1)
.flatMap { (number) -> Observable<String> in
return foo(number)
}
you need that returned value push more than one value in the stream
func updates() -> Observable<String> {
// Something that generates updates
}
func shouldListenToUpdated() -> Observable<Bool> {
return Observable.just(true)
}
shouldListenToUpdated()
.flatMap { (listenToUpdated) -> Observable<String> in
return listenToUpdated ? updates() : Observable.empty()
}
While map will just transform next value in the stream.
Hope this clarifies things a bit.
To keep it simple
Use flatMap when you want return Observable down the stream.
Use map is simply transform the value of the observable and pass down the stream
Flatmap:
response.flatMap { response, _ -> Observable<NSString> in
guard let value = response.allHeaderFields["string"] as? NSString
else {
return Observable.empty()
}
return Observable.just(value)
}.subscribe(onNext: { [weak self] string in
print(string)
}).disposed(by: bag)
Map:
response.filter { response, _ in
return 200..<300 ~= response.statusCode
}.map { _ , data -> [[String: Any]] in
guard let jsonObject = try? JSONSerialization.jsonObject(with: data, options: []),
let result = jsonObject as? [[String: Any]] else {
return []
}
return result
}.subscribe(onNext: { [weak self] objects in
print(objects)
}).disposed(by: bag)
flatMap is similar to map, but it transforms element of observable to an observable of sequences. The example you use is relatively simple, it is simply sending and Observable mapped into something else.
Here is quote from Reactive extension documentation,
The FlatMap operator transforms an Observable by applying a function
that you specify to each item emitted by the source Observable, where
that function returns an Observable that itself emits items. FlatMap
then merges the emissions of these resulting Observables, emitting
these merged results as its own sequence.
This method is useful, for example, when you have an Observable that
emits a series of items that themselves have Observable members or are
in other ways transformable into Observables, so that you can create a
new Observable that emits the complete collection of items emitted by
the sub-Observables of these items.
If you extend the example a bit, you will know that flatMap actually transforms each element into a sequence.
Notice that you used,
student.onNext(ryan)
Remove your dename2 and add this code below,
let studentObservable: PublishSubject<Student> = PublishSubject()
let deneme2 = student.flatMap({ val -> Observable<Student> in
return studentObservable.map { val in Student(score: val.score + 10) }
})
deneme2.subscribe(onNext: {
print("StudentFlatMAP: \($0.score)")
})
student.onNext(ryan)
studentObservable.onNext(Student(score: 80))
studentObservable.onNext(Student(score: 90))
studentObservable.onNext(Student(score: 100))
Now, you can see that map would simply transform a value from sequence and new Observable is created, while flatMap transforms it into sequence. Now, each of the flatMapped elements can themselves emit values since they are stream themselves.
I have a network request that can Succeed or Fail
I have encapsulated it in an observable.
I have 2 rules for the request
1) There can never be more then 1 request at the same time
-> there is a share operator i can use for this
2) When the request was Succeeded i don't want to repeat the same
request again and just return the latest value
-> I can use shareReplay(1) operator for this
The problem arises when the request fails, the shareReplay(1) will just replay the latest error and not restart the request again.
The request should start again at the next subscription.
Does anyone have an idea how i can turn this into a Observable chain?
// scenario 1
let obs: Observable<Int> = request().shareReplay(1)
// outputs a value
obs.subscribe()
// does not start a new request but outputs the same value as before
obs.subscribe()
// scenario 2 - in case of an error
let obs: Observable<Int> = request().shareReplay(1)
// outputs a error
obs.subscribe()
// does not start a new request but outputs the same value as before, but in this case i want it to start a new request
obs.subscribe()
This seems to be a exactly doing what i want, but it consists of keeping state outside the observable, anyone know how i can achieve this in a more Rx way?
enum Err: Swift.Error {
case x
}
enum Result<T> {
case value(val: T)
case error(err: Swift.Error)
}
func sample() {
var result: Result<Int>? = nil
var i = 0
let intSequence: Observable<Result<Int>> = Observable<Int>.create { observer in
if let result = result {
if case .value(let val) = result {
return Observable<Int>.just(val).subscribe(observer)
}
}
print("do work")
delay(1) {
if i == 0 {
observer.onError(Err.x)
} else {
observer.onNext(1)
observer.onCompleted()
}
i += 1
}
return Disposables.create {}
}
.map { value -> Result<Int> in Result.value(val: value) }
.catchError { error -> Observable<Result<Int>> in
return .just(.error(err: error))
}
.do(onNext: { result = $0 })
.share()
_ = intSequence
.debug()
.subscribe()
delay(2) {
_ = intSequence
.debug()
.subscribe()
_ = intSequence
.debug()
.subscribe()
}
delay(4) {
_ = intSequence
.debug()
.subscribe()
}
}
sample()
it only generates work when we don't have anything cached, but thing again we need to use side effects to achieve the desired output
As mentioned earlier, RxSwift errors need to be treated as fatal errors. They are errors your stream usually cannot recover from, and usually errors that would not even be user facing.
For that reason - a stream that emits an .error or .completed event, will immediately dispose and you won't receive any more events there.
There are two approaches to tackling this:
Using a Result type like you just did
Using .materialize() (and .dematerialize() if needed). These first operator will turn your Observable<Element> into a Observable<Event<Element>>, meaning instead of an error being emitted and the sequence terminated, you will get an element that tells you it was an error event, but without any termination.
You can read more about error handling in RxSwift in Adam Borek's great blog post about this: http://adamborek.com/how-to-handle-errors-in-rxswift/
If an Observable sequence emits an error, it can never emit another event. However, it is a fairly common practice to wrap an error-prone Observable inside of another Observable using flatMap and catch any errors before they are allowed to propagate through to the outer Observable. For example:
safeObservable
.flatMap {
Requestor
.makeUnsafeObservable()
.catchErrorJustReturn(0)
}
.shareReplay(1)
.subscribe()