How do I recover from a failed rx chain? - ios

Rx seem a bit fragile in that it closes down an entire chain if one single thing doesn't work. That has become a real problem in my code as I have a chain that requests parameters through ble. First we ask for ids, then definitions which is sort of mapping min and max values, lastly it asks for the actual parameters:
override func getParameters() -> Single<[ParameterModel?]> {
parameterCounter = 0
parameterDefinitionCounter = 0
return getParamterIds().do(onSuccess: { [weak self] values in
self?.numberOfParameterIds = Float(values?.count ?? 0)
})
.flatMap { ids in
Single.zip(ids!.compactMap { self.getParamterDefinition(id: $0) }) }
.flatMap { parameters in
Single.zip(parameters.compactMap { self.getParameter(id: $0!.id) }) }
}
So if we get an array with 30 parameter ids, it goes into getParamterDefinition(id: $0). And if it fails on a single one of those, which it does, the whole thing closes down and self.getParameter(id: $0!.id) is never run. So even though 29 parameters pass through getParamterDefinition(id: $0) nothing is passed to self.getParameter(id: $0!.id).
How do I recover from an error and keep going in the chain so that those that were successful in getParamterDefinition(id: $0) gets passed to self.getParameter(id: $0!.id) and gets displayed to the user?
*** UPDATE ***
This is the final result for anyone interested in solving issues like these:
override func getParameters() -> Single<[ParameterModel?]> {
parameterCounter = 0
parameterDefinitionCounter = 0
func getFailedParameter(id: Int) -> ParameterModel {
return ParameterModel(id: id, name: String(format: "tech_app_failed_getting_parameter".localized(), "\(id)"), min: 2000,
max: 21600000, defaultValue: 2500, value: 2500,
unit: "ms", access: 0, freezeFlag: 0,
multiplicator: 1, operatorByte: 1, brand: 0,
states: nil, didFailRetrievingParameter: true)
}
return getParamterIds().do(onSuccess: { [weak self] values in
self?.numberOfParameterIds = Float(values?.count ?? 0)
}).catchError { _ in return Single.just([]) }
.flatMap { ids in
Single.zip(ids!.compactMap { id in
self.getParamterDefinition(id: id).catchError { [weak self] _ in
self?.parameterErrorStatusRelay.accept(String(format: "tech_app_parameter_definition_error_status".localized(), "\(id)"))
return Single.just(getFailedParameter(id: id))
}
})
}
.flatMap { parameters in
Single.zip(parameters.compactMap { parameter in
guard let parameter = parameter, !(parameter.didFailRetrievingParameter) else {
return Single.just(parameter)
}
return self.getParameter(id: parameter.id).catchError { [weak self] _ in
self?.parameterErrorStatusRelay.accept(String(format: "tech_app_parameter_error_status".localized(), "\(parameter.id)"))
return Single.just(getFailedParameter(id: parameter.id))
}
})
}
}

You should use the Catch methods to handle errors, you can use these to stop your sequence from terminating when an error event occurs.
A simple example that just ignores any errors would be to return nil whenever your getParamterDefinition observable emits an error:
override func getParameters() -> Single<[ParameterModel?]> {
return getParameterIds()
.do(onSuccess: { [weak self] values in
self?.numberOfParameterIds = Float(values?.count ?? 0)
})
.flatMap { ids in
Single.zip(
ids!.compactMap {
self.getParameterDefinition(id: $0)?
.catchAndReturn(nil)
}
)
}
.flatMap { parameters in
Single.zip(
parameters.compactMap { parameter in
parameter.flatMap { self.getParameter(id: $0.id) }
}
)
}
}

Related

How to wait for an API request to finish before storing data from function callback?

In a personal project of mine, I have created an API caller to retrieve a user's saved tracks from the Spotify API. The Spotify endpoint which I am using has a limit (maximum of 50 tracks per request) as well as an offset (starting index of first track in request), which is why I decided to use a FOR loop to get a series of track pages (each 50 tracks) and append them to a global array. The data is loaded from the main thread, and while the data is being requested, I display a child view controller with a spinner view. Once the data request has completed, I remove the spinner view, and transition to another view controller (passing the data as a property).
I have tried many things, but the array of tracks is always empty following the API request. I have a feeling it has to do with the synchronicity of my request, or maybe its possible that I'm not handling it correctly. Ideally, I would like to wait until the request from my API finishes, then append the result to the array. Do you have any suggestions on how I could solve this? Any help is much appreciated!
func createSpinnerView() {
let loadViewController = LoadViewController.instantiateFromAppStoryboard(appStoryboard: .OrganizeScreen)
add(asChildViewController: loadViewController)
DispatchQueue.main.async { [weak self] in
if (self?.dropdownButton.dropdownLabel.text == "My saved music") {
self?.fetchSavedMusic() { tracksArray in
self?.tracksArray = tracksArray
}
}
...
self?.remove(asChildViewController: loadViewController)
self?.navigateToFilterScreen(tracksArray: self!.tracksArray)
}
}
private func fetchSavedMusic(completion: #escaping ([Tracks]) -> ()) {
let limit = 50
var offset = 0
var total = 200
for _ in stride(from: 0, to: total, by: limit) {
getSavedTracks(limit: limit, offset: offset) { tracks in
//total = tracks.total
self.tracksArray.append(tracks)
}
print(offset, limit)
offset = offset + 50
}
completion(tracksArray)
}
private func getSavedTracks(limit: Int, offset: Int, completion: #escaping (Tracks) -> ()) {
APICaller.shared.getUsersSavedTracks(limit: limit, offset: offset) { (result) in
switch result {
case .success(let model):
completion(model)
print("success")
case .failure(let error):
print("Error retrieving saved tracks: \(error.localizedDescription)")
print(error)
}
}
}
private func navigateToFilterScreen(tracksArray: [Tracks]) {
let vc = FilterViewController.instantiateFromAppStoryboard(appStoryboard: .OrganizeScreen)
vc.paginatedTracks = tracksArray
show(vc, sender: self)
}
First you need to call completion when all of your data is loaded. In your case you call completion(tracksArray) before any of the getSavedTracks return.
For this part I suggest you to recursively accumulate tracks by going through all pages. There are multiple better tools to do so but I will give a raw example of it:
class TracksModel {
static func fetchAllPages(completion: #escaping ((_ tracks: [Track]?) -> Void)) {
var offset: Int = 0
let limit: Int = 50
var allTracks: [Track] = []
func appendPage() {
fetchSavedMusicPage(offset: offset, limit: limit) { tracks in
guard let tracks = tracks else {
completion(allTracks) // Most likely an error should be handled here
return
}
if tracks.count < limit {
// This was the last page because we got less than limit (50) tracks
completion(allTracks+tracks)
} else {
// Expecting another page to be loaded
offset += limit // Next page
allTracks += tracks
appendPage() // Recursively call for next page
}
}
}
appendPage() // Load first page
}
private static func fetchSavedMusicPage(offset: Int, limit: Int, completion: #escaping ((_ tracks: [Track]?) -> Void)) {
APICaller.shared.getUsersSavedTracks(limit: limit, offset: offset) { result in
switch result {
case .success(let model):
completion(model)
case .failure(let error):
print(error)
completion(nil) // Error also needs to call a completion
}
}
}
}
I hope comments will clear some things out. But the point being is that I nested an appendPage function which is called recursively until server stops sending data. In the end either an error occurs or the last page returns fewer tracks than provided limit.
Naturally it would be nicer to also forward an error but I did not include it for simplicity.
In any case you can now anywhere TracksModel.fetchAllPages { } and receive all tracks.
When you load and show your data (createSpinnerView) you also need to wait for data to be received before continuing. For instance:
func createSpinnerView() {
let loadViewController = LoadViewController.instantiateFromAppStoryboard(appStoryboard: .OrganizeScreen)
add(asChildViewController: loadViewController)
TracksModel.fetchAllPages { tracks in
DispatchQueue.main.async {
self.tracksArray = tracks
self.remove(asChildViewController: loadViewController)
self.navigateToFilterScreen(tracksArray: tracks)
}
}
}
A few components may have been removed but I hope you see the point. The method should be called on main thread already. But you are unsure what thread the API call returned on. So you need to use DispatchQueue.main.async within the completion closure, not outside of it. And also call to navigate within this closure because this is when things are actually complete.
Adding situation for fixed number of requests
For fixed number of requests you can do all your requests in parallel. You already did that in your code.
The biggest problem is that you can not guarantee that responses will come back in same order than your requests started. For instance if you perform two request A and B it can easily happen due to networking or any other reason that B will return before A. So you need to be a bit more sneaky. Look at the following code:
private func loadPage(pageIndex: Int, perPage: Int, completion: #escaping ((_ items: [Any]?, _ error: Error?) -> Void)) {
// TODO: logic here to return a page from server
completion(nil, nil)
}
func load(maximumNumberOfItems: Int, perPage: Int, completion: #escaping ((_ items: [Any], _ error: Error?) -> Void)) {
let pageStartIndicesToRetrieve: [Int] = {
var startIndex = 0
var toReturn: [Int] = []
while startIndex < maximumNumberOfItems {
toReturn.append(startIndex)
startIndex += perPage
}
return toReturn
}()
guard pageStartIndicesToRetrieve.isEmpty == false else {
// This happens if maximumNumberOfItems == 0
completion([], nil)
return
}
enum Response {
case success(items: [Any])
case failure(error: Error)
}
// Doing requests in parallel
// Note that responses may return in any order time-wise (we can not say that first page will come first, maybe the order will be [2, 1, 5, 3...])
var responses: [Response?] = .init(repeating: nil, count: pageStartIndicesToRetrieve.count) { // Start with all nil
didSet {
// Yes, Swift can do this :D How amazing!
guard responses.contains(where: { $0 == nil }) == false else {
// Still waiting for others to complete
return
}
let aggregatedResponse: (items: [Any], errors: [Error]) = responses.reduce((items: [], errors: [])) { partialResult, response in
switch response {
case .failure(let error): return (partialResult.items, partialResult.errors + [error])
case .success(let items): return (partialResult.items + [items], partialResult.errors)
case .none: return (partialResult.items, partialResult.errors)
}
}
let error: Error? = {
let errors = aggregatedResponse.errors
if errors.isEmpty {
return nil // No error
} else {
// There was an error.
return NSError(domain: "Something more meaningful", code: 500, userInfo: ["all_errors": errors]) // Or whatever you wish. Perhaps just "errors.first!"
}
}()
completion(aggregatedResponse.items, error)
}
}
pageStartIndicesToRetrieve.enumerated().forEach { requestIndex, startIndex in
loadPage(pageIndex: requestIndex, perPage: perPage) { items, error in
responses[requestIndex] = {
if let error = error {
return .failure(error: error)
} else {
return .success(items: items ?? [])
}
}()
}
}
}
The first method is not interesting. It just loads a single page. The second method now collects all the data.
First thing that happens is we calculate all possible requests. We need a start index and per-page. So the pageStartIndicesToRetrieve for case of 145 items using 50 per page will return [0, 50, 100]. (I later found out we only need count 3 in this case but that depends on the API, so let's stick with it). We expect 3 requests starting with item indices [0, 50, 100].
Next we create placeholders for our responses using
var responses: [Response?] = .init(repeating: nil, count: pageStartIndicesToRetrieve.count)
for our example of 145 items and using 50 per page this means it creates an array as [nil, nil, nil]. And when all of the values in this array turn to not-nil then all requests have returned and we can process all of the data. This is done by overriding the setter didSet for a local variable. I hope the content of it speaks for itself.
Now all that is left is to execute all requests at once and fill the array. Everything else should just resolve by itself.
The code is not the easiest and again; there are tools that can make things much easier. But for academical purposes I hope this approach explains what needs to be done to accomplish your task correctly.

Make a network call every 10 seconds with RxSwift

I am completely new to RxSwift. I managed to load my table view but now I would like to make a call every 10 seconds.
I was reading here that I should probably use Observable<Int>.interval(10, scheduler: MainScheduler.instance), I tried without much success.
class MarketService: MarketServiceProtocol {
func fetchMarkets() -> Observable <[Market]> {
return Observable.create { observer -> Disposable in
RestManager.shared.makeRequest(withEndPoint: "market/v2/get-summary?region=US" , withHttpMethod: .get) { result in
if let error = result.error {
observer.onError(error)
return
}
guard let response = result.response,
200 ... 299 ~= response.httpStatusCode else {
return
}
guard let data = result.data else {
return
}
do {
let decodedData = try JSONDecoder().decode(MarketResult.self, from: data)
observer.onNext(decodedData.marketSummaryAndSparkResponse.markets)
} catch {
observer.onError(error)
}
}
return Disposables.create { }
}
}
}
then I call in my view controller:
viewModel.fetchMarketViewModels().observe(on: MainScheduler.instance).bind(to: tableView.rx.items(cellIdentifier: HomeTableViewCell.cellIdentifier)) {
index, viewModel, cell in
guard let cell = cell as? HomeTableViewCell else { return }
cell.setupData(viewModel: viewModel)
}.disposed(by: self.disposableBag)
There are a couple of problems with your Observable.create closure. You have to make sure that something is sent to the observer in every path, otherwise the Observable will call the function and then not emit anything and you will not know why.
Also, you want to minimize the amount of logic being performed in the closure passed to create. You are doing way too much in there.
So let's simplify the code in the create closure as much as possible first:
extension RestManager {
func rx_makeRequest(withEndPoint endPoint: String, withHttpMethod method: HttpMethod) -> Observable<(response: MyHTTPURLResponse, data: Data)> {
Observable.create { observer in
self.makeRequest(withEndPoint: endPoint, withHttpMethod: method) { result in
if let response = result.response, let data = result.data {
observer.onNext((response, data))
observer.onCompleted()
}
else {
observer.onError(result.error ?? RxError.unknown)
}
}
return Disposables.create() // is there some way of canceling a request? If so, it should be done here.
}
}
}
This does the bare minimum. Just wraps the underlying callback and nothing else. Now your fetchMarkets call is much simpler:
class MarketService: MarketServiceProtocol {
func fetchMarkets() -> Observable <[Market]> {
return RestManager.shared.rx_makeRequest(withEndPoint: "market/v2/get-summary?region=US", withHttpMethod: .get)
.do(onNext: { result in
guard 200...299 ~= result.response.httpStatusCode
else { throw URLError.httpRequestFailed(response: result.response, data: result.data) }
})
.map { try JSONDecoder().decode(MarketResult.self, from: $0.data).marketSummaryAndSparkResponse.markets }
}
}
Now to the meat of your question. How to make the network call every 10 seconds... Just wrap your network call in a flatMap like this:
Observable<Int>.interval(.seconds(10), scheduler: MainScheduler.instance)
.flatMapLatest { _ in
viewModel.fetchMarketViewModels()
}
.observe(on: MainScheduler.instance)
.bind(to: tableView.rx.items(cellIdentifier: HomeTableViewCell.cellIdentifier)) { index, viewModel, cell in
guard let cell = cell as? HomeTableViewCell else { return }
cell.setupData(viewModel: viewModel)
}
.disposed(by: self.disposableBag)
Learn more about flatMap and its variants from this article.

CoreStore creating object and return it in success

I use this code to create an object:
CoreStore.perform(
asynchronous: { (transaction) -> Void in
let summary = transaction.create(Into<SummaryEntity>())
},
completion: { _ in }
)
In completion I would like to return back just created summary object.
How to do this?
I did something like that, but not sure for what we need a lot of unwraps and fetchExisting function
CoreStore.perform(
asynchronous: { (transaction) -> Routine? in
let routine = try! transaction.importUniqueObject(
Into<Routine>(),
source: routineDictionary)
return routine
},
success: { (transactionRoutine) in
guard let unwrappedTransactionRoutine = transactionRoutine else {
return
}
let routine = CoreStore.fetchExisting(unwrappedTransactionRoutine)
guard let unwrappedRoutine = routine else {
return
}
completion(.data(unwrappedRoutine))
},
failure: { (error) in
// ...
}
)
This is an error of unwrapping from fetchExisting:
I am not exactly sure what you are asking as your examples are completely different.
If you want to create an object, you would do the same as for importing.
transaction.create does not return an optional, so just have your closure in the first one return the object:
CoreStore.perform(asynchronous: { (transaction) -> SummaryEntity in
let summary = transaction.create(Into<SummaryEntity>())
return summary
}, success: { (summary) in
let fetchedSummary = CoreStore.fetchExisting(summary)
completion(.data(fetchedSummary))
}, failure: { (error) in
// ...
})

Best way to call multiple API requests in a for loop in RxSwift

I have to make several api calls (approx 100) using a for loop and on completion of this I need to complete the Observable. I am using it as following:
func getMaterialInfo(materialNo:[String]) -> Observable<[String: Material]>{
return Observable.create({ (observable) -> Disposable in
for (index,mat) in materialNo.enumerated(){
// Pass the material number one by one to get the Material object
self.getMaterialInfo(materialNo: mat).subscribe(onNext: { material in
var materialDict: [String: Material] = [:]
materialDict[material.materialNumber] = material
observable.onNext(materialDict)
if index == (materialNo.count-1){
observable.onCompleted()
}
}, onError: { (error) in
observable.onError(error)
}, onCompleted: {
}).disposed(by: self.disposeBag)
}
return Disposables.create()
})
}
Although loop is working fine and observable.onCompleted() is called but the caller method does not receive it.
I am calling it like following:
private func getImage(materialNo:[String]){
if materialNo.isEmpty {
return
}
var dictMaterials = [String:String]()
materialService.getMaterialInfo(materialNo: materialNo).subscribe(onNext: { (materials) in
for (key,value) in materials{
if (value.imageUrl != nil){
dictMaterials[key] = value.imageUrl
}
}
}, onError: { (error) in
}, onCompleted: {
self.view?.updateToolImage(toolImageList: dictMaterials)
}, onDisposed: {}).disposed(by: disposeBag)
}
OnCompleted block of Rx is not executing. How can I fix it?
Edit (5-March)
I revisited this answer, because I'm not sure what my brain was doing when I wrote the code sample below. I'd do something like this instead:
func getMaterialInfo(materialNo: String) -> Observable<[String: Material]> {
// ...
}
func getMaterialInfo(materialNumbers:[String]) -> Observable<[String: Material]>{
let allObservables = materialNumbers
.map { getMaterialInfo(materialNo: $0) }
return Observable.merge(allObservables)
}
Original answer
From your code, I interpret that all individual getMaterialInfo calls are done concurrently. Based on that, I would rewrite your getMaterialInfo(:[_]) method to use the .merge operator.
func getMaterialInfo(materialNo:[String]) -> Observable<[String: Material]>{
return Observable.create({ (observable) -> Disposable in
// a collection of observables that we haven't yet subscribed to
let allObservables = materialNo
.map { getMaterialInfo(materialNo: $0) }
return Observable.merge(allObservables)
}
return Disposables.create()
}
Note that using merge subscribes to all observable simultaneously, triggering 100 network requests at the same time. For sequential subscription, use concat instead!

repeat-while operator in rx-swift

I have a function that fires an API request to the server. I want to loop over it until it returns false (no more data).
func getData(id: Int) -> Observable<Bool> {
return Observable.create { observer in
// Alamofire request
// parse data
// if can decode,
// return true and increment page's property
// otherwise false
// error, if there's a problem
}
}
First try: I've tried using takeWhile, like : getData(id).takeWhile {$0}. It only iterate over my function 1x only.
Second try: using a range. The problem here is that even if my getData function errors out, instead of stopping, the loop continues !
Observable.range(start: 1, count: 100)
.enumerated()
.flatMapLatest({ _ in
self.getData(someID)
})
.subscribe(onNext: { _ in
// save to DB
observer.onNext(true)
observer.onCompleted()
}, onError: { error in
observer.onError(error)
})
.disposed(by: self.disposeBag)
Is there a way to do it, rx style ?
Something like this?
let callApiTrigger = BehaviorRelay<Bool>(value: true)
let callApiEnoughTimes = callApiTrigger.asObservable()
.takeWhile { $0 }
.flatMap { _ in
return getData(someId)
}
.do(onNext: { (apiResult: Bool) in
callApiTrigger.accept(apiResult)
})
The reason why takeWhile and take(X) do not work, is because they do not resubscribe to the Observable. A network request observable typically emits one value at the most.
What you are looking for requires some form of recursion / resubscribing. If you want to it hardcore Rx I suggest you reverse engineer the retry operator to suit your needs. Although I consider myself experienced with RxSwift, that seems like a bridge too far.
Fortunately, I whipped up a recursive approach that works just fine too :)
class PageService {
typealias Page = (pageNumber: Int, items: [String]?)
private func getPage(_ pageNumber: Int) -> Observable<Page> {
let pageToReturn = Page(pageNumber: pageNumber, items: (pageNumber < 3) ? ["bert, henk"] : nil)
return Observable<Page>
.just(pageToReturn)
.delay(0.5, scheduler: MainScheduler.instance)
}
func allPagesFollowing(pageNumber: Int) -> Observable<Page> {
let objectToReturnInCaseOfError = Page(pageNumber: pageNumber + 1, items: nil)
return getPage(pageNumber + 1)
// in order to error out gracefully, you could catch the network error and interpret it as 0 results
.catchErrorJustReturn(objectToReturnInCaseOfError)
.flatMap { page -> Observable<Page> in
// if this page has no items, do not continue the recursion
guard page.items != nil else { return .empty() }
// glue this single page together with all the following pages
return Observable<Page>.just(page)
.concat(self.allPagesFollowing(pageNumber: page.pageNumber))
}
}
}
_ = PageService().allPagesFollowing(pageNumber: 0)
.debug("get page")
.subscribe()
This will print:
2018-03-30 11:56:24.707: get page -> subscribed
2018-03-30 11:56:25.215: get page -> Event next((pageNumber: 1, data: Optional(["bert, henk"])))
2018-03-30 11:56:25.718: get page -> Event next((pageNumber: 2, data: Optional(["bert, henk"])))
2018-03-30 11:56:26.223: get page -> Event completed
2018-03-30 11:56:26.223: get page -> isDisposed

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