Parsing a JSON and handling error response from API in RxSwift - ios

Question

I'm new to RxSwift. I have a BackendProvider with handles the communication with my API. I want to have a config file sync so that I can retrieve some parameters dynamically. I have a fallback case with a local stored JSON file that I can to access in case my API is not reachable or my JSON parsing fails:
ConfigFileBackendService
open func getLatestConfig() -> Observable<ConfigFile?> {
let urlString = IoC.urlProviderService.getConfigFileUrl()?.absoluteString ?? ""
let configFileJSONData = IoC.backendCommunicationService.getJsonData(url: urlString)
return configFileJSONData.map { data in
if let configFile = try? JSONDecoder().decode(ConfigFile.self, from: data) {
return configFile
} else {
return nil
}
}
}
ConfigFileProcessService
This is the one that falls back to the local stored file:
func getConfigFile() -> Observable<ConfigFile> {
return IoC.configFileBackendService.getLatestConfig()
.map { configFile in
guard let configFile = configFile else { fatalError() }
return configFile
}
.catchError { error in
// Use default config
let localURL = IoC.urlProviderService.getLocalConfigFileUrl()
do {
let data = try Data(contentsOf: localURL)
let configFile = try JSONDecoder().decode(ConfigFile.self, from: data)
return Observable.just(configFile)
} catch {
fatalError("Error loading local config")
}
}
}
This approach works, but I'm having doubts with the .map / .catchError blocks. Is there a better way to handle the error case? Maybe should I go with, onNext and then onError? Thanks in advance!

Answer 1

What you have looks good except for the multiple approaches to handling errors. In one case you use a try? and another uses do... catch and presumably your getJsonData(url:) can emit an Observable error. You are all over the place. I suggest you pick one error handling system and stick to it. The most flexible one is Event.error. So something like this:
func getLatestConfig() -> Observable<ConfigFile> {
let urlString = IoC.urlProviderService.getConfigFileUrl()?.absoluteString ?? ""
let configFileJSONData = IoC.backendCommunicationService.getJsonData(url: urlString)
return configFileJSONData.map { try JSONDecoder().decode(ConfigFile.self, from: $0) }
}
Note that I'm just letting decoding errors route into an Observable error event. No need to deal with nil that way.
func getConfigFile() -> Observable<ConfigFile> {
return IoC.configFileBackendService.getLatestConfig()
.catchError { _ in
let localURL = IoC.urlProviderService.getLocalConfigFileUrl()
let data = try! Data(contentsOf: localURL)
let configFile = try! JSONDecoder().decode(ConfigFile.self, from: data)
return Observable.just(configFile)
}
}
Since you are crashing if either try fails anyway, just put a ! on them. It has the same effect. You should consider putting the error block into a separate, testable, function because there's no guarantee you are going to hit it during a regular run of the program and it could be broken without you ever realizing it.
Lastly, with the above there is no reason to provide an onError: handler in a subscribe because the getConfigFile() observable will never emit an error. You might want to have the function return a Driver instead to make the fact more explicit.

Answer 2

Following #Daniel T. approach, I got rid of nil and made the code crash only if the local config file loading process fails:
open class ConfigFileBackendService: ConfigFileBackendServiceProtocol {
open func getLatestConfig() -> Observable<ConfigFile> {
let urlString = IoC.urlProviderService.getConfigFileUrl()
let configFileJSONData = IoC.backendCommunicationService.getJsonData(url: urlString)
return configFileJSONData.map { try JSONDecoder().decode(ConfigFile.self, from: $0) }
}
}
class ConfigFileProcessService: ConfigFileProcessServiceProtocol {
func getConfigFile() -> Observable<ConfigFile> {
return IoC.configFileBackendService.getLatestConfig()
.catchError { [weak self] error in
// Load local config file
NSLog(error.localizedDescription)
guard let strongSelf = self else { fatalError() }
return Observable.just(strongSelf.getLocalFile())
}
}
private func getLocalFile() -> ConfigFile {
let localURL = IoC.urlProviderService.getLocalConfigFileUrl()
guard
let data = try? Data(contentsOf: localURL),
let configFile = try? JSONDecoder().decode(ConfigFile.self, from: data)
else { fatalError("Error loading local config") }
return configFile
}
}