Update view after async API call with RxSwift - swift

Question

I'm trying to add RxSwift to a peace of code using MVVM pattern. My app need to get a list of FoodType (desert, meal, etc.) an Food from an API and save them to Realm database. Then, I have a view with an UITextField and a UIButton.
The user write a food type (ex: desert):
Background: the app should get FoodType and FoodList from Api if not in Realm DB
On button click: show list of Food that have the FoodType chosen by the user from Realm
ViewModel
struct FoodTypeViewModel {
// Get datas from API
private func getFoods() {
foodService.getAll(completionHandler: { result in
switch result {
case .Success(let foods):
for food in foods {
food.save()
}
break
case .Failure(let error):
debugPrint(error)
break
}
})
}
// Get datas from API
private func getFoodTypes() {
foodService.getAll(completionHandler: { result in
switch result {
case .Success(let foodTypes):
for type in types {
type.save()
}
break
case .Failure(let error):
debugPrint(error)
break
}
})
}
}
ViewController
class SetupViewController: UIViewController {
#IBOutlet weak var foodTypeTextField: UITextField!
#IBOutlet weak var foodTypeButton: UIButton!
}
Model
class FoodType: Object {
dynamic var identifier: String = ""
dynamic var fullName: String?
let foods = List<Food>()
}
I would like to add RxSwift to that code but how can I handle the asynchronous API. On first start the app have no datas (I don't want to populate at start) but when the user click the button. So on button click, UI should wait the response from the service (using waiting animation) and ViewModel should update UI when service respond. Any idea ?

Answer 1

First, create a generic return object to wrap communication errors.
enum APIResult<T> {
case success(T)
case error(Error)
}
Then, convert your completion handler to return an Observable:
func getFoods() -> Observable<APIResult<[FoodType]>> {
return Observable<APIResult<[FoodType]>>.create { observer -> Disposable in
self.foodService.getAll(completionHandler: { result in
switch result {
case .Success(let foods):
observer.onNext(.success(foods))
break
case .Failure(let error):
observer.onNext(.error(error))
break
}
observer.onCompleted()
return Disposables.create()
})
}
}
Now simply process the observable as any other in RxSwift.
getFoods().subscribe(onNext: { result in
switch result {
case .success(let foods):
print("Received foods: \(foods)")
break
case .error(let error):
print("Received error: \(error)")
break
}
}.addDisposableTo(disposeBag)
Using these utility classes will help you mapping success results and split error and success signals to different observables. For example:
let foodsRequest = getFoods().splitSuccess
foodsRequest.error.subscribe(onNext: { error in
print("Received error: \(error)")
})
foodsRequest.success.subscribe(onNext: { foods in
print("Received foods: \(foods)")
}
You can also convert Realm objects to RxSwift observables:
let realm = try! Realm()
realm.objects(Lap).asObservable()
.subscribeNext {[weak self] laps in
self?.tableView.reloadData()
}
Take a look at Using Realm Seamlessly in an RxSwift App for more information and examples.