swift combine flatmap 顺序操作

Question

extension CLGeocoder {
    func reverseGeocodeLocationPublisher(_ location: CLLocation, preferredLocale locale: Locale? = nil) -> AnyPublisher<CLPlacemark, Error> {
        Future<CLPlacemark, Error> { promise in
            self.reverseGeocodeLocation(location, preferredLocale: locale) { placemarks, error in
                guard let placemark = placemarks?.first else {
                    return promise(.failure(error ?? CLError(.geocodeFoundNoResult)))
                }
                return promise(.success(placemark))
            }
        }.eraseToAnyPublisher()
    }
}

$stations.flatMap(maxPublishers: .max(1)) { (station) -> AnyPublisher<CLPlacemark, Error> in
            let location = CLLocation(latitude: station.latitude, longitude: station.longitude)
            self.geocoder.reverseGeocodeLocationPublisher(location)

        }.eraseToAnyPublisher().sink(receiveCompletion: { completion in
            print("done")
        }, receiveValue: { placemark in
            print("placemark:", placemark)
        }).store(in: &cancellableSet)

错误： Instance method 'flatMap(maxPublishers:_:)' requires the types 'Published<[Station]>.Publisher.Failure' (aka 'Never') and 'Error' be equivalent

我想要实现的是我有一个包含location 的电台列表，我想依次reverseGeocodeLocation

Answer 1

您看到的错误是由于Publishers.FlatMap 要求上游和新发布者的Failure 相同。 FlatMap 返回一个 Failure 的 Error 类型，但 $stations 有一个 Failure 的 Never，所以你需要调整它：

.setFailureType(to: Error.self)

由于stations 是一个数组，并且您希望按顺序处理每个元素，因此您需要使用Publishers.Sequence 发布者（可通过Array.publisher 获得），它将逐个发出每个元素：

stations.publisher

把这两件事放在一起：

$stations.flatMap { stations in
            stations.publisher
         }
         .setFailureType(to: Error.self)
         .flatMap { station in 
            let location = CLLocation(latitude: station.latitude, ...)
            return self.geocoder.reverseGeocodeLocationPublisher(location)
         }