Combine - fetching data using URLSession publishers

CombineFetcher.swift

import Foundation
import Combine

enum APIError: Error, LocalizedError {
    case unknown, apiError(reason: String)

    var errorDescription: String? {
        switch self {
        case .unknown:
            return "Unknown error"
        case .apiError(let reason):
            return reason
        }
    }
}

func fetch(url: URL) -> AnyPublisher<Data, APIError> {
    let request = URLRequest(url: url)

    return URLSession.DataTaskPublisher(request: request, session: .shared)
        .tryMap { data, response in
            guard let httpResponse = response as? HTTPURLResponse, 200..<300 ~= httpResponse.statusCode else {
                throw APIError.unknown
            }
            return data
        }
        .mapError { error in
            if let error = error as? APIError {
                return error
            } else {
                return APIError.apiError(reason: error.localizedDescription)
            }
        }
        .eraseToAnyPublisher()
}

// Usage

guard let url = URL(string: "https://www.amazon.com") else { return }
fetch(url: url)
    .sink(receiveCompletion: { completion in
        switch completion {
        case .finished:
            break
        case .failure(let error):
            print(error.localizedDescription)
        }
    }, receiveValue: { data in
        guard let response = String(data: data, encoding: .utf8) else { return }
        print(response)
    })

Thanks, I've been looking for a good tryMap and mapError with URLSession.DataTaskPublisher example for awhile!

Do you need to try fetch(url: url) at line 40?

I tried this but I am getting finished with error [-999] Error Domain=NSURLErrorDomain Code=-999 "cancelled". It got cancelled immediately after starting. Anyone has an idea why? I use a .print("test") and i am getting

test: receive subscription: (DataTaskPublisher)
test: request unlimited
test: receive cancel

same here, getting cancelled immediately after starting.

@workingDog I dont know what is the cause but I found the solution for this

Dont call .sink like that, declare a subscriber and call it

like this:

let subscriber = AnyCancellable? 
subscriber.sink() 

Thanks for the update.
It seemed an elegant way of fetching data. But after so much frigging around I
change tack and went with PromiseKit and the normal URLSession.
That works very well.

@junweimah The reason is that sink is returning an Cancellable. If this cancellable is not referenced it gets deallocated immediately and is calling cancel. If you hold a reference to it this is not the case.

let sub = fetch(url: url)
    .sink(receiveCompletion: { completion in
        switch completion {
        case .finished:
            break
        case .failure(let error):
            print(error.localizedDescription)
        }
    }, receiveValue: { data in
        guard let response = String(data: data, encoding: .utf8) else { return }
        print(response)
    })

Should work here

It does not work when you encapsulate the fetch inside a function and have the Cancellable let sub reside on the same scope because the completion is called long after the function is destroyed!
It is necessary to have a global Cancellable optional variable available that survives the entire scope of the caller of fetch!

@junweimah The reason is that sink is returning an Cancellable. If this cancellable is not referenced it gets deallocated immediately and is calling cancel. If you hold a reference to it this is not the case.

let sub = fetch(url: url)
    .sink(receiveCompletion: { completion in
        switch completion {
        case .finished:
            break
        case .failure(let error):
            print(error.localizedDescription)
        }
    }, receiveValue: { data in
        guard let response = String(data: data, encoding: .utf8) else { return }
        print(response)
    })

Should work here