URLSession with URLSession and Combine normally

Overview

I investigated how to write URLSession normally and how to write in Combine.

Subject

We will use Qiita's LGTM acquisition API as an example. Reference: Get LGTM and View of your article with Qiita API. --Qiita

Currently LGTM gets about 5 articles LGTM.

https://qiita.com/api/v2/items/a9ead7285d10aadf5643/likes

You can get it by hitting this URL with GET.

Ordinary URLSession

import Foundation

let qiitaURL = URL(string: "https://qiita.com/api/v2/items/a9ead7285d10aadf5643/likes")!

struct LGTM: Codable {
    var created_at: String
    var user: User
}
struct User: Codable {
    var id: String
}

let session = URLSession.shared.dataTask(with: qiitaURL) {data, response, error in
    if let error = error {
        fatalError("Error: \(error.localizedDescription)")
    }
    guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
        fatalError("Error: invalid HTTP response code")
    }
    guard let data = data else {
        fatalError("Error: missing response data")
    }

    do {
        let lgtms = try JSONDecoder().decode([LGTM].self, from: data)
        lgtms.forEach{ lgtm in
            print("username: \(lgtm.user.id), created_at \(lgtm.created_at)")
        }
    }
    catch {
        print("Error: \(error.localizedDescription)")
    }
}
session.resume()

The type of let session is ʻURLSessionDataTask. ʻCommunicate by calling resume of URLSessionDataTask.

The definition of the URLSession method dataTask is as follows.

open func dataTask(with url: URL, completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask

URL Session with Combine

The description in Combine is as follows.

import Foundation
import Combine

var cancellable = URLSession.shared
    .dataTaskPublisher(for: qiitaURL)
    .tryMap { element -> Data in
        guard let httpResponse = element.response as? HTTPURLResponse, httpResponse.statusCode == 200 else {
            throw URLError(.badServerResponse)
        }
        return element.data
    }
    .decode(type: [LGTM].self, decoder: JSONDecoder())
    .sink(receiveCompletion: {
        print("complete! \($0)")
    }, receiveValue: { lgtms in
        lgtms.forEach{ lgtm in
            print("username: \(lgtm.user.id), created_at \(lgtm.created_at)")
        }
    })

The processing has changed a little, but I feel that Combine has less processing in the completionHandler and has a better outlook.

Tips

Specify a name that you do not know whether it is JsonDecoder or not.

If you specify something that is not in the Json field name, ʻError: The data could n’t be read because it is missing.` and JsonDecoder gives an error.

For example, if you define a struct as follows.

struct User: Codable {
    var id: String
    var aaa: String
}

In such a case, it is possible to prevent an error by entering nil by setting the field name that may not exist to Optional.

struct User: Codable {
    var id: String
    var aaa: String?
}

What is URLSession.shared?

URLSession.shared is a singleton to use when you want to write quickly. Since it works with the default behavior, advanced processing cannot be performed.

• Sequential data acquisition from the server is not possible
• Cannot be greatly customized
• There are restrictions on authentication
• Cannot download or upload when the app is not running

shared | Apple Developer Documentation

What is element-> Data?

It just specifies the closure arguments. It works even if there is no -> Data part.

Closures — The Swift Programming Language (Swift 5.3)

I want to process the elements of the array one by one

In Combine, the URLSession subscribe process is currently nested by forEach.

.decode(type: [LGTM].self, decoder: JSONDecoder())
.sink(receiveValue: { lgtms in
        lgtms.forEach{ lgtm in
            print("username: \(lgtm.user.id), created_at \(lgtm.created_at)")
        }
    }

You can use flatMap to get rid of this nesting.

.decode(type: [LGTM].self, decoder: JSONDecoder())
.flatMap {$0.publisher}
.sink(receiveCompletion: {
    print("complete! \($0)")
}, receiveValue: { lgtm in
    print("username: \(lgtm.user.id), created_at \(lgtm.created_at)")
})

the end