CombineでURLSessionを連結するやつ

chain-publisher-in-combine.swift

import SwiftUI
import Combine

let decoder: JSONDecoder = {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    return decoder
}()

struct User: Decodable, Equatable, Hashable {
    let login: String
    let id: Int
    let avatarUrl: URL
}

enum UsersError: Error {
    case empty
}

struct UsersPublisher {
    let request: URLRequest = {
        var request = URLRequest(url: URL(string: "https://api.github.com/users")!)
        request.httpMethod = "GET"
        request.allHTTPHeaderFields = ["Accept": "application/vnd.github.v3+json"]
        return request
    }()

    func publish() -> AnyPublisher<[User], Error> {
        URLSession.shared
            .dataTaskPublisher(for: request)
            .map(\.data)
            .decode(type: [User].self, decoder: decoder)
            .eraseToAnyPublisher()
    }
}

enum ImageError: Error {
    case url(URLError)
    case other
}

struct ImagePublisher {
    let url: URL

    func publish() -> AnyPublisher<UIImage, Error> {
        URLSession.shared
            .dataTaskPublisher(for: url)
            .map { UIImage(data: $0.data)! }
            // 連結するときFailureの型が一致しないとダメっぽい
            // https://stackoverflow.com/questions/66713123/why-cant-i-use-flatmap-after-trymap-in-swift-combine/66715459#66715459
            .mapError { $0 as Error }
            .eraseToAnyPublisher()
    }
}

var cancellables: Set<AnyCancellable> = []

UsersPublisher().publish()
    .tryMap { users -> URL in
        guard let first = users.first else {
            throw UsersError.empty
        }
        return first.avatarUrl
    }
    .flatMap { url in
        ImagePublisher(url: url).publish()
    }
    .sink(receiveCompletion: { print($0) }, receiveValue: { print($0) })
    .store(in: &cancellables)