Combine中dataTaskPublisher(for:)无输出，如何分离网络层代码？

问题 #

使用Combine分离网络层代码，创建HTTPAsyncManager类处理网络请求时，发现dataTaskPublisher(for: url)未触发输出。将private var requests = Set<AnyCancellable>()移到ContentView中并声明为@State private var requests = Set<AnyCancellable>()后能正常输出数据，但希望保持网络层与ContentView分离，询问修改方案。

相关代码 #

HTTPAsyncManager 实现

import Combine
import SwiftUI


class HTTPAsyncManager {
    private var requests = Set<AnyCancellable>()
    
    func fetch(_ url: URL) {
        let decoder = JSONDecoder()
        
        URLSession.shared.dataTaskPublisher(for: url)
            .handleEvents(receiveSubscription: { print("Receive subscription: \($0)") },
                              receiveOutput: { print("Receive output: \($0)") },
                              receiveCompletion: { print("Receive completion: \($0)") },
                              receiveCancel: { print("Receive cancel") },
                              receiveRequest: { print("Receive request: \($0)") })
            .map(\.data)
            .decode(type: User.self, decoder: decoder)
            .replaceError(with: User.default)
            .sink(receiveValue: { print($0.name) })
            .store(in: &requests)
    }
}

视图与模型定义

import Combine
import SwiftUI

struct User: Decodable {
    var id: UUID
    var name: String

    static let `default` = User(id: UUID(), name: "Anonymous")
}

struct ContentView: View {
    
    var body: some View {
        VStack {
            Button("Fetch user") {
                let url = URL(string: "https://www.hackingwithswift.com/samples/user-24601.json")!
                HTTPAsyncManager().fetch(url)
            }
        }
        .padding()
    }
}

控制台输出 #

Receive subscription: DataTaskPublisher
Receive request: unlimited
Receive cancel

解决方案 #

问题根源在于：每次点击按钮时创建的HTTPAsyncManager是局部变量，方法执行完毕后该实例会被立即销毁，其内部的requests集合也随之释放，导致Combine订阅被取消，网络请求尚未完成就被终止。

只需让ContentView持有HTTPAsyncManager的实例，避免其被提前销毁即可，修改后的代码如下：

修改后的 ContentView

import Combine
import SwiftUI

struct ContentView: View {
    // 使用@StateObject持有HTTPAsyncManager实例，保证其生命周期与视图一致
    @StateObject private var httpManager = HTTPAsyncManager()
    
    var body: some View {
        VStack {
            Button("Fetch user") {
                let url = URL(string: "https://www.hackingwithswift.com/samples/user-24601.json")!
                httpManager.fetch(url)
            }
        }
        .padding()
    }
}

HTTPAsyncManager 保持不变

原HTTPAsyncManager的代码无需修改，现在实例被视图持有，requests集合不会被提前释放，订阅可以正常完成，网络请求的输出也能正常触发。

内容的提问来源于stack exchange，提问作者Zhou Haibo