如果您在Playgroud中尝试以下代码：

import Combine
import Foundation

struct User {
    let name: String
}

private var subscriptions = Set<AnyCancellable>()
var didAlreadyImportUsers = false
var users = [User]()

func importUsers() -> Future<Bool,Never> {
    Future { promise in
        DispatchQueue.global(qos: .userInitiated).async {
            sleep(5)
            users = [User(name: "John"),User(name: "Jack")]
            promise(.success(true))
        }
    }
}

func getUsers(age: Int? = nil) ->Future<[User],Error> {
    Future { promise in
        promise(.success(users))
    }
}

var usersPublisher: AnyPublisher<[User],Error> {
    if didAlreadyImportUsers {
        return getUsers().eraseToAnyPublisher()
    } else {
        return importUsers()
            .setfailureType(to: Error.self)
            .combineLatest(getUsers())
            .map { $0.1 }
            .eraseToAnyPublisher()
    }
}

usersPublisher
    .sink(receiveCompletion: { completion in
    print(completion)
},receiveValue: { value in
    print(value)
}).store(in: &subscriptions)

它将打印：

[]
finished

但我希望：

[User(name: "John"),User(name: "Jack")]
finished

如果我删除带有sleep(5)的行，则它将正确打印结果。似乎是异步问题。似乎.combineLatest(getUsers())不在等待importUsers()我以为combineLatest正在照顾那个？我在这里想念什么？

（在我的真实代码中，Core Data操作长期运行，而不是sleep）