合并从模型到 viewModel 的转换会给出错误“当运行 .eraseToAnyPublisher() 时表达式类型不明确且没有更多上下文

Question

我想弄清楚为什么最后一个 .eraseToAnyPublisher() 会出现上述错误，对我来说似乎所有类型都已明确指定，不是吗？

static func searchUsers(query: String) -> AnyPublisher<[UserViewModel], Never> {
            // prepare URL
            let urlString = "\(baseURL)/search/users?q=\(query)"
            guard let url = URL(string: urlString) else  {
                return Just([]).eraseToAnyPublisher()
            }
            // get handle of native data task publisher
            let publisher = URLSession.shared.dataTaskPublisher(for: url)
                .handleEvents(
                    receiveSubscription: { _ in
                        activityIndicatorPublisher.send(true)
                    }, receiveCompletion: { _ in
                        activityIndicatorPublisher.send(false)
                    }, receiveCancel: {
                        activityIndicatorPublisher.send(false)
                    })
                .tryMap { data, response -> Data in
                    guard let httpResponse = response as? HTTPURLResponse,
                          httpResponse.statusCode == 200 else {
                              throw NetworkError.httpError
                          }
                    print(String(data: data, encoding: .utf8) ?? "")
                    return data
                }
                .decode(type: SearchUserResponse.self, decoder: JSONDecoder())
                .map { [=11=].items }
                .flatMap({ users in
                    var userViewModels = [UserViewModel]()
                    users.forEach { user in
                        userViewModels.append(contentsOf: UserViewModel(with: user))
                    }
                    return userViewModels
                })
                .catch { err -> Just<[UserViewModel]> in
                    print(err)
                    return Just([])
                }
                .eraseToAnyPublisher() // <-- HERE IS THE ERROR
            return publisher
        }

Answer 1

不幸的是，对于那些复杂的 Combine 管道，有时编译器错误会显示在错误的行上。在你的情况下有两个问题，但不是编译器指向的地方。

一个是使用 flatMap 而不是 map。

管道的这一部分：

let publisher = URLSession.shared.dataTaskPublisher(for: url)
    .handleEvents(
        receiveSubscription: { _ in
            activityIndicatorPublisher.send(true)
        }, receiveCompletion: { _ in
            activityIndicatorPublisher.send(false)
        }, receiveCancel: {
            activityIndicatorPublisher.send(false)
        })
        .tryMap { data, response -> Data in
             guard let httpResponse = response as? HTTPURLResponse,
                 httpResponse.statusCode == 200 else {
                 throw NetworkError.httpError
             }
             print(String(data: data, encoding: .utf8) ?? "")
             return data
        }
        .decode(type: SearchUserResponse.self, decoder: JSONDecoder())
        .map { [=10=].items }

returns Publisher<[User], Error>.

接下来您要将其转换为 Publisher<[UserViewModel], Error>，为此您需要一个函数 map:

func map<T>(_ transform: @escaping (Output) -> T) -> Publishers.Map<Upstream, T>

将一种类型的 Output 转换为另一种类型的 Output 而不是 flatMap:

func flatMap<T, P>(maxPublishers: Subscribers.Demand = .unlimited, _ transform: @escaping (Self.Output) -> P) -> Publishers.FlatMap<P, Self> where T == P.Output, P : Publisher, Self.Failure == P.Failure

将 Output 转换为新的 Publisher

第二个问题是 append(contentsOf;)，它需要 Sequence 个元素，如果是单个元素，您应该使用 append()，但更简单的方法是映射 [User] 到 [UserViewModel]:

{ users in
    users.map { user in
        UserViewModel(with: user)
    }
}

所以整个功能应该适用于这些更改：

static func searchUsers(query: String) -> AnyPublisher<[UserViewModel], Never> {
    // prepare URL
    let urlString = "\(baseURL)/search/users?q=\(query)"
    guard let url = URL(string: urlString) else  {
        return Just([]).eraseToAnyPublisher()
    }
    // get handle of native data task publisher
    let publisher = URLSession.shared.dataTaskPublisher(for: url)
        .handleEvents(
            receiveSubscription: { _ in
                activityIndicatorPublisher.send(true)
            }, receiveCompletion: { _ in
                activityIndicatorPublisher.send(false)
            }, receiveCancel: {
                activityIndicatorPublisher.send(false)
            })
        .tryMap { data, response -> Data in
            guard let httpResponse = response as? HTTPURLResponse,
                  httpResponse.statusCode == 200 else {
                      throw NetworkError.httpError
                  }
            print(String(data: data, encoding: .utf8) ?? "")
            return data
        }
        .decode(type: SearchUserResponse.self, decoder: JSONDecoder())
        .map { [=14=].items }
        .map { users in
            users.map { user in
                UserViewModel(with: user)
            }
        }
        .catch { err -> Just<[UserViewModel]> in
            print(err)
            return Just([])
        }
        .eraseToAnyPublisher()
    return publisher
}

合并从模型到 viewModel 的转换会给出错误“当运行 .eraseToAnyPublisher() 时表达式类型不明确且没有更多上下文

Combine conversion from model to viewModel gives error "Type of expression is ambiguous without more context when running .eraseToAnyPublisher()

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合并从模型到 viewModel 的转换会给出错误“当 运行 .eraseToAnyPublisher() 时表达式类型不明确且没有更多上下文

Combine conversion from model to viewModel gives error "Type of expression is ambiguous without more context when running .eraseToAnyPublisher()

ios

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合并从模型到 viewModel 的转换会给出错误“当运行 .eraseToAnyPublisher() 时表达式类型不明确且没有更多上下文