結合從模型到視圖模型的轉換會出現錯誤“在運行.eraseToAnyPublisher()時，運算式型別不明確，沒有更多背景關系

我試圖弄清楚為什么最后一個 .eraseToAnyPublisher() 給出了上述錯誤，對我來說似乎所有型別都指定得很好，不是嗎？

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 { $0.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
        }

uj5u.com熱心網友回復：

不幸的是，對于那些復雜的Combine管道，有時編譯器錯誤會顯示在錯誤的行上。在您的情況下，有兩個問題，但不是編譯器指向的問題。

一種是使用 offlatMap而不是map。

這部分管道：

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 { $0.items }

回傳Publisher<[User], Error>。

接下來，您要將其轉換Publisher<[UserViewModel], Error>為您需要的功能map：

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

它將一種型別的Output轉換為另一種型別的Output非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

它轉變Output為一個新的Publisher

第二個問題是append(contentsOf;)which 需要 aSequence的元素，如果你應該使用單個元素append()，但更簡單的是將其映射[User]到[UserViewModel]：

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

所以整個功能應該與這些變化一起作業：

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 { $0.items }
        .map { users in
            users.map { user in
                UserViewModel(with: user)
            }
        }
        .catch { err -> Just<[UserViewModel]> in
            print(err)
            return Just([])
        }
        .eraseToAnyPublisher()
    return publisher
}

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