无法将“Publishers.FlatMap<AnyPublisher<>, AnyPublisher<>>”类型的返回表达式转换为返回类型“AnyPublisher<>”答案

【问题标题】：Cannot convert return expression of type 'Publishers.FlatMap<AnyPublisher<>, AnyPublisher<>>' to return type 'AnyPublisher<>'无法将“Publishers.FlatMap<AnyPublisher<>, AnyPublisher<>>”类型的返回表达式转换为返回类型“AnyPublisher<>”
【发布时间】：2021-10-15 08:31:57
【问题描述】：

我很难完全理解Combine。这里有一个问题，我似乎无法返回正确的输出类型。

我该怎么做？

func test(ticketId: String) -> AnyPublisher<Void, Error> {

    campaignByTicketIdUseCase.execute(ticketId: ticketId)     // this is AnyPublisher<Campaign,Error>
    .flatMap { (campaign) -> AnyPublisher<Void, Error> in     // this is where the error is thrown

        guard let url = URL(string: "url"),
        validator.isParticipationValid(campaignIdentifier: campaign.identifier) else {
            return Result<Void, Error>.failure(HttpError()).publisher.eraseToAnyPublisher()
        }

        var request = URLRequest(url: url)
        request.httpMethod = "POST"

        return AuthSession.shared.doRequest(request: request)
        .tryMap({ (_: Data, response: URLResponse) -> Void in
            if let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode != 200 {
                throw HttpError()
            }
        }).eraseToAnyPublisher()
    }
}

【问题讨论】：

如果不出意外，您将返回campaignByTicketIdUseCase.execute(...).flatMap { ... }，并且似乎缺少.eraseToAnyPublisher()。

标签： swift reactive-programming combine

【解决方案1】：

就像 Rob 在他的评论中所说的那样，在 flatMap 操作之后，我错过了 .eraseToAnyPublisher()。

func test(ticketId: String) -> AnyPublisher<Void, Error> {

    campaignByTicketIdUseCase.execute(ticketId: ticketId)     // this is AnyPublisher<Campaign,Error>
    .flatMap { (campaign) -> AnyPublisher<Void, Error> in     // this is where the error is thrown

        guard let url = URL(string: "url"),
        validator.isParticipationValid(campaignIdentifier: campaign.identifier) else {
            return Result<Void, Error>.failure(HttpError()).publisher.eraseToAnyPublisher()
        }

        var request = URLRequest(url: url)
        request.httpMethod = "POST"

        return AuthSession.shared.doRequest(request: request)
        .tryMap({ (_: Data, response: URLResponse) -> Void in
            if let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode != 200 {
                throw HttpError()
            }
        }).eraseToAnyPublisher()
    }.eraseToAnyPublisher()       // <-- this was needed to solve the issue
}

【讨论】：