如果首次解码失败,则使用Combine和Swift解码另一个响应 [英] Decode another response if first decoding failed using Combine and Swift
问题描述
我有以下模型:
struct Response: Decodable {
let message: String
}
struct ErrorResponse: Decodable {
let errorMessage: String
}
enum APIError: Error {
case network(code: Int, description: String)
case decoding(description: String)
case api(description: String)
}
我正在尝试使用以下流程获取网址并解析JSON响应:
I'm trying to fetch an url and parse the JSON response using this flow:
func fetch(url: URL) -> AnyPublisher<Response, APIError> {
URLSession.shared.dataTaskPublisher(for: URLRequest(url: url))
// #1 URLRequest fails, throw APIError.network
.mapError { .network(code: $0.code.rawValue, description: $0.localizedDescription) }
// #2 try to decode data as a `Response`
.tryMap { JSONDecoder().decode(Response.self, from: $0.data) }
// #3 if decoding fails, decode as an `ErrorResponse`
// and throw `APIError.api(description: errorResponse.errorMessage)`
// #4 if both fail, throw APIError.decoding
// #5 return
.eraseToAnyPublisher()
}
我对#3
有问题:如何在 tryMap
部分之后解码原始数据?
I have a problem with #3
: how can I decode the original data after the tryMap
part?
似乎唯一可以访问的值是来自 tryMap
的错误,但是我需要原始数据来解码 ErrorRepsonse
.
It seems like the only value I can access there is the error coming from tryMap
but I need the original data to decode an ErrorRepsonse
.
注意:不幸的是,错误响应带有200状态,区分它们的唯一方法是对它们进行解码.
Note: unfortunately the error response comes with the 200 status and the only way to differentiate them is to decode them.
推荐答案
您可以使用 flatMap
并对其内部进行解码:
You can use a flatMap
and deal with decoding inside it:
URLSession.shared.dataTaskPublisher(for: URLRequest(url: url))
// #1 URLRequest fails, throw APIError.network
.mapError {
APIError.network(code: $0.code.rawValue, description: $0.localizedDescription)
}
.flatMap { data -> AnyPublisher<Response, APIError> in
// #2 try to decode data as a `Response`
if let response = try? JSONDecoder().decode(Response.self, from: data) {
return Just(response).setFailureType(to: APIError.self)
.eraseToAnyPublisher()
}
do {
// #3 if decoding fails, decode as an `ErrorResponse`
let error = try decoder.decode(ErrorResponse.self, from: data)
// and throw `APIError.api(description: errorResponse.errorMessage)`
return Fail(error: APIError.api(description: errorResponse.errorMessage))
.eraseToAnyPublisher()
} catch {
// #4 if both fail, throw APIError.decoding
return Fail(error: APIError.decoding(description: error.localizedDescription))
.eraseToAnyPublisher()
}
}
编辑
如果您想以纯"方式执行此操作,结合方式,那么您仍然想要使用 flatMap
来访问原始数据并避开原始可能的网络错误,然后使用 tryCatch
来处理失败路径.
If you want to do this in a "pure" Combine way, then you'd still want to use a flatMap
to have access the original data and to sidestep the original possible network error, and then use tryCatch
to deal with the failure path.
请注意,步骤4位于步骤3的两个部分之间:
Note that step #4 comes between two parts of step #3:
URLSession.shared.dataTaskPublisher(for: URLRequest(url: url))
// #1 URLRequest fails, throw APIError.network
.mapError {
APIError.network(code: $0.code.rawValue, description: $0.localizedDescription)
}
.flatMap { v in
Just(v)
// #2 try to decode data as a `Response`
.decode(type: Response.self, decoder: JSONDecoder())
// #3 if decoding fails,
.tryCatch { _ in
Just(v)
// #3.1 ... decode as an `ErrorResponse`
.decode(type: ErrorResponse.self, decoder: JSONDecoder())
// #4 if both fail, throw APIError.decoding
.mapError { _ in APIError.decoding(description: "error decoding") }
// #3.2 ... and throw `APIError.api
.tryMap { throw APIError.api(description: $0.errorMessage) }
}
// force unwrap is not terrible here, since you know
// that `tryCatch` only ever throws APIError
.mapError { $0 as! APIError }
}
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