pattern-datataskpublisher-decode.adoc

Making a network request with dataTaskPublisher

Goal

• One common use case is requesting JSON data from a URL and decoding it.

References

• URLSession.dataTaskPublisher

• map

• decode

• sink

• subscribe

See also

• Stricter request processing with dataTaskPublisher

• Using catch to handle errors in a one-shot pipeline

• Retrying in the event of a temporary failure

Code and explanation

This can be readily accomplished with Combine using URLSession.dataTaskPublisher followed by a series of operators that process the data. Minimally, dataTaskPublisher on URLSession uses map and decode before going to the subscriber.

The simplest case of using this might be:

let myURL = URL(string: "https://postman-echo.com/time/valid?timestamp=2016-10-10")
// checks the validity of a timestamp - this one returns {"valid":true}
// matching the data structure returned from https://postman-echo.com/time/valid
fileprivate struct PostmanEchoTimeStampCheckResponse: Decodable, Hashable { (1)
    let valid: Bool
}

let remoteDataPublisher = URLSession.shared.dataTaskPublisher(for: myURL!) (2)
    // the dataTaskPublisher output combination is (data: Data, response: URLResponse)
    .map { $0.data } (3)
    .decode(type: PostmanEchoTimeStampCheckResponse.self, decoder: JSONDecoder()) (4)

let cancellableSink = remoteDataPublisher
    .sink(receiveCompletion: { completion in
            print(".sink() received the completion", String(describing: completion))
            switch completion {
                case .finished: (5)
                    break
                case .failure(let anError): (6)
                    print("received error: ", anError)
            }
    }, receiveValue: { someValue in (7)
        print(".sink() received \(someValue)")
    })

1. Commonly you will have a struct defined that supports at least Decodable (if not the full Codable protocol). This struct can be defined to only pull the pieces you are interested in from the JSON provided over the network. The complete JSON payload does not need to be defined.

2. dataTaskPublisher is instantiated from URLSession. You can configure your own options on URLSession, or use a shared session.

3. The data that is returned is a tuple: (data: Data, response: URLResponse). The map operator is used to get the data and drops the URLResponse, returning just Data down the pipeline.

4. decode is used to load the data and attempt to parse it. Decode can throw an error itself if the decode fails. If it succeeds, the object passed down the pipeline will be the struct from the JSON data.

5. If the decoding completed without errors, the finished completion will be triggered and the value will be passed to the receiveValue closure.

6. If the a failure happens (either with the original network request or the decoding), the error will be passed into with the failure closure.

7. Only if the data succeeded with request and decoding will this closure get invoked, and the data format received will be an instance of the struct PostmanEchoTimeStampCheckResponse.

---