pattern-constrained-network.adoc

Requesting data from an alternate URL when the network is constrained

Goal

• From Apple’s WWDC 2019 presentation Advances in Networking, Part 1, a sample pattern was provided using tryCatch and tryMap operators to react to the specific error of the network being constrained.

References

• URLSession.dataTaskPublisher

• tryCatch

• tryMap

See also

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

• Retrying in the event of a temporary failure

Code and explanation

// Generalized Publisher for Adaptive URL Loading
func adaptiveLoader(regularURL: URL, lowDataURL: URL) -> AnyPublisher<Data, Error> {
    var request = URLRequest(url: regularURL) (1)
    request.allowsConstrainedNetworkAccess = false (2)
    return URLSession.shared.dataTaskPublisher(for: request) (3)
        .tryCatch { error -> URLSession.DataTaskPublisher in (4)
            guard error.networkUnavailableReason == .constrained else {
               throw error
            }
            return URLSession.shared.dataTaskPublisher(for: lowDataURL) (5)
        .tryMap { data, response -> Data in
            guard let httpResponse = response as? HTTPUrlResponse, (6)
                   httpResponse.statusCode == 200 else {
                       throw MyNetworkingError.invalidServerResponse
            }
            return data
}
.eraseToAnyPublisher() (7)

This example, from Apple’s WWDC, provides a function that takes two URLs - a primary and a fallback. It returns a publisher that will request data and fall back requesting a secondary URL when the network is constrained.

1. The request starts with an attempt requesting data.

2. Setting request.allowsConstrainedNetworkAccess will cause the dataTaskPublisher to error if the network is constrained.

3. Invoke the dataTaskPublisher to make the request.

4. tryCatch is used to capture the immediate error condition and check for a specific error (the constrained network).

5. If it finds an error, it creates a new one-shot publisher with the fall-back URL.

6. The resulting publisher can still fail, and tryMap can map this a failure by throwing an error on HTTP response codes that map to error conditions

7. eraseToAnyPublisher enables type erasure on the chain of operators so the resulting signature of the adaptiveLoader function is AnyPublisher<Data, Error>

In the sample, if the error returned from the original request wasn’t an issue of the network being constrained, it passes on the .failure completion down the pipeline. If the error is that the network is constrained, then the tryCatch operator creates a new request to an alternate URL.

---