Only send error event if not from cancelling the task

[eliotfowler]

Closes #20.

Upon cancelling a Kingfisher task, the Kingfisher library calls the completion block twice:

• once for the request timing out
• once for the task being cancelled

This causes issues in Rx because it detects a re-entrancy anomaly. I'm not actually sure if it causes a real issue given that the task is being cancelled from the stream being disposed of anyway, but it prints a scary message to the console and it's best to clean things up.

Important note: I don't have enough familiarity with the Kingfisher library to be confident that there's no time they would only send the task cancelled error and not from us explicitly calling task.cancel(). If that were ever to happen, we would want to re-think this solution.

[cddjr]

My solution

public func retrieveImage(with source: Source,
                              options: KingfisherOptionsInfo? = nil) -> Single<Image> {
        return Single.create { [base] single in
            var task: DownloadTask?
            let cancelTask = Disposables.create { task?.cancel() }
            
            task = base.retrieveImage(with: source,
                                          options: options) { result in
                guard !cancelTask.isDisposed else {
                    return
                }
                switch result {
                case .success(let value):
                    single(.success(value.image))
                case .failure(let error):
                    single(.error(error))
                }
            }

            return cancelTask
        }
    }

[freak4pc]

Apparently changing the primary branch name closes all PRs. Do you mind opening a new PR with this fix? Thanks!

[freak4pc]

I think you can just do case .failure(let error) where !error.isTaskCancelledError