MBL-1016: Add custom operators for common API patterns #1900
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,48 @@ | ||
| import Combine | ||
| import Foundation | ||
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| extension Publisher { | ||
| /// A convenience method for mapping the results of your fetch to another data type. Any unknown errors are returned in the error as `ErrorEnvelope.couldNotParseJSON`. | ||
| func mapFetchResults<NewOutputType>(_ convertData: @escaping ((Output) -> NewOutputType?)) | ||
| -> AnyPublisher<NewOutputType, ErrorEnvelope> { | ||
| return self.tryMap { (data: Output) -> NewOutputType in | ||
| guard let envelope = convertData(data) else { | ||
| throw ErrorEnvelope.couldNotParseJSON | ||
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There was a problem hiding this comment. We could consider a different generic error here, if we wished, but I think this is fine. |
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| } | ||
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| return envelope | ||
| } | ||
| .mapError { rawError in | ||
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| if let error = rawError as? ErrorEnvelope { | ||
| return error | ||
| } | ||
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| return ErrorEnvelope.couldNotParseJSON | ||
| } | ||
| .eraseToAnyPublisher() | ||
| } | ||
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| /// A convenience method for gracefully catching API failures. | ||
| /// If you handle your API failure in receiveCompletion:, that will actually cancel the entire pipeline, which means the failed request can't be retried. | ||
| /// This is a wrapper around the .catch operator, which just makes it a bit easier to read. | ||
| /// | ||
| /// An example: | ||
| /// ``` | ||
| /// self.somethingHappened | ||
| /// .flatMap() { _ in | ||
| /// self.doAnAPIRequest | ||
| /// .handleFailureAndAllowRetry() { e in | ||
| /// showTheError(e) | ||
| /// } | ||
| /// } | ||
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| public func handleFailureAndAllowRetry(_ onFailure: @escaping (Self.Failure) -> Void) | ||
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There was a problem hiding this comment. I had an "interesting" learning experience writing this operator. I spent about two hours trying to wrangle the types (with generics) for this function, and got completely stuck. I ended up changing things around a bit so that the types were simpler, which worked - but I think it's worth noting that any custom operators that handle complex type transformations (say, with a |
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| -> AnyPublisher<Self.Output, Never> { | ||
| return self.catch { e in | ||
| onFailure(e) | ||
| return Empty<Self.Output, Never>() | ||
| } | ||
| .eraseToAnyPublisher() | ||
| } | ||
| } | ||
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Did this a while ago but forgot the
//TODO.