Fix nested choice array #50
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## Introduction In merging in CoreOffice#119, we fixed most but not quite all of CoreOffice#91! Decoding of _null_ choice elements (represented as enums with empty struct associated values on the Swift side) still results in errors. This PR adds a test to demonstrate and fixes this issue by wrapping each `NullBox` inside of a `SingleKeyedBox` at the `merge` phase (called by `transformToBoxTree`). ## Motivation One of the main lessons from CoreOffice#119 was that we have to wrap choice elements in the decoding phase to hold onto their keys. The keys are needed for directing us to the correct branch of the do-catch pyramid used for decoding. ```swift private enum Entry: Equatable { case run(Run) case properties(Properties) case br(Break) } extension Entry: Decodable { private enum CodingKeys: String, XMLChoiceCodingKey { case run, properties, br } public init(from decoder: Decoder) throws { let container = try decoder.container(keyedBy: CodingKeys.self) do { self = .run(try container.decode(Run.self, forKey: .run)) } catch { do { self = .properties(try container.decode(Properties.self, forKey: .properties)) } catch { self = .br(try container.decode(Break.self, forKey: .br)) } } } } ``` where one of the associated values could be an empty struct (represented by null): ```swift private struct Break: Decodable {} ``` Although we _can_ throw out keys for non-choice null elements, a mechanism is needed for holding onto the keys while transforming from the `XMLCoderElement` tree to the `boxTree`. Only later will we know if the key is needed (if this wrapped element is transformed to a `ChoiceBox`); if not, we will be able to throw out the key. ## Proposed solution The Public API is unchanged. On the implementation side, we catch `NullBox` values in `merge` and wrap them in `SingleKeyedBox` instances. ## Detailed Design In `merge`, we wrap each `NullBox` in a `SingleKeyedBox` with the appropriate key bundled in. An `XMLChoiceDecodingContainer` can be constructed from the `SingleKeyedBox` by converting it to a `ChoiceBox` (just transferring over the contents) - as normal. In `XMLKeyedDecodingContainer`, when preparing the `elements` for concrete decoding, we unwrap all `SingleKeyedBox` values that may be contained therein, as any choice elements contained would have already been transformed to a `ChoiceBox` by this point in decoding: any stray `SingleKeyedBox` wrappers can thus be thrown out. ## Source compatibility This is purely an additive change.
* Add Xcode 11, Swift 5.1 to azure-pipelines.yml * Add FoundationXML import to XMLStackParser * Clean up xcodebuild.sh, use iPhone 8 for iOS 13 * Check for the Linux compiler version * Fix old compiler versions not handling comparisons
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It looks like #51 took this all along with it. |
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See CoreOffice/pull/126