Expandable collection support.

[Andrewangeta]

Fixes #117

[gwynne]

Define this extension somewhere (in this file if you like), or elsewhere:

internal extension Decoder {
    func singleValueContainerIfPresentAndNotNull() throws -> SingleValueDecodingContainer? {
        do {
            let container = try self.singleValueContainer()
            
            if container.decodeNil() {
                return nil
            }
            return container
        }
        catch DecodingError.keyNotFound(_, let context)
            where context.codingPath.count == self.codingPath.count
        {
            return nil
        }
    }
}

(n.b.: I tinkered with the code formatting for the catch part about a dozen times, I just can't figure out something that feels idiomatic for that 😅)

Then you can do this and avoid some of the convoluted boilerplate:

Suggested change

	do {
	let container = try decoder.singleValueContainer()
	do {
	if container.decodeNil() {
	_state = .empty
	} else {
	_state = .unexpanded(try container.decode([String].self))
	}
	} catch DecodingError.typeMismatch(let type, _) where type is String.Type {
	_state = .expanded(try container.decode([Model].self))
	}
	} catch DecodingError.keyNotFound(_, let context) where context.codingPath.count == codingPath.count {
	_state = .empty
	}
	if let container = try decoder.singleValueContainerIfPresentAndNotNull() {
	do {
	self._state = .unexpanded(try container.decode([String].self))
	} catch DecodingError.typeMismatch(let type, _) where type is String.Type {
	self._state = .expanded(try container.decode([Model].self))
	}
	} else {
	self._state = .empty
	}

Obviously you can do the same in @Expandable as well.

(Disclaimer: There are no functional changes in this code versus your version; it's entirely a stylistic choice. I personally don't like nested do blocks, especially when the errors being explicitly caught are as complex as DecodingErrors usually are. And of course there's always something to appreciate in factoring out reused logic. Codable's API makes things hard enough to read already.)

[gwynne]

LGTM!

I keep wanting to write this rant about how you should make the ExpandableState enums Codable and put the switches in those decode/encode methods instead of on the property wrappers (which could then let the compiler synthesize their conformance), and I keep having to remind myself that it actually doesn't change anything whatsoever so why not keep the enums clean? 🤣 But yeah, this seems great, ship it! 💯

[gwynne]

Darn auto-dismiss 😆