When ContainerWrapper asks decoder to decode a [String: Int], that goes through a special-case code path. It calls this overload of decode:
public func decode<T : Decodable>(_ type: T.Type) throws -> T {
try expectNonNull(type)
return try self.unbox(self.storage.topContainer, as: type)!
}
which calls this overload of unbox:
func unbox<T : Decodable>(_ value: Any, as type: T.Type) throws -> T? {
return try unbox_(value, as: type) as? T
}
which calls unbox_:
func unbox_(_ value: Any, as type: Decodable.Type) throws -> Any? {
if type == Date.self || type == NSDate.self {
return try self.unbox(value, as: Date.self)
} else if type == Data.self || type == NSData.self {
return try self.unbox(value, as: Data.self)
} else if type == URL.self || type == NSURL.self {
guard let urlString = try self.unbox(value, as: String.self) else {
return nil
}
guard let url = URL(string: urlString) else {
throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: self.codingPath,
debugDescription: "Invalid URL string."))
}
return url
} else if type == Decimal.self || type == NSDecimalNumber.self {
return try self.unbox(value, as: Decimal.self)
} else if let stringKeyedDictType = type as? _JSONStringDictionaryDecodableMarker.Type {
return try self.unbox(value, as: stringKeyedDictType)
} else {
self.storage.push(container: value)
defer { self.storage.popContainer() }
return try type.init(from: self)
}
}
which detects the conformance of [String: Int] to _JSONStringDictionaryDecodableMarker and so calls this overload of unbox:
func unbox<T>(_ value: Any, as type: _JSONStringDictionaryDecodableMarker.Type) throws -> T? {
guard !(value is NSNull) else { return nil }
var result = [String : Any]()
guard let dict = value as? NSDictionary else {
throw DecodingError._typeMismatch(at: self.codingPath, expectation: type, reality: value)
}
let elementType = type.elementType
for (key, value) in dict {
let key = key as! String
self.codingPath.append(_JSONKey(stringValue: key, intValue: nil))
defer { self.codingPath.removeLast() }
result[key] = try unbox_(value, as: elementType)
}
return result as? T
}
which iterates over the keys of the internal representation (an NSDictionary) without performing snake-case conversion.
When DirectWrapper calls the init(from: Decoder) method of [String: Int], it's calling this initializer:
public init(from decoder: Decoder) throws {
self.init()
if Key.self == String.self {
// The keys are Strings, so we should be able to expect a keyed container.
let container = try decoder.container(keyedBy: _DictionaryCodingKey.self)
for key in container.allKeys {
let value = try container.decode(Value.self, forKey: key)
self[key.stringValue as! Key] = value
}
} else if Key.self == Int.self {
... many more lines omitted ...
which (because Key is String) asks the decoder for a KeyedDecodingContainer by calling this method:
public func container<Key>(keyedBy type: Key.Type) throws -> KeyedDecodingContainer<Key> {
guard !(self.storage.topContainer is NSNull) else {
throw DecodingError.valueNotFound(KeyedDecodingContainer<Key>.self,
DecodingError.Context(codingPath: self.codingPath,
debugDescription: "Cannot get keyed decoding container -- found null value instead."))
}
guard let topContainer = self.storage.topContainer as? [String : Any] else {
throw DecodingError._typeMismatch(at: self.codingPath, expectation: [String : Any].self, reality: self.storage.topContainer)
}
let container = _JSONKeyedDecodingContainer<Key>(referencing: self, wrapping: topContainer)
return KeyedDecodingContainer(container)
}
which calls the init of _JSONKeyedDecodingContainer:
init(referencing decoder: __JSONDecoder, wrapping container: [String : Any]) {
self.decoder = decoder
switch decoder.options.keyDecodingStrategy {
case .useDefaultKeys:
self.container = container
case .convertFromSnakeCase:
// Convert the snake case keys in the container to camel case.
// If we hit a duplicate key after conversion, then we'll use the first one we saw. Effectively an undefined behavior with JSON dictionaries.
self.container = Dictionary(container.map {
key, value in (JSONDecoder.KeyDecodingStrategy._convertFromSnakeCase(key), value)
}, uniquingKeysWith: { (first, _) in first })
case .custom(let converter):
self.container = Dictionary(container.map {
key, value in (converter(decoder.codingPath + [_JSONKey(stringValue: key, intValue: nil)]).stringValue, value)
}, uniquingKeysWith: { (first, _) in first })
}
self.codingPath = decoder.codingPath
}
which performs snake-case conversion. This is the only way to get snake-case conversion while decoding.
Since ContainerWrapper doesn't end up going through a keyed container, it doesn't get snake-case conversion.
