wip separate thing for encoding to object

simplifies parsing from object (you don't have to do the "is it an
object" check yourself) and also lets you get something that's
ARTJSONWhateverCompatible (which will be needed when we start passing as
`extras` to `publish`)

TODO move this test out to make it more general

TODO update protocol name in test?

Sources/AblyChat/JSONCodable.swift

@@ -7,3 +7,35 @@ internal protocol JSONDecodable {
 }
 
 internal typealias JSONCodable = JSONDecodable & JSONEncodable
+
+internal protocol JSONObjectEncodable: JSONEncodable {
+    var toJSONObject: [String: JSONValue] { get }
+}
+
+// Default implementation of `JSONEncodable` conformance for `JSONObjectEncodable`
+internal extension JSONObjectEncodable {
+    var toJSONValue: JSONValue {
+        .object(toJSONObject)
+    }
+}
+
+internal protocol JSONObjectDecodable: JSONDecodable {
+    init(jsonObject: [String: JSONValue]) throws
+}
+
+internal enum JSONValueDecodingError: Error {
+    case valueIsNotObject
+}
+
+// Default implementation of `JSONDecodable` conformance for `JSONObjectDecodable`
+internal extension JSONObjectDecodable {
+    init(jsonValue: JSONValue) throws {
+        guard case let .object(jsonObject) = jsonValue else {
+            throw JSONValueDecodingError.valueIsNotObject
+        }
+
+        self = try .init(jsonObject: jsonObject)
+    }
+}
+
+internal typealias JSONObjectCodable = JSONObjectDecodable & JSONObjectEncodable

Sources/AblyChat/JSONValue.swift

@@ -169,7 +169,7 @@ internal extension JSONValue {
     var toAblyCocoaData: Any {
         switch self {
         case let .object(underlying):
-            underlying.mapValues(\.toAblyCocoaData)
+            underlying.toAblyCocoaDataDictionary
         case let .array(underlying):
             underlying.map(\.toAblyCocoaData)
         case let .string(underlying):
@@ -183,3 +183,15 @@ internal extension JSONValue {
         }
     }
 }
+
+internal extension [String: JSONValue] {
+    /// Creates an ably-cocoa deserialized JSON object from a dictionary that has string keys and `JSONValue` values.
+    ///
+    /// Specifically, the value of this property can be used as:
+    ///
+    /// - `ARTPresenceMessage`’s `data` property
+    /// - the `data` argument that’s passed to `ARTRealtime`’s `request(…)` method
+    var toAblyCocoaDataDictionary: [String: Any] {
+        mapValues(\.toAblyCocoaData)
+    }
+}

Sources/AblyChat/PresenceDataDTO.swift

@@ -3,32 +3,24 @@ internal struct PresenceDataDTO: Equatable {
     internal var userCustomData: PresenceData?
 }
 
-// MARK: - JSONCodable
+// MARK: - JSONObjectCodable
 
-extension PresenceDataDTO: JSONCodable {
+extension PresenceDataDTO: JSONObjectCodable {
     internal enum JSONKey: String {
         case userCustomData
     }
 
-    internal enum DecodingError: Error {
-        case topLevelValueHasWrongType
-    }
-
-    internal init(jsonValue: JSONValue) throws {
-        guard case let .object(jsonObject) = jsonValue else {
-            throw DecodingError.topLevelValueHasWrongType
-        }
-
+    internal init(jsonObject: [String: JSONValue]) throws {
         userCustomData = jsonObject[JSONKey.userCustomData.rawValue]
     }
 
-    internal var toJSONValue: JSONValue {
+    internal var toJSONObject: [String: JSONValue] {
         var result: [String: JSONValue] = [:]
 
         if let userCustomData {
             result[JSONKey.userCustomData.rawValue] = userCustomData
         }
 
-        return .object(result)
+        return result
     }
 }

Tests/AblyChatTests/PresenceDataDTOTests.swift

@@ -18,7 +18,7 @@ struct PresenceDataDTOTests {
 
     @Test
     func initWithJSONValue_failsIfNotObject() {
-        #expect(throws: PresenceDataDTO.DecodingError.self) {
+        #expect(throws: JSONValueDecodingError.self) {
             try PresenceDataDTO(jsonValue: "hello")
         }
     }