Swift - passing types dynamically to JSONDecoder - json

Question

I am trying to decode from json objects with generic nested object, and for this I want to pass the type of class dynamically when decoding.
For example, my classes are EContactModel and ENotificationModel which extend ObjectModel (and :Codable)s. ENotificationModel can contain a nested ObjectModel (which can be a contact, notification or other objectmodel).
I have a dictionary of types like this:
static let OBJECT_STRING_CLASS_MAP = [
"EContactModel" : EContactModel.self,
"ENotificationModel" : ENotificationModel.self
...
]
My decoding init method in ENotificationModel looks like this:
required init(from decoder: Decoder) throws
{
try super.init(from: decoder)
let values = try decoder.container(keyedBy: CodingKeys.self)
...
//decode some fields here
self.message = try values.decodeIfPresent(String.self, forKey: .message)
...
//decode field "masterObject" of generic type ObjectModel
let cls = ObjectModelTypes.OBJECT_STRING_CLASS_MAP[classNameString]!
let t = type(of: cls)
print(cls) //this prints "EContactModel"
self.masterObject = try values.decodeIfPresent(cls, forKey: .masterObject)
print(t) //prints ObjectModel.Type
print(type(of: self.masterObject!)) //prints ObjectModel
}
I also tried passing type(of: anObjectInstanceFromADictionary) and still not working, but if I pass type(of: EContactModel()) it works. I cannot understand this, because both objects are the same (ie. instance of EContactModel)
Is there a solution for this?

Answer 1

You could declare your object models with optional variables and let JSONDecoder figure it out for you.
class ApiModelImage: Decodable {
let file: String
let thumbnail_file: String
...
}
class ApiModelVideo: Decodable {
let thumbnail: URL
let duration: String?
let youtube_id: String
let youtube_url: URL
...
}
class ApiModelMessage: Decodable {
let title: String
let body: String
let image: ApiModelImage?
let video: ApiModelVideo?
...
}
Then all you have to do is....
if let message = try? JSONDecoder().decode(ApiModelMessage.self, from: data) {
if let image = message.image {
print("yay, my message contains an image!")
}
if let video = message.video {
print("yay, my message contains a video!")
}
}
Alternatively, you could use generics and specify the type when calling your API code:
func get<T: Decodable>(from endpoint: String, onError: #escaping(_: Error?) -> Void, onSuccess: #escaping (_: T) -> Void) {
getData(from: endpoint, onError: onError) { (data) in
do {
let response = try JSONDecoder().decode(T.self, from: data)
onSuccess(response)
} catch {
onError(error)
}
}
}
Used this way, you just have to make sure you define your expected response type:
let successCb = { (_ response: GetUnreadCountsResponse) in
...
}
ApiRequest().get(from: endpoint, onError: { (_) in
...
}, onSuccess: successCb)
Since you define successCb as requiring a GetUnreadCountsResponse model, the API get method generic will be of type GetUnreadCountsResponse at runtime.
Good Luck!