Swift 4 Using Generics as Return Value - swift

Question

I have a protocol like so:
protocol ModelProtocol{
func parse<T.Model>(data: Data) -> Array<T>? {}
}
The return is an array of option values. The method takes in data, parses it and returns the array of parsed objects from API data.
The type of data that the API returns is called MyData that has an array as the value of the dictionary.
I parse the JSON like so
func parse<T>(data: Data) -> Array<T>? {
do {
let newJSONDecoder = JSONDecoder()
let menu = try newJSONDecoder.decode(MyData.self, from:data)
let dataArray = menu.data //array
let modelArray = Array<T>()
for object in dataArray {
modelArray.append(object)
}
return modelArray
}
catch {
print("error while parsing:\(error.localizedDescription)")
return nil
}
}
I get error on the line where I append into the array to be returned.
Cannot invoke 'append' with an argument list of type '(MyData.Drinks)'
Ultimately I want the returned array to have objects of the type that is in the array MyData.data- in this case, the type is Drinks. But more broadly, I want the method to return any type that is in any JSON payload. The goal is to create a method that can take in any data and return any object as parsed object of type X in the array.
How do I do this?

Answer 1

First of all the code does not compile:
Protocol methods must not have bodies
so you have to remove the braces:
protocol ModelProtocol{
func parse<T : Decodable>(data: Data) -> Array<T>?
}
To solve your problem create MyData also as generic
struct MyData<T : Decodable> : Decodable {
let data : [T]?
}
and declare parse
func parse<T : Decodable>(data: Data) -> Array<T>? {
do {
let newJSONDecoder = JSONDecoder()
let menu = try newJSONDecoder.decode(MyData<T>.self, from:data)
return menu.data
}
catch {
print("error while parsing: ", error)
return nil
}
}
print always the entire error to get detailed information about the decoding error. localizedDescription is too broad.
If data is expected to be non-optional make parse throw and hand over the decoding error
protocol ModelProtocol{
func parse<T : Decodable>(data: Data) throws -> Array<T>
}
struct MyData<T : Decodable> : Decodable {
let data : [T]
}
func parse<T : Decodable>(data: Data) throws -> Array<T> {
let newJSONDecoder = JSONDecoder()
let menu = try newJSONDecoder.decode(MyData<T>.self, from:data)
return menu.data
}