Approach 1: Swift 4's Decodable when you don't know the type you're decoding

1.swift

import Foundation

protocol KeyedStruct {
    var type_: String { get set }
}

struct Inputs : Codable {
    struct KeyStruct : KeyedStruct, Codable {
        var type_: String
    }
}

extension Inputs {
    struct FooInput : KeyedStruct, Codable {
        var type_: String = "FooInput"
        var foo: Int
        var floaty: Float
    }
    
    struct BarInput : KeyedStruct, Codable {
        var type_: String = "BarInput"
        var bar: Int
        var floaty: Float
    }
}

class Foo {
    var input = Inputs.FooInput(type_: "FooInput", foo: 3, floaty: 3.14)
}

class Bar {
    var input = Inputs.BarInput(type_: "BarInput", bar: 5, floaty: 5.58)
}

let fooinput = Foo.init().input
let data = (try? JSONEncoder().encode(fooinput))!
let jsonStr = String.init(data: data, encoding: .utf8)!
print(jsonStr)

let keyed = (try? JSONDecoder().decode(Inputs.KeyStruct.self, from: data))!
print(keyed.type_)

if keyed.type_ == "FooInput" {
    let foo2 = (try? JSONDecoder().decode(Inputs.FooInput.self, from: data))!
    print(foo2)
}