Merging inner object fields during encoding

merge_json_objects.swift

public struct Object: Codable {
    let context: String
}

public struct Activity: Codable {

    var base: Object

    var hello: String

    public func encode(to encoder: Encoder) throws {
        try base.encode(to: encoder)
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(hello, forKey: .hello)
    }
}

let a = Activity(base: Object(context: "foobar"), hello: "hello")
let data = try! JSONEncoder().encode(a)
print(String(data: data, encoding: .utf8)!)     // {"context":"foobar","hello":"hello"}