extension Encodable {
    subscript(key: String) -> Any? {
        return dictionary[key]
    }
    var jsonData: Data? {
        return try? JSONEncoder().encode(self)
    }
    var dictionary: [String: Any] {
        guard let data = jsonData else {return [:]}
        return (try? JSONSerialization.jsonObject(with: data)) as? [String: Any] ?? [:]
    }
    var jsonStr: String? {
        guard let data = jsonData else {return nil}
        return String(data: data, encoding: .utf8)
    }
}

extension Decodable {
    static func getModel(data: Data) -> Self? {
        return try? JSONDecoder().decode(self, from: data)
    }
    static func getModel(jsonStr: String) -> Self? {
        guard let data = jsonStr.data(using: .utf8) else {return nil}
        return try? JSONDecoder().decode(self, from: data)
    }
}

test:

struct Teacher: Codable {
    let name: String
    let age: Int
}

struct Test: Codable {
    let name: String
    let age: Int
    let height: Double
    let teacher: Teacher
}

let test = Test(name: "aaa", age: 11, height: 120, teacher: Teacher(name: "bbb", age: 30))
test.jsonStr //"{\"age\":11,\"name\":\"aaa\",\"height\":120,\"teacher\":{\"name\":\"bbb\",\"age\":30}}"
test.dictionary //["age":11,"name":"aaa","height":120,"teacher":["name":"bbb","age":30]]
test.jsonData //

let jsonStr = "{\"name\":\"ccc\",\"age\":20,\"height\":170,\"teacher\":{\"name\":\"ddd\",\"age\":40}}"
let jsonData = jsonStr.data(using: .utf8)!

if let test2 = Test.getModel(data: jsonData) {
    test2.name //"ccc"
    test2.age //20
    test2.height //170
    test2.teacher.name //"ddd"
    test2.teacher.age //40
}

if let test3 = Test.getModel(jsonStr: jsonStr) {
    test3.name //"ccc"
    test3.age //20
    test3.height //170
    test3.teacher.name //"ddd"
    test3.teacher.age //40
}

Demo:
Demo