ResultContainerCodingKey.swift

import UIKit

let json1 = """
{"results": [{"name": "bob"}]}
""".data(using: .utf8)!

let json2 = """
{"people": [{"name": "Jane"}]}
""".data(using: .utf8)!

struct Person: Decodable {
    let name: String
}

protocol ResultContainerCodingKey: CodingKey {
    static var resultCodingKey: String { get }

    init()
}

extension ResultContainerCodingKey {
    init() {
        self.init()
    }

    // MARK: CodingKey impl

    var intValue: Int? { nil }

    init?(intValue: Int) {
        return nil // unused
    }

    var stringValue: String {
        Self.resultCodingKey
    }

    init?(stringValue: String) {
        return nil
    }
}

struct ResultsContainerCodingKey: ResultContainerCodingKey {
    static var resultCodingKey = "results"
}

struct PeopleContainerCodingKey: ResultContainerCodingKey {
    static var resultCodingKey = "people"
}

struct ResultsContainer<ResultKey, ResultType>: Decodable where ResultKey: ResultContainerCodingKey,  ResultType: Decodable {
    let results: ResultType

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: ResultKey.self)
        results = try container.decode(ResultType.self, forKey: ResultKey())
    }
}

let decodedResult1 = try! JSONDecoder().decode(ResultsContainer<ResultsContainerCodingKey, [Person]>.self,
                                               from: json1)
decodedResult1.results

let decodedResult2 = try! JSONDecoder().decode(ResultsContainer<PeopleContainerCodingKey, [Person]>.self,
                                               from: json2)
decodedResult2.results

// uncomment to see decoding errors when keys don't exist in the json data:

//let decodedResult3 = try! JSONDecoder().decode(ResultsContainer<ResultsContainerCodingKey, [Person]>.self, from: json2)
//decodedResult3.results

//let decodedResult4 = try! JSONDecoder().decode(ResultsContainer<PeopleContainerCodingKey, [Person]>.self, from: json1)
//decodedResult4.results