Swift4_JSON_07.swift

import Foundation

let json = """
{
    "nodes": [
        {
            "extensions": {
                "KHR_materials_common": {
                    "light": 0
                }
            }
        }
    ],
    "materials": [
        {
            "extensions": {
                "KHR_materials_common": {
                    "technique": "BLINN"
                }
            }
        }
    ]
}
""".data(using: .utf8)!

struct KHRMaterialsCommon_NodeExtension: Codable {
    let light: Int
}

struct KHRMaterialsCommon_MaterialExtension: Codable {
    let technique: String
}

struct KHRMaterialsCommon: Codable {
    let data: Any
    
    init(from decoder: Decoder) throws {
        // 上の階層が nodes なら NodeExtension、materials なら MaterialExtension を読み込みたい。
        // codingPath[3] は KHR_materials_common、[2] は extensions、[1] は nil、[0] は nodes か materials
        let level = decoder.codingPath.count
        guard level >= 4 else {
            throw DecodingError.typeMismatch(KHRMaterialsCommon.self, .init(codingPath: decoder.codingPath, debugDescription: "something is wrong"))
        }
        
        guard let parentName = decoder.codingPath[level - 4]?.stringValue else {
            throw DecodingError.typeMismatch(KHRMaterialsCommon.self, .init(codingPath: decoder.codingPath, debugDescription: "something is wrong"))
        }
        
        switch parentName {
        case "nodes":
            self.data = try KHRMaterialsCommon_NodeExtension(from: decoder)
        case "materials":
            self.data = try KHRMaterialsCommon_MaterialExtension(from: decoder)
        default:
            throw DecodingError.typeMismatch(KHRMaterialsCommon.self, .init(codingPath: decoder.codingPath, debugDescription: "unknown parent: \(parentName)"))
        }
    }
    
    func encode(to encoder: Encoder) throws {
        // do something
    }
}

struct Extensions: Codable {
    let materialsCommon: KHRMaterialsCommon?
    
    enum CodingKeys: String, CodingKey {
        case materialsCommon = "KHR_materials_common"
    }
}

struct Material: Codable {
    let extensions: Extensions?
}

struct Node: Codable {
    let extensions: Extensions?
}

struct GLTF: Codable {
    let nodes: [Node]
    let materials: [Material]
}


do {
    let decoder = JSONDecoder()
    let gltf = try decoder.decode(GLTF.self, from: json)
    
    if let nodeExtension = gltf.nodes[0].extensions?.materialsCommon?.data as? KHRMaterialsCommon_NodeExtension {
        print("light: \(nodeExtension.light)")
    }
    if let materialExtension = gltf.materials[0].extensions?.materialsCommon?.data as? KHRMaterialsCommon_MaterialExtension {
        print("technique: \(materialExtension.technique)")
    }
} catch DecodingError.keyNotFound(let key, let context) {
    print("keyNotFound: \(key): \(context)")
} catch DecodingError.typeMismatch(let type, let context) {
    print("typeMismatch: \(type): \(context)")
} catch {
    print("\(error.localizedDescription)")
}

/*
 light: 0
 technique: BLINN
*/