Swift 4 Decodable: struct from nested array

问题:

Given the following JSON document I'd like to create a struct with four properties: filmCount (Int), year (Int), category (String), and actor (Actor array).

{    
    "filmCount": 5,
    "year": 2018,
    "category": "Other",
    "actors":{  
        "nodes":[  
            {  
                "actor":{  
                    "id":0,
                    "name":"Daniel Craig"
                }
            },
            {  
                "actor":{  
                    "id":1,
                    "name":"Naomie Harris"
                }
            },
            {  
                "actor":{  
                    "id":2,
                    "name":"Rowan Atkinson"
                }
            }
        ]
    }
}

---

PlacerholderData is a struct storing the three main properties and the list of actors which should be retrieved from the nested nodes container within the actors property from the JSON object.

PlacerholderData:

struct PlaceholderData: Codable {
    let filmCount: Int
    let year: Int
    let category: String
    let actors: [Actor]
}

Actor.swift:

struct Actor: Codable {
    let id: Int
    let name: String
}

---

I am attempting to do this through providing my own init to initialise the values from the decoder's container manually. How can I go about fixing this without having to have an intermediate struct storing a nodes object?

回答1:

You can use nestedContainer(keyedBy:) and nestedUnkeyedContainer(forKey:) for decoding nested array and dictionary like this to turn it into your desired structure. Your decoding in init(decoder: ) might look something like this,

Actor extension for decoding,

extension Actor: Decodable {

    enum CodingKeys: CodingKey { case id, name }

    enum ActorKey: CodingKey { case actor }

    init(from decoder: Decoder) throws {
        let rootKeys        = try decoder.container(keyedBy: ActorKey.self)
        let actorContainer  = try rootKeys.nestedContainer(keyedBy: CodingKeys.self,
                                                           forKey: .actor)
        try id =  actorContainer.decode(Int.self,
                                       forKey: .id)
        try name =  actorContainer.decode(String.self,
                                         forKey: .name)
    }
}

PlaceholderData extension for decoding,

extension PlaceholderData: Decodable {

    enum CodingKeys: CodingKey { case filmCount, year, category, actors }

    enum NodeKeys: CodingKey { case nodes }

    init(from decoder: Decoder) throws {
        let rootContainer   = try decoder.container(keyedBy: CodingKeys.self)
        try filmCount       =  rootContainer.decode(Int.self,
                                                    forKey: .filmCount)
        try year            =  rootContainer.decode(Int.self,
                                                    forKey: .year)
        try category        =  rootContainer.decode(String.self,
                                                    forKey: .category)
        let actorsNode      = try rootContainer.nestedContainer(keyedBy: NodeKeys.self,
                                                                forKey: .actors)
        var nodes = try actorsNode.nestedUnkeyedContainer(forKey: .nodes)
        var allActors: [Actor] = []

        while !nodes.isAtEnd {
            let actor = try nodes.decode(Actor.self)
            allActors += [actor]
        }
        actors = allActors
    }
}

Then, you can decode it like this,

let decoder = JSONDecoder()
do {
    let placeholder = try decoder.decode(PlaceholderData.self, from: jsonData)
    print(placeholder)
} catch {
    print(error)
}

Here, the basic idea is to decode dictionary container using nestedContainer(keyedBy:) and array container using nestedUnkeyedContainer(forKey:)