json - 如何使用 Codable 在 Swift 4 中解析复杂的 JSON

Question

我需要有关从服务器解析 JSON 的帮助。这是 JSON:

{
"response": {
    "items": [
        {
            "type": "post",
            "source_id": -17507435,
            "date": 1514538602,
            "post_id": 4105,
            "post_type": "post",
            "text": "Some text here",
            "marked_as_ads": 0,
            "attachments": [
                {
                    "type": "photo",
                    "photo": {
                        "id": 456239655,
                        "album_id": -7,
                        "owner_id": -17507435,
                        "user_id": 100,
                        "photo_75": "https://sun1-3.userapi.com/c840632/v840632924/3b7e7/4YUS7DlaLK8.jpg",
                        "photo_130": "https://sun1-3.userapi.com/c840632/v840632924/3b7e8/Ffpb4ZUlulI.jpg",
                        "photo_604": "https://sun1-3.userapi.com/c840632/v840632924/3b7e9/-pkl6Qdb9hk.jpg",
                        "width": 439,
                        "height": 312,
                        "text": "",
                        "date": 1514538602,
                        "post_id": 4105,
                        "access_key": "6a61a49570efd9c39c"
                    }
                }
            ],
            "post_source": {
                "type": "api"
            },
            "comments": {
                "count": 0,
                "groups_can_post": true,
                "can_post": 1
            },
            "likes": {
                "count": 0,
                "user_likes": 0,
                "can_like": 1,
                "can_publish": 1
            },
            "reposts": {
                "count": 0,
                "user_reposted": 0
            },
            "views": {
                "count": 2
            }
        }
    ],
    "profiles": [],
    "groups": [
        {
            "id": 17507435,
            "name": "Literature Museum",
            "screen_name": "samlitmus",
            "is_closed": 0,
            "type": "group",
            "is_admin": 0,
            "is_member": 1,
            "photo_50": "https://pp.userapi.com/c615722/v615722068/e58c/d5Y8E_5689s.jpg",
            "photo_100": "https://pp.userapi.com/c615722/v615722068/e58b/Hm05ga3x2J8.jpg",
            "photo_200": "https://pp.userapi.com/c615722/v615722068/e589/yoG_DDalFII.jpg"
        },
        {
            "id": 27711883,
            "name": "E:\\music\\melodic hardcore",
            "screen_name": "e_melodic_hc",
            "is_closed": 0,
            "type": "page",
            "is_admin": 0,
            "is_member": 1,
            "photo_50": "https://pp.userapi.com/c628220/v628220426/47092/xepNnC7pSBw.jpg",
            "photo_100": "https://pp.userapi.com/c628220/v628220426/47091/uAokr-c3NQ8.jpg",
            "photo_200": "https://pp.userapi.com/c628220/v628220426/4708f/eNY4vzooz4E.jpg"
        },
        {
            "id": 81574241,
            "name": "DOS4GW.EXE",
            "screen_name": "dos4gw",
            "is_closed": 0,
            "type": "page",
            "is_admin": 0,
            "is_member": 1,
            "photo_50": "https://pp.userapi.com/c622118/v622118651/e045/vlhV6QxtoLI.jpg",
            "photo_100": "https://pp.userapi.com/c622118/v622118651/e044/P9mVUhXBV58.jpg",
            "photo_200": "https://pp.userapi.com/c622118/v622118651/e043/Soq8oxCMB0I.jpg"
        },
        {
            "id": 76709587,
            "name": "Prosvet",
            "screen_name": "prosvet_pub",
            "is_closed": 0,
            "type": "page",
            "is_admin": 0,
            "is_member": 0,
            "photo_50": "https://pp.userapi.com/c630431/v630431500/b24a/GHox8AmDTXU.jpg",
            "photo_100": "https://pp.userapi.com/c630431/v630431500/b249/H3mcC-K7htM.jpg",
            "photo_200": "https://pp.userapi.com/c630431/v630431500/b248/9fyvB8gkcwc.jpg"
        }
    ],
    "next_from": "1/4105_1514494800_5"
}

我需要从这个 JSON 中获取的是行:“文本”、“评论”、“喜欢”、“转发”、“附件”。

在“附件”字段中，我想获取“photo_604”行。

这是我的代码:

class NewsItems: Decodable {
    var text: String?
    var comments: Comments
    var likes: Likes
    var reposts: Reposts
    var attachments: [Attachments]
}

class Comments: Decodable {
    var count: Int?
}

class Likes: Decodable {
    var count: Int?
}

class Reposts: Decodable {
    var count: Int?
}

class Attachments: Decodable {
    var attachments: AttachmentPhoto
}

class AttachmentPhoto: Decodable {
    var photo: WhatIsInsideAttachmentsPhoto
}

class WhatIsInsideAttachmentsPhoto: Decodable {
    var photo: String?

    enum CodingKeys: String, CodingKey {
        case photo = "photo_604"
    }
}


class WhatIsIsideResponseNewsFeed: Decodable {
    var items: [NewsItems]
}

public class ResponseNewsFeed: Decodable {
    var response: WhatIsIsideResponseNewsFeed
}

但是发出请求后:

Alamofire.request(baseURL+methodName, parameters: parameters).responseData(completionHandler: { response in
        if let result = response.result.value {
            let decoder = JSONDecoder()
            let myResponse = try! decoder.decode(ResponseNewsFeed.self, from: result)
            completion(myResponse.response.items)

我得到一个错误:

Fatal error: 'try!' expression unexpectedly raised an error: Swift.DecodingError.keyNotFound(_UL_PetrovLeonid.NewsItems.(CodingKeys in _FA9A2FC8130449AA328C19ACD9506C2D).attachments, Swift.DecodingError.Context(codingPath: [_UL_Leonid.ResponseNewsFeed.(CodingKeys in _FA9A2FC8130449AA328C19ACD9506C2D).response, _UL_PetrovLeonid.WhatIsIsideResponseNewsFeed.(CodingKeys in _FA9A2FC8130449AA328C19ACD9506C2D).items, Foundation.(_JSONKey in _12768CA107A31EF2DCE034FD75B541C9)(stringValue: "Index 0", intValue: Optional(0))], debugDescription: "No value associated with key attachments (\"attachments\").", underlyingError: nil))

为什么会发生这种情况，我需要做什么来解决它？从现在开始我才开始编码三个月，所以如果我的问题看起来很愚蠢，请事先原谅我。

谢谢。

Answer 1

错误信息很清楚

... Swift.DecodingError.keyNotFound ... NewsItems.(CodingKeys in _FA9A2FC8130449AA328C19ACD9506C2D).attachments ... No value associated with key attachments (\"attachments\").

• keyNotFound 是 key 丢失
• NewsItems.(CodingKeys ... ).attachments 是 NewsItems 中关键 attachments 的对象，即 Attachments
• 没有与关键附件相关联的值 (\"attachments\") 是所说的。

简短地说:Attachments 中没有关键的attachments，这是真的。

看看你的 JSON

      "attachments": [
            {
                "type": "photo",
                "photo": {

等价类是

class Attachments: Decodable {
    let type : String
    let photo : AttachmentPhoto
}

AttachmentPhoto 应该是

class AttachmentPhoto: Decodable {

    private enum CodingKeys : String, CodingKey {
        case photo604 = "photo_604"
        case id
    }

    let id : Int
    let photo604 : String // or even URL
    // etc.
}

其实没有必要使用class，大多数情况下一个struct就足够了。