如何使用 Codable 在 Swift 4 中解析復雜的 JSON

Question

我需要從服務器解析 JSON 的幫助。 這是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"
}

}

我需要從這個 JSON 中得到幾行：“文本”、“評論”、“喜歡”、“轉發”、“附件”。

在“附件”字段中，我想得到“photo_604”行。

這是我的代碼：

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
}

但是在提出請求之后：

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)

我收到一個錯誤：

致命錯誤：“嘗試！” 表達意外加一個錯誤：Swift.DecodingError.keyNotFound（_UL_PetrovLeonid.NewsItems（在_FA9A2FC8130449AA328C19ACD9506C2D）.attachments CodingKeys，Swift.DecodingError.Context（codingPath：[在_FA9A2FC8130449AA328C19ACD9506C2D）.response _UL_Leonid.ResponseNewsFeed（CodingKeys，_UL_PetrovLeonid.WhatIsIsideResponseNewsFeed（ CodingKeys in _FA9A2FC8130449AA328C19ACD9506C2D).items, Foundation.(_JSONKey in _12768CA107A31EF2DCE034FD75B541C9)(stringValue: "Index 0", intValue: "Index 0", intValue: "Optional\\Debug" with) 。底層錯誤：無））

為什么會發生這種情況，我需要做什么來解決它？ 從現在起我才開始編碼三個月，所以如果我的問題看起來很愚蠢，請事先原諒我。

謝謝你。

Answer 1

錯誤信息很清楚

... Swift.DecodingError。 keyNotFound ... NewsItems。（_FA9A2FC8130449AA328C19ACD9506C2D中的編碼鍵）。 附件...沒有與關鍵附件 (\\"attachments\\") 關聯的值。

• keyNotFound是密鑰丟失
• NewsItems.(CodingKeys ... ).attachments是NewsItems中關鍵attachments的對象，它是Attachments
• 沒有與關鍵附件（\\“附件\\”）相關聯的值是所說的。

不久：沒有鑰匙attachments在Attachments這是真的。

看看你的 JSON

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

等價的類是

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

而AttachmentPhoto應該是

class AttachmentPhoto: Decodable {

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

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

其實沒有必要使用class上課，在大多數情況下， struct就足夠了。