在Swift中使用JSONDecoder时，我不断收到“没有与键相关的值”错误。 有人可以解释这里出了什么问题吗？

Question

I am trying to read from a local JSON file and populate my tableView with cells to be retrieved from the Decoder . Since my table view was still empty, I added a breakpoint on the JSONDecoder.decode line to see what is going on. I get this error, even though I made sure that my naming convention is the same in both my structs and JSON file. Is there something I am missing here.

Since my naming convention was consistent across the files, at first I did not try add CodingKeys enum inside my structs. After a few failed runs I added that in but it feels kind of obsolete.

Where I run the decoder:

let fileName = "settings"

...


   if let url = Bundle.main.url(forResource: fileName, withExtension: "json") {
       do {
            let data = try Data(contentsOf: url)
            let list = try JSONDecoder().decode(SettingsPayload.self, from: data)
            completion(list.sections)
       } catch {
            completion(nil)
       }
   }

fileprivate struct SettingsPayload: Decodable {
    let sections: [Section]
}

My structs that will be used to store the data when retrieved:

struct Item: Decodable {
    let title: String
    let type: String
    let url: String

    private enum CodingKeys: String, CodingKey {
        case title = "title"
        case type = "type"
        case url = "url"
    }
}

struct Section: Decodable {
    let title: String
    let items: [Item]

    private enum CodingKeys: String, CodingKey {
        case title = "title"
        case items = "items"
    }
}

and my .json file:

{
    "sections": [{
        "section": {
            "title": "Main Settings",
            "items": [{
                "item": {
                    "title": "Notifications",
                    "type": "notification",
                    "url": ""
                },
                "item": {
                    "title": "Log Out",
                    "type": "",
                    "url": ""
                }
            }]
        },
        "section": {
            "title": "Feedback",
            "items": [{
                "item": {
                    "title": "Contact Us",
                    "type": "email",
                    "url": ""
                },
                "item": {
                    "title": "Rate on App Store",
                    "type": "webView",
                    "url": "https://www.apple.com/uk/ios/app-store/"
                }
            }]
        },
        "section": {
            "title": "About",
            "items": [{
                "item": {
                    "title": "Terms of Service",
                    "type": "webView",
                    "url": ""
                },
                "item": {
                    "title": "Privacy Policy",
                    "type": "webView",
                    "url": "https://www.apple.com/uk/ios/app-store/"
                },
                "item": {
                    "title": "Version Info",
                    "type": "webView",
                    "url": ""
                }
            }]
        }
    }]
}

This is the error message I get:

     - debugDescription : "No value associated with key CodingKeys(stringValue: \"title\", intValue: nil) (\"title\")."

Answer 1

I think the problem is that the decoder expects an Array of the item Section, but your json has an Array of dictionaries with a key "section" and an item Section in there.

Maybe try to modify your json like this:

{
    "sections": [{
            "title": "Main Settings",
            "items": [ {
                    "title": "Notifications",
                    "type": "notification",
                    "url": ""
                }, {
                    "title": "Log Out",
                    "type": "",
                    "url": ""
                }]}, {
            "title": "Feedback",
            "items": [{
                    "title": "Contact Us",
                    "type": "email",
                    "url": ""
                },{
                    "title": "Rate on App Store",
                    "type": "webView",
                    "url": "https://www.apple.com/uk/ios/app-store/"
                }]}, {
            "title": "About",
            "items": [{
                    "title": "Terms of Service",
                    "type": "webView",
                    "url": ""
                }, {
                    "title": "Privacy Policy",
                    "type": "webView",
                    "url": "https://www.apple.com/uk/ios/app-store/"
                }, {
                    "title": "Version Info",
                    "type": "webView",
                    "url": ""
                }
  ]}]
}

---

Update

JSONDecoder doesn't look for the name of your decodable struct in the JSON, it only looks for the name of the properties.