Swift Codable：如何将顶级数据编码到嵌套容器中

我的应用程序使用返回 JSON 的服务器，如下所示：

{
    "result":"OK",
    "data":{

        // Common to all URLs
        "user": {
            "name":"John Smith" // ETC...
        },

        // Different for each URL
        "data_for_this_url":0
    }
}

正如您所看到的，特定于 URL 的信息与常见的信息存在于同一字典中。user字典。

GOAL:

1. Decode this JSON into classes/structs.
   • Because user很常见，我希望它位于顶级类/结构中。
2. Encode to new format (e.g. plist).
   • 我需要保留原来的结构。 （即重新创建data来自顶级的字典user信息和子对象的信息）

PROBLEM:

重新编码数据时，我不能同时写入user字典（来自顶级对象）和特定于 URL 的数据（来自子对象）到编码器。

Either user覆盖其他数据，或者其他数据覆盖user。我不知道如何将它们结合起来。

这是我到目前为止所拥有的：

// MARK: - Common User
struct User: Codable {
    var name: String?
}

// MARK: - Abstract Response
struct ApiResponse<DataType: Codable>: Codable {
    // MARK: Properties
    var result: String
    var user: User?
    var data: DataType?

    // MARK: Coding Keys
    enum CodingKeys: String, CodingKey {
        case result, data
    }
    enum DataDictKeys: String, CodingKey {
        case user
    }

    // MARK: Decodable
    init(from decoder: Decoder) throws {
        let baseContainer = try decoder.container(keyedBy: CodingKeys.self)
        self.result = try baseContainer.decode(String.self, forKey: .result)
        self.data = try baseContainer.decodeIfPresent(DataType.self, forKey: .data)

        let dataContainer = try baseContainer.nestedContainer(keyedBy: DataDictKeys.self, forKey: .data)
        self.user = try dataContainer.decodeIfPresent(User.self, forKey: .user)
    }

    // MARK: Encodable
    func encode(to encoder: Encoder) throws {
        var baseContainer = encoder.container(keyedBy: CodingKeys.self)
        try baseContainer.encode(self.result, forKey: .result)

        // MARK: - PROBLEM!!

        // This is overwritten
        try baseContainer.encodeIfPresent(self.data, forKey: .data)

        // This overwrites the previous statement
        var dataContainer = baseContainer.nestedContainer(keyedBy: DataDictKeys.self, forKey: .data)
        try dataContainer.encodeIfPresent(self.user, forKey: .user)
    }
}

EXAMPLE:

在下面的示例中，重新编码的 plist 不包括order_count，因为它被包含的字典覆盖了user.

// MARK: - Concrete Response
typealias OrderDataResponse = ApiResponse<OrderData>

struct OrderData: Codable {
    var orderCount: Int = 0
    enum CodingKeys: String, CodingKey {
        case orderCount = "order_count"
    }
}


let orderDataResponseJson = """
{
    "result":"OK",
    "data":{
        "user":{
            "name":"John"
        },
        "order_count":10
    }
}
"""

// MARK: - Decode from JSON
let jsonData = orderDataResponseJson.data(using: .utf8)!
let response = try JSONDecoder().decode(OrderDataResponse.self, from: jsonData)

// MARK: - Encode to PropertyList
let plistEncoder = PropertyListEncoder()
plistEncoder.outputFormat = .xml

let plistData = try plistEncoder.encode(response)
let plistString = String(data: plistData, encoding: .utf8)!

print(plistString)

// 'order_count' is not included in 'data'!

/*
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>data</key>
    <dict>
        <key>user</key>
        <dict>
            <key>name</key>
            <string>John</string>
        </dict>
    </dict>
    <key>result</key>
    <string>OK</string>
</dict>
</plist>
*/

我刚刚在查看编码器协议时顿悟了。

KeyedEncodingContainerProtocol.superEncoder(forKey:)方法正是针对这种情况。

该方法返回一个单独的Encoder可以收集多个项目和/或嵌套容器，然后将它们编码为单个密钥。

对于这个具体案例，顶层user数据可以通过简单地调用自己的编码来编码encode(to:)方法，用新的superEncoder。然后，还可以使用编码器创建嵌套容器，并正常使用。

问题的解答

// MARK: - Encodable
func encode(to encoder: Encoder) throws {

    var baseContainer = encoder.container(keyedBy: CodingKeys.self)
    try baseContainer.encode(self.result, forKey: .result)

    // MARK: - PROBLEM!!
//    // This is overwritten
//    try baseContainer.encodeIfPresent(self.data, forKey: .data)
//
//    // This overwrites the previous statement
//    var dataContainer = baseContainer.nestedContainer(keyedBy: DataDictKeys.self, forKey: .data)
//    try dataContainer.encodeIfPresent(self.user, forKey: .user)

    // MARK: - Solution
    // Create a new Encoder instance to combine data from separate sources.
    let dataEncoder = baseContainer.superEncoder(forKey: .data)

    // Use the Encoder directly:
    try self.data?.encode(to: dataEncoder)

    // Create containers for manually encoding, as usual:
    var userContainer = dataEncoder.container(keyedBy: DataDictKeys.self)
    try userContainer.encodeIfPresent(self.user, forKey: .user)
}

Output:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>data</key>
    <dict>
        <key>order_count</key>
        <integer>10</integer>
        <key>user</key>
        <dict>
            <key>name</key>
            <string>John</string>
        </dict>
    </dict>
    <key>result</key>
    <string>OK</string>
</dict>
</plist>