How to exclude properties from Swift 4's Codable

问题

Swfit 4's new Encodable/Decodable protocols make JSON (de)serialization quite pleasant. However, I have not yet found a way to have fine-grained control over which properties should be encoded and which should get decoded.

I have noticed that excluding the property from the accompanying CodingKeys enum excludes the property from the process altogether, but is there a way to have more fine-grained control?

回答1:

The list of keys to encode/decode is controlled by a type called CodingKeys (note the s at the end). The compiler can synthesize this for you but can always override that.

Let's say you want to exclude the property nickname from both encoding and decoding:

struct Person: Codable {
    var firstName: String
    var lastName: String
    var nickname: String?

    private enum CodingKeys: String, CodingKey {
        case firstName, lastName
    }
}

If you want it to be asymmetric (i.e. encode but not decode or vice versa), you have to provide your own implementations of encode(with encoder: ) and init(from decoder: ):

struct Person: Codable {
    var firstName: String
    var lastName: String

    // Since fullName is a computed property, it's excluded by default
    var fullName: String {
        return firstName + " " + lastName
    }

    private enum CodingKeys: String, CodingKey {
        case firstName
        case lastName
        case fullName
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        firstName = try container.decode(String.self, forKey: .firstName)
        lastName = try container.decode(String.self, forKey: .lastName)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(firstName, forKey: .firstName)
        try container.encode(lastName, forKey: .lastName)
        try container.encode(fullName, forKey: .fullName)
    }
}

回答2:

If we need to exclude decoding of a couple of properties from a large set of properties in the structure, declare them as optional properties. Code to unwrapping optionals is less than writing a lot of keys under CodingKey enum.

I would recommend using extensions to add computed instance properties and computed type properties. It separates codable comforming properties from other logic hence provides better readability.

回答3:

While this can be done it ultimately ends up being very unSwifty and even unJSONy. I think I see where you are coming from, the concept of #ids is prevalent in HTML, but it is rarely transported over to the world of JSON which I consider a good thing (TM).

Some Codable structs will be able to parse your JSON file just fine if you restructure it using recursive hashes, i.e. if your recipe just contains an array of ingredients which in turn contains (one or several) ingredient_info. That way the parser will help you to stitch your network together in the first place and you only have to provide some backlinks through a simple traversal the structure if you really need them. Since this requires a thorough rework of your JSONand your data structure I only sketch out the idea for you to think about it. If you deem it acceptable please tell me in the comments then I could elaborate it further, but depending on the circumstances you may not be at the liberty to change either one of them.

回答4:

I have used protocol and its extension along with AssociatedObject to set and get image (or any property which needs to be excluded from Codable) property.

With this we dont have to implement our own Encoder and Decoder

Here is the code, keeping relevant code for simplicity:

protocol SCAttachmentModelProtocol{
    var image:UIImage? {get set}
    var anotherProperty:Int {get set}
}
extension SCAttachmentModelProtocol where Self: SCAttachmentUploadRequestModel{
    var image:UIImage? {
        set{
            //Use associated object property to set it
        }
        get{
            //Use associated object property to get it
        }
    }
}
class SCAttachmentUploadRequestModel : SCAttachmentModelProtocol, Codable{
    var anotherProperty:Int
}

Now, whenever we want to access the Image property we can use on the object confirming to protocol (SCAttachmentModelProtocol)

来源：https://stackoverflow.com/questions/44655562/how-to-exclude-properties-from-swift-4s-codable

标签

json

swift

codable