在Swift中使用可编码扩展更新对象属性

Question

let me start by saying that I have already implemented Decodable which decodes JSON into several objects with these two Integer values:

public class ARBufferData: DecoderUpdatable {

    private var previousStation: Int
    private var numberOfElements: Int

    func update(from decoder: Decoder) throws {
    //Still needs work
    }
}

What I am now trying to achieve is making the created objects updatable so that when a value in the JSON changes (eg numberOfElements) only the value is changed in the corresponding object. I believe this guide can enable me to do it, but I am having trouble implementing it: Understanding and Extending Swift 4's Codable

This is the extension of KeyedDecodingContainer:

extension KeyedDecodingContainer {
    func update<T: DecoderUpdatable>(_ value: inout T, forKey key: Key, userInfo: Any) throws {
        let nestedDecoder = NestedDecoder(from: self, key: key)
        try value.update(from: nestedDecoder)
    }
}

The reason this would be helpful is that I can then set a property observer on that value and trigger a redraw of the visualisation.

I would be very grateful, if anyone can help or point me in the right direction.

Thank you!

Cheers

Answer 1

There are two ways to update the class. One, you decode each int by itself and compare. Two, you implement DecoderUpdatable for Int and call container.update with them as argument.

public class ARBufferData: NSObject, Decodable, DecoderUpdatable {
    init(previousStation: Int, numberOfElements: Int) {
        self.previousStation = previousStation
        self.numberOfElements = numberOfElements
    }

    @objc dynamic var previousStation: Int
    @objc dynamic var numberOfElements: Int

    private enum CodingKeys: String, CodingKey {
        case previousStation, numberOfElements
    }

    public func update(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        try container.update(&previousStation, forKey: .previousStation)
        try container.update(&numberOfElements, forKey: .numberOfElements)
    }
}

extension Int: DecoderUpdatable {
    public mutating func update(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let result = try container.decode(Int.self)
        guard result != self else { return }
        self = result
    }
}

I do not know whether the blogpost-writer intended it this way though. If he did, then generating the DecoderUpdatable conformances for the basic types could be a use case for Sourcery, but that's off topic here.

In Swift4 there is an interesting way to observe which you may also be interested in:

let token = buffer.observe(\.numberOfElements, options: [.new, .old]) {
    object, change in
    if change.oldValue != change.newValue {
        // Act on change of buffer.numberOfElements.
    }
}

Source