Expanding on Generic Json Parsing

Thu Apr 12 2018 | Mark Struzinski

Expanded JSON Parsing with Generics

In the last post, I went over a very simple way to use generics when parsing a small object that had a value type determined at runtime. Today, I’d like to expand on that concept a bit and show how I implemented that concept into a parent struct that contained arrays of this object.

As a quick recap, here is the original object:

struct PropertyEntry<T: Codable>: Codable {
    let appVersion: String
    let value: T

The value property in this struct is a generic, and is specified at runtime.

Here is an example of using this from my unit tests:

let jsonString = "{\"appVersion\": \"1.0.1\", \"defaultValue\": true}"
let data = jsonString.data(using: .utf8)!
let decoder = JSONDecoder()

let response: BCPAppVersionValue<Bool> = 
	try! decoder.decode(BCPAppVersionValue<Bool>.self, from: data)

Keep in mind I’m doing a lot of force unwrapping here because I’m in a test case. This is not a safe practice in production code! Moving on from this, I needed an object that would represent the setting for a property, including all potentials values for each version number of the app. The buisiness requirement here is to use the default value unless it is overridden. A default value is overridden if the optional array of valuesByVersion contains a setting that matches the current app version.

The object declaration looks like this:

public struct PropertySetting<T: Codable>: Codable {
    public let defaultValue: T
    public let valuesByVersion: [PropertyEntry<T>]?

The defaultValue in this object is decoded exactly the same way as the object in the last post. The interesting part comes in when decoding valuesByVersion.

Here is the business logic I need to run on the array of PropertySetting objects:

  1. Parse the initial result (default value)
  2. Check the optional array of values by version
  3. Compare the app version(s) contained in this array to the current app version
  4. If the current app version matches one of the values in the array, then that means there is a custom setting for this app version. Ensure that version overrides the default and use it.

This item is typed as an array of PropertyEntry<T>. For this one, I’ll have to drop into a custom init to get the object decoded properly.

Here’s the relevant code block, then I’ll walk through and explain everything:

public init(from decoder: Decoder) throws {
	let container = try decoder.container(keyedBy: CodingKeys.self)
    // 1
	var defaultValue = try container.decode(T.self, forKey: .defaultValue)
	// 2
	let valuesByVersion = try? container.decode(Array<PropertySetting<T>>.self, forKey: .valuesByVersion)
    // 3
	if let versions = valuesByVersion,
    	let appVersion = decoder.userInfo["appVersion"] as? String {
    	if let updatedDefault = versions.filter({ version -> Bool in
        	version.appVersion == appVersion
    	}).first {
        	defaultValue = updatedDefault.defaultValue
   // 4
	self.defaultValue = defaultValue
	self.valuesByVersion = valuesByVersion

With the new Decodable protocol, anytime you want to run custom logic when decoding an object (rather than just mapping properties to values), you need to implement init(from decoder: Decoder). After this point you, are reponsible for initializing all non-optional properties of your model object prior to exiting the method. I’ll use the init method here to check for the specific version of the running app instance, and see if that version is in the list of PropertyEntry objects coming back in the list. If it is, then I know there is a custom setting for this version of the app and I’ll use it. This is accomplished by passing in some context to the JSON Decoder.

  1. Decode an initial defaultValue from the single entry in the base JSON response
  2. Try to decode an array of PropertyEntry objects from the response
  3. If an array exists, check the userInfo dictionary attached to the decoder. If an appVersion exists in here, use it as a filter value.
  4. Assign the result of all that logic to the defaultValue of the model object

Like a lot of other objects in Cocoa Touch APIs, you are allowed to attach a userInfo dictionary to the decoder. For this decoding scenario, I’m attaching the current app version to the decoder that parses this object. When creating this dictionary, the keys are of type CodingUserInfoKey.

Here is the declaration of CodingUserInfoKey in Swift:

/// A user-defined key for providing context during encoding and decoding.
public struct CodingUserInfoKey : RawRepresentable, Equatable, Hashable 

Here is how to attach that dictionary to a decoder:

let decoder = JSONDecoder()
guard let infoKey = CodingUserInfoKey(rawValue: "appVersion") else {
    preconditionFailure("Invalid info key")

let infoDict: [CodingUserInfoKey: Any] = [
    infoKey: "1.4.0"

decoder.userInfo = infoDict

This was a lot, but I hope it shows how flexible and powerful we can make parsing logic now with Decodable. It also reduces a ton of boilerplate code, which is a solid improvement.