Can I use both init() and initial value in Decoder? - ios

Question

I have lots of values in my codable struct. I have URLs coming in as ""(empty string) so I need custom Decoder to convert "" as nil. So I made a propertyWrapper to solve this.
For example, I have values like ["https://google.com", "", "https://google.com"] and I want to make it as [URL?]. This works well with my decoder. It's converted as [URL("https://google.com"), nil, URL("https://google.com")]
However, I found a problem that, when I use init(from decoder: Decoder) throws, I also have to initialize all other values in struct. Is there any way to use just courseImages and use other values in struct as set?
struct CabinetCourse: Codable {
let courseId: String
let title: String
let planStartDate: Date
let planEndDate: Date
let companionTypeCd: String
let courseCategory: String
let planId: String
let nickname: String?
let isCabinet: Bool
let isFavorite: Bool?
let course: String
let score: String
let shareCnt: Int
let favoriteCnt: Int
let cabinetCnt: Int
let placeCount: Int
let createDt: Date
let childPlaceCount: Int
let wheelChairPlaceCount: Int
let elderPlaceCount: Int
#OptionalObject
var courseImages: [URL?]
public init(from decoder: Decoder) throws {
let values = try decoder.container(keyedBy: CodingKeys.self)
let emptyURLS = try values.decode([OptionalObject<URL>].self, forKey: .courseImages)
courseImages = emptyURLS.map { $0.wrappedValue }
// => these are the lines I don't want to write
courseId = try values.decode(String.self, forKey: .courseId)
title = try values.decode(String.self, forKey: .title)
planStartDate = try values.decode(Date.self, forKey: .planStartDate)
planEndDate = try values.decode(Date.self, forKey: .planEndDate)
companionTypeCd = try values.decode(String.self, forKey: .companionTypeCd)
courseCategory = try values.decode(String.self, forKey: .courseCategory)
planId = try values.decode(String.self, forKey: .planId)
nickname = try values.decode(String.self, forKey: .nickname)
isCabinet = try values.decode(Bool.self, forKey: .isCabinet)
isFavorite = try values.decode(Bool.self, forKey: .isCabinet)
course = try values.decode(String.self, forKey: .isCabinet)
score = try values.decode(String.self, forKey: .isCabinet)
}
#propertyWrapper
struct OptionalObject<Base: Decodable>: Decodable {
var wrappedValue: Base?
init(from decoder: Decoder) throws {
do {
let container = try decoder.singleValueContainer()
wrappedValue = try container.decode(Base.self)
} catch {
wrappedValue = nil
}
}
}

Answer 1

You are misunderstanding property wrappers. They "decorate" the entire property type - [URL?], not just the array element type URL?. [URL?] doesn't match the type of wrappedValue. [URL]? does (Base == [URL]), but that's not what you want.
One way to create a property wrapper that can be applied to an array of optionals is:
#propertyWrapper
struct OptionalArray<Base: Decodable>: Decodable {
var wrappedValue: [Base?]
...
Now Base == URL matches [URL?], and in init, you have to decode a [Base?]:
init(from decoder: Decoder) throws {
var arr = [Base?]()
var container = try decoder.unkeyedContainer()
for _ in 0..<(container.count ?? 0) {
if let element = try? container.decode(Base.self) {
arr.append(element)
} else {
arr.append(nil)
_ = try container.decode(String.self) // advances the decoder to the next position
}
}
wrappedValue = arr
}
Once you have the property wrapper, you don't need the custom decoding code at all. Swift figures it out.
struct CabinetCourse: Decodable {
let courseId: String
let title: String
let planStartDate: Date
let planEndDate: Date
let companionTypeCd: String
let courseCategory: String
let planId: String
let nickname: String?
let isCabinet: Bool
let isFavorite: Bool?
let course: String
let score: String
let shareCnt: Int
let favoriteCnt: Int
let cabinetCnt: Int
let placeCount: Int
let createDt: Date
let childPlaceCount: Int
let wheelChairPlaceCount: Int
let elderPlaceCount: Int
#OptionalArray
var courseImages: [URL?]
}
// That's it!
For the encoding part, it depends on how you want to encode the nils. But either way, the code is very similar to the decoding code.