A small problem in Swift using Decodable to parse JSON

Preface

Swift 4 is the latest version Apple plans to launch in fall 2017, with its main focus on providing source compatibility with Swift 3 code and working to achieve ABI stability. Decodable parsing JSON from Swift4 is indeed very convenient, but when you encounter a small problem, please record it.

When the value of a key in JSON is {} or an empty string "" and the value that needs to be parsed is not the basic type, even if it is marked as Optional, the entire parsing will still fail:

//: Playground
import Foundation

//
struct Book: Codable {
 var id: Int
 var name: String
 var cover: BookCover?

 enum CodingKeys: String, CodingKey {
 case id
 case name
 case cover
 }

 struct BookCover: Codable {

 var url: String
 var thumbURL: String

 enum CodingKeys: String, CodingKey {
 case url
 case thumbURL = "thumb_url"
 }
 }
}

//JSON
let bookJSON1 =
"""
{
 "id": 1,
 "name": "fake name 1",
 "cover": {
 "url": "",
 "thumb_url": ""
 }
}
"""

let bookJSON2 =
"""
{
 "id": 2,
 "name": "fake name 2",
 "cover": {

 }
}
"""

//Analysislet decoder = JSONDecoder()
 = .iso8601
let book1 = try? (, from: (using: .utf8)!)

// The parsing is normalprint(book1)
let book2 = try? (, from: (using: .utf8)!)
// Output nil, cover is already Optional, why does the entire book fail to parse?print(book2)

reason:

Because cover is Optional, decodeIfPresent will be called for parsing, and the two keys of url and thumb_url are not found in the cover node, resulting in the default resolution failure and an error is directly thrown.

solve:

Reimplement decodeIfPresent, returning nil when parsing fails instead of throwing an error causing the entire parsing to fail:

extension KeyedDecodingContainer {
 public func decodeIfPresent<T>(_ type: , forKey key: K) throws -> T? where T : Decodable {
 return try? decode(type, forKey: key)
 }
}

refer to:/2017/08/16/jsondecoder-in-the-real-world

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