Better Decoding Error Messages

BetterDecodingError.swift

import Foundation

enum BetterDecodingError: CustomStringConvertible {

    case dataCorrupted(_ message: String)
    case keyNotFound(_ message: String)
    case typeMismatch(_ message: String)
    case valueNotFound(_ message: String)
    case any(_ error: Error)

    init(with error: Error) {
        guard let decodingError = error as? DecodingError else {
            self = .any(error)
            return
        }

        switch decodingError {
        case let .dataCorrupted(context):
            let debugDescription = (context.underlyingError as NSError?)?.userInfo["NSDebugDescription"] ?? ""
            self = .dataCorrupted("Data corrupted. \(context.debugDescription) \(debugDescription)")
        case let .keyNotFound(key, context):
            self = .keyNotFound("Key not found. Expected -> \(key.stringValue) <- at: \(context.prettyPath())")
        case let .typeMismatch(_, context):
            self = .typeMismatch("Type mismatch. \(context.debugDescription), at: \(context.prettyPath())")
        case let .valueNotFound(_, context):
            self = .valueNotFound("Value not found. -> \(context.prettyPath()) <- \(context.debugDescription)")
        }
    }
    var description: String {
        switch self {
        case let .dataCorrupted(message), let .keyNotFound(message), let .typeMismatch(message), let .valueNotFound(message):
            return message
        case let .any(error):
            return error.localizedDescription
        }
    }
}

extension DecodingError.Context {
    func prettyPath(separatedBy separator: String = ".") -> String {
        codingPath.map { $0.stringValue }.joined(separator: ".")
    }
}

let jsonKeyNotFound = """
{
    "name": "pedro",
    "age": 20,
    "job": {
        "description": "Enginner",
        "yearsActive": 2,
        "boss": {
            "name": null,
            "age": 20
        }
    }
}
"""

let invalidJson = """
{
    "name: "pedro"
}
"""

let jsonValueNotFound = """
{
    "name": "pedro",
    "age": 20,
    "job": {
        "description": "Enginner",
        "yearsActive": 2,
        "boss": {
            "name": null,
            "age": 20
        }
    }
}
"""

let jsonValueTypeMismatch = """
{
    "name": "pedro",
    "age": 20,
    "job": {
        "description": "Enginner",
        "yearsActive": 2,
        "boss": {
            "name": "Jose",
            "age": "20"
        }
    }
}
"""

struct Boss: Decodable {
    let name: String
    let age: Int
}

struct Job: Decodable {
    let description: String
    let yearsActive: Int
    let boss: Boss
}

struct Person: Decodable {
    let name: String
    let age: Int
    let job: Job
}

[jsonValueTypeMismatch, invalidJson, jsonValueNotFound, jsonKeyNotFound].forEach { json in
    do {
        try JSONDecoder().decode(Person.self, from: json.data(using: .utf8)!)
    } catch {

        print(
            BetterDecodingError(with: error),
            "\n", "\n", "vs", "\n", "\n",
            error,
            "\n","\n", "----------------------------------", "\n"
        )
    }
}

// Type mismatch. Expected to decode Int but found a string/data instead., at: job.boss.age 
//  
// vs 
// 
// typeMismatch(Swift.Int, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "job", 
// intValue: nil), CodingKeys(stringValue: "boss", intValue: nil), CodingKeys(stringValue: "age", 
// intValue: nil)], debugDescription: "Expected to decode Int but // found a string/data instead.", 
// underlyingError: nil)) 
// 
// ---------------------------------- 
// 
// Data currupted. The given data was not valid JSON. No value for key in object around character 14. 
//  
// vs 
// 
// dataCorrupted(Swift.DecodingError.Context(codingPath: [], debugDescription: "The given data was not
// valid JSON.", underlyingError: Optional(Error Domain=NSCocoaErrorDomain Code=3840 "No value for key 
// in object around character 14." UserInfo={NSDebugDescription=No // value for key in object around 
// character 14.}))) 
// 
// ---------------------------------- 
// 
// Value not found. -> job.boss.name <- Expected String value but found null instead. 
//  
// vs 
// 
// valueNotFound(Swift.String, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "job", 
// intValue: nil), CodingKeys(stringValue: "boss", intValue: nil), CodingKeys(stringValue: "name", 
// intValue: nil)], debugDescription: "Expected String value but // found null instead.", underlyingError: nil)) 
// 
// ---------------------------------- 
// 
// Value not found. -> job.boss.name <- Expected String value but found null instead. 
//  
// vs 
// 
// valueNotFound(Swift.String, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "job", 
// intValue: nil), CodingKeys(stringValue: "boss", intValue: nil), CodingKeys(stringValue: "name", 
// intValue: nil)], debugDescription: "Expected String value but // found null instead.", 
// underlyingError: nil)) 
// 
// ---------------------------------- 

Nice one!