How to add (Swift4) init to Decodable protocol
I am trying to create a Codable extension capable of initializing a Decodable object (Swift 4) with only json string. So what should work:
struct MyObject: Decodable {
var title: String?
}
let myObject = MyObject(json: "{\"title\":\"The title\"}")
I think this means that I have to create an init that calls self.init with the decoder. Here is the code I came up with:
public init?(json: String) throws {
guard let decoder: Decoder = GetDecoder.decode(json: json)?.decoder else { return }
try self.init(from: decoder) // Who what should we do to get it so that we can call this?
}
This code is able to get a decoder, but I get a compiler error when calling init. The error I am getting:
'self' used before calling self.init
Does this mean that there is no way to add init to the Decodable protocol?
For full source code see the extensible extension for github
update:
After debugging the solution below from @appzYourLive I found out that I had a conflict with the initializers init(json:
on Decodable and Array. I just posted a new version of the extension for GitHub. I've also added a solution as a new answer to this question.
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3 answers
Possible workaround
DecodableFromString
Possible solution is defining another protocol
protocol DecodableFromString: Decodable { }
with its own initializer
extension DecodableFromString {
init?(from json: String) throws {
guard let data = try json.data(using: .utf8) else { return nil }
guard let value = try? JSONDecoder().decode(Self.self, from: data) else { return nil }
self = value
}
}
Corresponds to DecodableFromString
Now you need to match the type DecodableFromString
struct Person:Codable, DecodableFromString {
let firstName: String
let lastName: String
}
Result
And finally got the JSON
let json = """
{
"firstName": "Luke",
"lastName": "Skywalker"
}
"""
you can create your value
if let luke = try? Person(from: json) {
print(luke)
}
Person (firstName: "Luke", lastName: "Skywalker")
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This is what can be done:
extension Decodable {
init?(jsonString: String) throws {
guard let jsonData = jsonString.data(using: .utf8) else { return nil }
self = try JSONDecoder().decode(Self.self, from: jsonData)
}
}
See: https://bugs.swift.org/browse/SR-5356
[UPD] Problem fixed in XCode 9 beta 3 (see link above for details).
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My original problem was caused by two reasons:
- Conflict between init with identical signature, which I added as an extension to an array, which is also Codable when its internal objects are Encoded.
- A quick compiler error that causes a problem when using a failover initializer. See https://bugs.swift.org/browse/SR-5356
Since there is a problem using the failable initialiser, I ended up with:
public extension Decodable {
init(json: String) throws {
guard let data = json.data(using: .utf8) else { throw CodingError.RuntimeError("cannot create data from string") }
try self.init(data: data, keyPath: keyPath)
}
init(data: Data) throws {
self = try JSONDecoder().decode(Self.self, from: data)
}
}
enum CodingError : Error {
case RuntimeError(String)
}
I also made an option where you can use keyPath to navigate to a specific section:
public extension Decodable {
init(json: String, keyPath: String? = nil) throws {
guard let data = json.data(using: .utf8) else { throw CodingError.RuntimeError("cannot create data from string") }
try self.init(data: data, keyPath: keyPath)
}
init(data: Data, keyPath: String? = nil) throws {
if let keyPath = keyPath {
let topLevel = try JSONSerialization.jsonObject(with: data, options: JSONSerialization.ReadingOptions.mutableContainers)
guard let nestedJson = (topLevel as AnyObject).value(forKeyPath: keyPath) else { throw CodingError.RuntimeError("Cannot decode data to object") }
let nestedData = try JSONSerialization.data(withJSONObject: nestedJson)
self = try JSONDecoder().decode(Self.self, from: nestedData)
return
}
self = try JSONDecoder().decode(Self.self, from: data)
}
}
Below you can find the complete code for the Decodable and Encodable extensions. This is also on my GitHub subtype. With this extension, you can use code like:
struct YourCodableObject : Codable {
var naam: String?
var id: Int?
}
let json = yourEncodableObjectInstance.toJsonString()
let data = yourEncodableObjectInstance.toJsonData()
let newObject = try? YourCodableObject(json: json)
let newObject2 = try? YourCodableObject(data: data)
let objectArray = try? [YourCodableObject](json: json)
let objectArray2 = try? [YourCodableObject](data: data)
let newJson = objectArray.toJsonString()
let innerObject = try? TestCodable(json: "{\"user\":{\"id\":1,\"naam\":\"Edwin\"}}", keyPath: "user")
try initialObject.saveToDocuments("myFile.dat")
let readObject = try? TestCodable(fileNameInDocuments: "myFile.dat")
try objectArray.saveToDocuments("myFile2.dat")
let objectArray3 = try? [TestCodable](fileNameInDocuments: "myFile2.dat")
And here are 2 extensions:
//
// Codable.swift
// Stuff
//
// Created by Edwin Vermeer on 28/06/2017.
// Copyright © 2017 EVICT BV. All rights reserved.
//
enum CodingError : Error {
case RuntimeError(String)
}
public extension Encodable {
/**
Convert this object to json data
- parameter outputFormatting: The formatting of the output JSON data (compact or pritty printed)
- parameter dateEncodinStrategy: how do you want to format the date
- parameter dataEncodingStrategy: what kind of encoding. base64 is the default
- returns: The json data
*/
public func toJsonData(outputFormatting: JSONEncoder.OutputFormatting = .compact, dateEncodingStrategy: JSONEncoder.DateEncodingStrategy = .deferredToDate, dataEncodingStrategy: JSONEncoder.DataEncodingStrategy = .base64Encode) -> Data? {
let encoder = JSONEncoder()
encoder.outputFormatting = outputFormatting
encoder.dateEncodingStrategy = dateEncodingStrategy
encoder.dataEncodingStrategy = dataEncodingStrategy
return try? encoder.encode(self)
}
/**
Convert this object to a json string
- parameter outputFormatting: The formatting of the output JSON data (compact or pritty printed)
- parameter dateEncodinStrategy: how do you want to format the date
- parameter dataEncodingStrategy: what kind of encoding. base64 is the default
- returns: The json string
*/
public func toJsonString(outputFormatting: JSONEncoder.OutputFormatting = .compact, dateEncodingStrategy: JSONEncoder.DateEncodingStrategy = .deferredToDate, dataEncodingStrategy: JSONEncoder.DataEncodingStrategy = .base64Encode) -> String? {
let data = self.toJsonData(outputFormatting: outputFormatting, dateEncodingStrategy: dateEncodingStrategy, dataEncodingStrategy: dataEncodingStrategy)
return data == nil ? nil : String(data: data!, encoding: .utf8)
}
/**
Save this object to a file in the temp directory
- parameter fileName: The filename
- returns: Nothing
*/
public func saveTo(_ fileURL: URL) throws {
guard let data = self.toJsonData() else { throw CodingError.RuntimeError("cannot create data from object")}
try data.write(to: fileURL, options: .atomic)
}
/**
Save this object to a file in the temp directory
- parameter fileName: The filename
- returns: Nothing
*/
public func saveToTemp(_ fileName: String) throws {
let fileURL = try FileManager.default.url(for: .cachesDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent(fileName)
try self.saveTo(fileURL)
}
#if os(tvOS)
// Save to documents folder is not supported on tvOS
#else
/**
Save this object to a file in the documents directory
- parameter fileName: The filename
- returns: true if successfull
*/
public func saveToDocuments(_ fileName: String) throws {
let fileURL = try FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent(fileName)
try self.saveTo(fileURL)
}
#endif
}
public extension Decodable {
/**
Create an instance of this type from a json string
- parameter json: The json string
- parameter keyPath: for if you want something else than the root object
*/
init(json: String, keyPath: String? = nil) throws {
guard let data = json.data(using: .utf8) else { throw CodingError.RuntimeError("cannot create data from string") }
try self.init(data: data, keyPath: keyPath)
}
/**
Create an instance of this type from a json string
- parameter data: The json data
- parameter keyPath: for if you want something else than the root object
*/
init(data: Data, keyPath: String? = nil) throws {
if let keyPath = keyPath {
let topLevel = try JSONSerialization.jsonObject(with: data, options: JSONSerialization.ReadingOptions.mutableContainers)
guard let nestedJson = (topLevel as AnyObject).value(forKeyPath: keyPath) else { throw CodingError.RuntimeError("Cannot decode data to object") }
let nestedData = try JSONSerialization.data(withJSONObject: nestedJson)
let value = try JSONDecoder().decode(Self.self, from: nestedData)
self = value
return
}
self = try JSONDecoder().decode(Self.self, from: data)
}
/**
Initialize this object from an archived file from an URL
- parameter fileNameInTemp: The filename
*/
public init(fileURL: URL) throws {
let data = try Data(contentsOf: fileURL)
try self.init(data: data)
}
/**
Initialize this object from an archived file from the temp directory
- parameter fileNameInTemp: The filename
*/
public init(fileNameInTemp: String) throws {
let fileURL = try FileManager.default.url(for: .cachesDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent(fileNameInTemp)
try self.init(fileURL: fileURL)
}
/**
Initialize this object from an archived file from the documents directory
- parameter fileNameInDocuments: The filename
*/
public init(fileNameInDocuments: String) throws {
let fileURL = try FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent(fileNameInDocuments)
try self.init(fileURL: fileURL)
}
}
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