Swift 4 added the new Codable protocol. When I use JSONDecoder it seems to require all the non-optional properties of my Codable class to have keys in the JSON or it throws an error.
Making every property of my class optional seems like an unnecessary hassle since what I really want is to use the value in the json or a default value. (I don't want the property to be nil.)
Is there a way to do this?
class MyCodable: Codable {
var name: String = "Default Appleseed"
}
func load(input: String) {
do {
if let data = input.data(using: .utf8) {
let result = try JSONDecoder().decode(MyCodable.self, from: data)
print("name: \(result.name)")
}
} catch {
print("error: \(error)")
// `Error message: "Key not found when expecting non-optional type
// String for coding key \"name\""`
}
}
let goodInput = "{\"name\": \"Jonny Appleseed\" }"
let badInput = "{}"
load(input: goodInput) // works, `name` is Jonny Applessed
load(input: badInput) // breaks, `name` required since property is non-optional
You can implement the init(from decoder: Decoder) method in your type instead of using the default implementation:
class MyCodable: Codable {
var name: String = "Default Appleseed"
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
if let name = try container.decodeIfPresent(String.self, forKey: .name) {
self.name = name
}
}
}
You can also make name a constant property (if you want to):
class MyCodable: Codable {
let name: String
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
if let name = try container.decodeIfPresent(String.self, forKey: .name) {
self.name = name
} else {
self.name = "Default Appleseed"
}
}
}
or
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
self.name = try container.decodeIfPresent(String.self, forKey: .name) ?? "Default Appleseed"
}
Re your comment: With a custom extension
extension KeyedDecodingContainer {
func decodeWrapper<T>(key: K, defaultValue: T) throws -> T
where T : Decodable {
return try decodeIfPresent(T.self, forKey: key) ?? defaultValue
}
}
you could implement the init method as
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
self.name = try container.decodeWrapper(key: .name, defaultValue: "Default Appleseed")
}
but that is not much shorter than
self.name = try container.decodeIfPresent(String.self, forKey: .name) ?? "Default Appleseed"
You can use a computed property that defaults to the desired value if the JSON key is not found.
class MyCodable: Decodable {
var name: String { return _name ?? "Default Appleseed" }
var age: Int?
// this is the property that gets actually decoded/encoded
private var _name: String?
enum CodingKeys: String, CodingKey {
case _name = "name"
case age
}
}
If you want to have the property read-write, you can also implement the setter:
var name: String {
get { _name ?? "Default Appleseed" }
set { _name = newValue }
}
This adds a little extra verbosity as you'll need to declare another property, and will require adding the CodingKeys enum (if not already there). The advantage is that you don't need to write custom decoding/encoding code, which can become tedious at some point.
Note that this solution only works if the value for the JSON key either holds a string or is not present. If the JSON might have the value under another form (e.g. it's an int), then you can try this solution.
Approach that I prefer is using so called DTOs - data transfer object.
It is a struct, that conforms to Codable and represents the desired object.
struct MyClassDTO: Codable {
let items: [String]?
let otherVar: Int?
}
Then you simply init the object that you want to use in the app with that DTO.
class MyClass {
let items: [String]
var otherVar = 3
init(_ dto: MyClassDTO) {
items = dto.items ?? [String]()
otherVar = dto.otherVar ?? 3
}
var dto: MyClassDTO {
return MyClassDTO(items: items, otherVar: otherVar)
}
}
This approach is also good since you can rename and change final object however you wish to.
It is clear and requires less code than manual decoding.
Moreover, with this approach you can separate networking layer from other app.
You can implement.
struct Source : Codable {
let id : String?
let name : String?
enum CodingKeys: String, CodingKey {
case id = "id"
case name = "name"
}
init(from decoder: Decoder) throws {
let values = try decoder.container(keyedBy: CodingKeys.self)
id = try values.decodeIfPresent(String.self, forKey: .id) ?? ""
name = try values.decodeIfPresent(String.self, forKey: .name)
}
}
I came across this question looking for the exact same thing. The answers I found were not very satisfying even though I was afraid that the solutions here would be the only option.
In my case, creating a custom decoder would require a ton of boilerplate that would be hard to maintain so I kept searching for other answers.
I ran into this article that shows an interesting way to overcome this in simple cases using a #propertyWrapper. The most important thing for me, was that it was reusable and required minimal refactoring of existing code.
The article assumes a case where you'd want a missing boolean property to default to false without failing but also shows other different variants.
You can read it in more detail but I'll show what I did for my use case.
In my case, I had an array that I wanted to be initialized as empty if the key was missing.
So, I declared the following #propertyWrapper and additional extensions:
#propertyWrapper
struct DefaultEmptyArray<T:Codable> {
var wrappedValue: [T] = []
}
//codable extension to encode/decode the wrapped value
extension DefaultEmptyArray: Codable {
func encode(to encoder: Encoder) throws {
try wrappedValue.encode(to: encoder)
}
init(from decoder: Decoder) throws {
let container = try decoder.singleValueContainer()
wrappedValue = try container.decode([T].self)
}
}
extension KeyedDecodingContainer {
func decode<T:Decodable>(_ type: DefaultEmptyArray<T>.Type,
forKey key: Key) throws -> DefaultEmptyArray<T> {
try decodeIfPresent(type, forKey: key) ?? .init()
}
}
The advantage of this method is that you can easily overcome the issue in existing code by simply adding the #propertyWrapper to the property. In my case:
#DefaultEmptyArray var items: [String] = []
Hope this helps someone dealing with the same issue.
UPDATE:
After posting this answer while continuing to look into the matter I found this other article but most importantly the respective library that contains some common easy to use #propertyWrappers for these kind of cases:
https://github.com/marksands/BetterCodable
If you don't want to implement your encoding and decoding methods, there is somewhat dirty solution around default values.
You can declare your new field as implicitly unwrapped optional and check if it's nil after decoding and set a default value.
I tested this only with PropertyListEncoder, but I think JSONDecoder works the same way.
If you think that writing your own version of init(from decoder: Decoder) is overwhelming, I would advice you to implement a method which will check the input before sending it to decoder. That way you'll have a place where you can check for fields absence and set your own default values.
For example:
final class CodableModel: Codable
{
static func customDecode(_ obj: [String: Any]) -> CodableModel?
{
var validatedDict = obj
let someField = validatedDict[CodingKeys.someField.stringValue] ?? false
validatedDict[CodingKeys.someField.stringValue] = someField
guard
let data = try? JSONSerialization.data(withJSONObject: validatedDict, options: .prettyPrinted),
let model = try? CodableModel.decoder.decode(CodableModel.self, from: data) else {
return nil
}
return model
}
//your coding keys, properties, etc.
}
And in order to init an object from json, instead of:
do {
let data = try JSONSerialization.data(withJSONObject: json, options: .prettyPrinted)
let model = try CodableModel.decoder.decode(CodableModel.self, from: data)
} catch {
assertionFailure(error.localizedDescription)
}
Init will look like this:
if let vuvVideoFile = PublicVideoFile.customDecode($0) {
videos.append(vuvVideoFile)
}
In this particular situation I prefer to deal with optionals but if you have a different opinion, you can make your customDecode(:) method throwable
Related
I have a base model called Requirements and another more specific model called AccountRequirements.
When I try to read the currentDeadline property, if i use Requirements it works fine. If I use AccountRequirements it comes out as nil.
I do not understand why. I'm guessing it has to do somehow with the class. I always use struct in my models but since I can not inherit from a struct I'm using class here.
class Requirements: Codable {
var commonProperty: String
// works
var currentDeadline: Int?
enum CodingKeys: String, CodingKey {
case commonProperty = "common_property"
case currentDeadline = "current_deadline"
}
}
class AccountRequirements: Requirements {
// doesnt work
var currentDeadline: Int?
enum CodingKeys: String, CodingKey {
case currentDeadline = "current_deadline"
}
}
I decode data like this:
documentReference.addSnapshotListener { [self] documentSnapshot, error in
guard let document = documentSnapshot else {
self.error = error!.localizedDescription
return
}
self.user = try? document.data(as: Requirements.self)
}
If you want to decode it as the subclass then you need to give that class and not the superclass to document.data(as:). You also need to implement init(from:) for the subclass to decode it properly
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
currentDeadline = try container.decodeIfPresent(Int.self, forKey: .currentDeadline)
try super.init(from: decoder)
}
Below is an example with a hardcoded json value
let data = """
{ "common_property": "Some text",
"current_deadline": 42
}
""".data(using: .utf8)!
do {
let result = try JSONDecoder().decode(Requirements.self, from: data)
print(type(of: result), result.commonProperty)
let result2 = try JSONDecoder().decode(AccountRequirements.self, from: data)
print(type(of: result2), result2.commonProperty, result2.currentDeadline ?? "")
} catch {
print(error)
}
Requirements Some text
AccountRequirements Some text 42
In my custom initializer I'd like to decode a dictionary from JSON and then assign its values to properties in the class. To my surprise, compiler does not allow me to decode the dictionary, I get the error:
Value of protocol type 'Any' cannot conform to 'Decodable'; only struct/enum/class types can conform to protocols
If I try to decode dictionary of type [String: Decodable] the error message says:
Value of protocol type 'Decodable' cannot conform to 'Decodable'; only struct/enum/class types can conform to protocols
My initializer looks like this:
public let total: Int
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
...
if let dict = try container.decodeIfPresent([String: Any].self, forKey: .tracks),
let value = dict["total"] as? Int { // Error is displayed at this line
total = value
} else {
total = 0
}
...
}
When I looked for the answer I found this answer and according to it the code above should not cause any problems.
What you are looking for is nestedContainer. It helps you "skip" a hierarchy level in your JSON. Ie: let's imagine that in your code, it's all in the same level (one struct), but in the JSON, it's not, there is a sub dictionary.
For test purpose, your JSON might look like:
{
"title": "normal",
"tracks": {
"name": "myName",
"total": 3
}
}
If we want in our model:
struct TestStruct: Codable {
let title: String
let name: String
let total: Int
}
We need to use nestedContainer(keyedBy: forKey:):
extension TestStruct {
enum TopLevelKeys: String, CodingKey {
case title
case tracks
}
enum NestedLevelCodingKeys: String, CodingKey {
case name
case total
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: TopLevelKeys.self)
self.title = try container.decode(String.self, forKey: .title)
let subcontainer = try container.nestedContainer(keyedBy: NestedLevelCodingKeys.self, forKey: TopLevelKeys.tracks)
self.name = try subcontainer.decode(String.self, forKey: .name)
self.total = try subcontainer.decode(Int.self, forKey: .total) //You can use here a `decodeIfPresent()` if needed, use default values, etc.
}
}
In your sample, you used decodeIfPresent() for the subdictionary. It's unclear if it was for test purpose, or if the sub dictionary was sometimes not present.
If that's the case and you can have a JSON like this:
{
"title": "normal"
}
Then, before calling nestedContainer(keyedBy: forKey:), use if container.contains(TopLevelKeys.tracks) {} and set default values if needed in the else case.
I am trying to parse some JSON in Swift using JSONDecoder where the JSON occasionally has null values. I would like to put in a default instead.
The following allows me to handle it but the nulls cause problems later.
struct Book: Codable {
let title : String
let author: String?
}
Is there a way to do something like (following does not compile), perhaps using an initializer?:
struct Book: Codable {
let title : String
let author: String ?? "unknown"
}
Thanks for any suggestions
This could be address by manually decoding as described here.
The other way to go would be to have the stored properties reflect the data exactly, and then have a computed var for the case of providing a non-optional value.
struct Book: Codable {
let title : String
let author: String?
var displayAuthor: String {
return author ?? "unknown"
}
}
The other reason this might be appealing is it preserves the optional value should you need to check if the value exists at all in the future.
You can achieve this using the custom init(decoder:) method definition. Use the decodeIfPresent API and give the property your desired default value if the try fails. Or you can use the computed property method mentioned by #dktaylor. Here the code you need:
struct Book {
let title : String
let author: String
enum CodingKeys: String, CodingKey {
case title, author
}
}
extension Book: Codable {
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
title = try container.decode(String.self, forKey: .title)
author = try container.decodeIfPresent(String.self, forKey: .author) ?? "unknown"
}
}
You can also achieve this with a property wrapper:
#propertyWrapper
struct OnNil<T> {
let value: T
init(_ value: T) {
self.value = value
}
private var _wrappedValue: T?
var wrappedValue: T! {
get { _wrappedValue ?? value }
set { _wrappedValue = newValue }
}
}
struct SomeStruct {
let title : String
#OnNil("unknown")
let author: String!
}
The benefits of using a property wrapper like this is that you don't have to soil your object with utility methods, and you don't have to fiddle with the decode function. The downside obviously is that the syntax looks kind of odd, you have to make some variables implicitly unwrapped optional, and it makes your code slightly harder to debug due to the nature of property wrappers.
I have the following object
struct Properties: Decodable {
var id: String?
var value: String?
var color: String?
}
In the first request to server I get the following response
{
"id":"1",
"color":"red"
}
And after another request I get
{
"id":"1", // the id of the object props is meant for
"props":{
"value":"my value" // I can get any property here
}
}
After the two requests I should have the object with all properties set
By now I decode the second request as following
struct SetAttr: Decodable {
let id: String
let props: [String : Any]
enum SetAttrCodingKeys: String, CodingKey {
case id
case props
}
init(from decoder: Decoder) throws {
let container = try! decoder.container(keyedBy: SetAttrCodingKeys.self)
props = try! container.decode([String : Any].self, forKey: .props)
id = try! container.decode(String.self, forKey: .id)
}
}
But I do not know how to parse props dictionary and set the properties on the first object. I am willing to use a decoding library, but I did not find any that can do this
EDIT:
This is how I tried to set the properties from dictionary, but the solution is not scalable
var myObject: Properties
properties = setAttr.props // [String:Any]
let keys = properties.keys
keys.forEach { key in
if let value = properties[key] {
switch key {
case "value":
myObject.value = value as? String
case "color":
myObject.color = value as? String
default:
break
}
}
}
Just use JSONSerialization which parses whatever you throw at it into arrays and dictionaries. That frees you from all the problems you have with strangely formatted JSON.
For example, the second request will be parsed as a dictionary with two keys "id" and "props", and "props" has a value which is again a dictionary with one key "value" and a value "my value".
And please stop using try! That will cause your app to crash instantly if any input is not expected. Unexpected inputs should be handled, not lead to a crash.
There are various way to do this, but one possible way could be something like this:
struct SecAttr: Decodable {
let id: String
var props: Properties?
private enum CodingKeys: String, CodingKey {
case id
case props
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
id = try container.decode(String.self, forKey: .id)
if let props = try container.decodeIfPresent(Properties.self, forKey: .props) {
self.props = props
} else {
// decode Properties from the same object
self.props = try Properties(from: decoder)
}
}
}
struct Properties: Decodable {
var value: String?
var color: String?
mutable update(from props: Properties) {
value = props.value ?? value
color = color.value ?? color
}
}
Now you can decode your original object and after getting updated properties, just update them on the original.
Here is the code I am using,
struct CreatePostResponseModel : Codable{
var transcodeId:String?
var id:String = ""
enum TopLevelCodingKeys: String, CodingKey {
case _transcode = "_transcode"
case _transcoder = "_transcoder"
}
enum CodingKeys:String, CodingKey{
case id = "_id"
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: TopLevelCodingKeys.self)
if let transcodeId = try container.decodeIfPresent(String.self, forKey: ._transcode) {
self.transcodeId = transcodeId
}else if let transcodeId = try container.decodeIfPresent(String.self, forKey: ._transcoder) {
self.transcodeId = transcodeId
}
}
}
Here, transcodeId is decided by either _transcode or _transcoder.
But I want id and rest of the keys (not included here) to be automatically decoded. How can I do it ?
You need to manually parse all the keys once you implement init(from:) in the Codable type.
struct CreatePostResponseModel: Decodable {
var transcodeId: String?
var id: String
enum CodingKeys:String, CodingKey{
case id, transcode, transcoder
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
id = try container.decodeIfPresent(String.self, forKey: .id) ?? ""
if let transcodeId = try container.decodeIfPresent(String.self, forKey: .transcode) {
self.transcodeId = transcodeId
} else if let transcodeId = try container.decodeIfPresent(String.self, forKey: .transcoder) {
self.transcodeId = transcodeId
}
}
}
In the above code,
In case you only want to decode the JSON, there is no need to use Codable. Using Decodable is enough.
Using multiple enums for CodingKey seems unnecessary here. You can use a single enum CodingKeys.
If the property name and the key name is an exact match, there is no need to explicitly specify the rawValue of that case in enum CodingKeys. So, there is no requirement of "_transcode" and "_transcoder" rawValues in TopLevelCodingKeys.
Apart from all that, you can use keyDecodingStrategy as .convertFromSnakeCase to handle underscore notation (snake case notation), i.e.
do {
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase //here.....
let model = try decoder.decode(CreatePostResponseModel.self, from: data)
print(model)
} catch {
print(error)
}
So, you don't need to explicitly handle all the snake-case keys. It'll be handled by the JSONDecoder on its own.
This can be one of the good solution for you wherever you want you can add multiple keys for one variable:
var transcodeId:String?
public init(from decoder: Decoder) throws {
do {
let container = try decoder.container(keyedBy: CodingKeys.self)
transcodeId = container.getValueFromAvailableKey(codingKeys: [CodingKeys._transcoder,CodingKeys._transcode])
} catch {
print("Error reading config file: \(error.localizedDescription)")
}
}
extension KeyedDecodingContainerProtocol{
func getValueFromAvailableKey(codingKeys:[CodingKey])-> String?{
for key in codingKeys{
for keyPath in self.allKeys{
if key.stringValue == keyPath.stringValue{
do{
return try self.decodeIfPresent(String.self, forKey: keyPath)
} catch {
return nil
}
}
}
}
return nil
}
}
Hope it helps.
The compiler-generated init(from:) is all-or-nothing. You can’t have it decode some keys and “manually” decode others.
One way to use the compiler-generated init(from:) is by giving your struct both of the possible encoded properties, and make transcodeId a computed property:
struct CreatePostResponseModel: Codable {
var transcodeId: String? {
get { _transcode ?? _transcoder }
set { _transcode = newValue; _transcoder = nil }
}
var _transcode: String? = nil
var _transcoder: String? = nil
var id: String = “”
// other properties
}