I am using struct in swift.
class Constants {
struct const {
static let signupFirstName = "signupFirstName"
}
}
I want to iterate the struct. For iterating I am using :
let mirrored_object = Mirror(reflecting: Constants.const())
for (index, attr) in mirrored_object.children.enumerate() {
if let property_name = attr.label as String! {
print("Attr \(index): \(property_name) = \(attr.value)")
}
}
But it does not enter into the code because of static value. Is there any way to iterate this struct?
Since static members are technically part of the type and not the instance, you would need to approach it this way to reflect on the type itself:
let mirrored_object = Mirror(reflecting: Constants.const.self)
However, Swift's automatic reflection for types themselves doesn't appear to be implemented at this time, so even the above line won't work.
That leaves one last option, which is defining your own custom mirror for reflecting on an instance of your type. That could look something like this:
class Constants {
struct const : CustomReflectable {
static let signupFirstName = "signupFirstName"
func customMirror() -> Mirror {
return Mirror(self, children: ["signupFirstName" : const.signupFirstName])
}
}
}
If you modify your code with a CustomReflectable implementation similar to the above, your loop to iterate through the struct members will now work.
Swift reflection will eventually get better out of the box, you might want to try a different approach until then.
You can do it directly by accessing the variable at class level
if let property_value = const.signupFirstName as String! {
print("hello \(property_value)")
}
Be sure to access it from the class it self const not an instance const(). Because the variable is static you can not access it from instance, you have to directly use the class
Related
I'm refactoring a project to use MVVM and using protocols to ensure that my view models have a consistent structure. This works fine for defining public properties relating to input and output (which are based on internal structs) but defining actions in the same way is proving problemmatic as, currently, they are defined as closures which have to refer to view model properties. If I use the same approach as I have to input and output properties, I don't think I can access properties of the containing instance.
Example:
protocol ViewModelType {
associatedtype Input
associatedtype Output
associatedtype Action
}
final class MyViewModel: ViewModelType {
struct Input { var test: String }
struct Output { var result: String }
struct Action {
lazy var createMyAction: Action<String, Void> = { ... closure to generate Action which uses a MyViewModel property }
}
var input: Input
var output: Output
var action: Action
}
It's not a deal breaker if I can't do it, but I was curious as I can't see any way of getting access to the parent's properties.
Answer to your question
Let's begin with a note that createMyAction: Action<String, Void> refers to the type (struct) named Action as if it was a generic, but you have not declared it as such and will thus not work.
And to answer your question of the nested struct Action can refer its outer class MyViewModel - yes you can refer static properties, like this:
struct Foo {
struct Bar {
let biz = Foo.buz
}
static let buz = "buz"
}
let foobar = Foo.Bar()
print(foobar.biz)
But you should probably avoid such circular references. And I will omit any ugly hack that might be able to achive such a circular reference on non static properties (would probably involve mutable optional types). It is a code smell.
Suggestion for MVVM
Sounds like you would like to declare Action as a function? I'm using this protocol myself:
protocol ViewModelType {
associatedtype Input
associatedtype Output
func transform(input: Input) -> Output
}
Originally inspired by SergDort's CleanArchitecture.
You can prepare an instance of input (containing Observables) from the UIViewController and call the transform function and then map the Output of transform (being Observabless) to update the GUI.
So this code assumes you have basic Reactive knowledge. As for Observables you can chose between RxSwift or ReactiveSwift - yes their names are similar.
If you are comfortable with Rx, it is an excellent way of achieving a nice MVVM architecture with simple async updates of the GUI. In the example below, you will find the type Driver which is documented here, but the short explanation is that is what you want to use for input from views and input to views, since it updates the views on the GUI thread and it is guaranteed to not error out.
CleanArchitecture contains e.g. PostsViewModel :
final class PostsViewModel: ViewModelType {
struct Input {
let trigger: Driver<Void>
let createPostTrigger: Driver<Void>
let selection: Driver<IndexPath>
}
struct Output {
let fetching: Driver<Bool>
let posts: Driver<[PostItemViewModel]>
let createPost: Driver<Void>
let selectedPost: Driver<Post>
let error: Driver<Error>
}
private let useCase: PostsUseCase
private let navigator: PostsNavigator
init(useCase: PostsUseCase, navigator: PostsNavigator) {
self.useCase = useCase
self.navigator = navigator
}
func transform(input: Input) -> Output {
let activityIndicator = ActivityIndicator()
let errorTracker = ErrorTracker()
let posts = input.trigger.flatMapLatest {
return self.useCase.posts()
.trackActivity(activityIndicator)
.trackError(errorTracker)
.asDriverOnErrorJustComplete()
.map { $0.map { PostItemViewModel(with: $0) } }
}
let fetching = activityIndicator.asDriver()
let errors = errorTracker.asDriver()
let selectedPost = input.selection
.withLatestFrom(posts) { (indexPath, posts) -> Post in
return posts[indexPath.row].post
}
.do(onNext: navigator.toPost)
let createPost = input.createPostTrigger
.do(onNext: navigator.toCreatePost)
return Output(fetching: fetching,
posts: posts,
createPost: createPost,
selectedPost: selectedPost,
error: errors)
}
}
I'm using structs instead of classes to store data in my iOS app because of the obvious advantage of value vs reference types. However, I'm trying to figure out how to architect groups of similar content. User posts may consist of images, text, and/or titles. If I were using classes the approach I would use is having a common Post superclass with different subclasses representing different types of posts. That way I could pass Post data around and cast as needed. However, structs don't allow for inheritance, so how could I architect something similar?
In Swift with struct you can create protocol for common task and also implement default implementation using protocol extension.
protocol Vehicle {
var model: String { get set }
var color: String { get set }
}
//Common Implementation using protocol extension
extension Vehicle {
static func parseVehicleFields(jsonDict: [String:Any]) -> (String, String) {
let model = jsonDict["model"] as! String
let color = jsonDict["color"] as! String
return (model, color)
}
}
struct Car : Vehicle {
var model:String
var color:String
let horsepower: Double
let license_plate: String
init(jsonDict: [String:Any]) {
(model, color) = Car.parseVehicleFields(jsonDict: jsonDict)
horsepower = jsonDict["horsepower"] as! Double
license_plate = jsonDict["license_plate"] as! String
}
}
struct Bicycle : Vehicle {
var model:String
var color:String
let chainrings: Int
let sprockets: Int
init(jsonDict: [String:Any]) {
(model, color) = Bicycle.parseVehicleFields(jsonDict: jsonDict)
chainrings = jsonDict["chainrings"] as! Int
sprockets = jsonDict["sprockets"] as! Int
}
}
There is a detailed answer at following Gist, with all possible approaches. I do not like any of them as I am fan of Classes. But structs are future of Swift, you have to understand, adopt and like :( it .
Link: https://gist.github.com/AliSoftware/9e4946c8b6038572d678
I'm struggling to understand class/reference type behavior and how this relates to changes as I try to upgrade and reduce code using Codable in Swift 4.
I have two classes – a SuperClass with all of the data that will be persistent and that I save to UserDefaults (a place name & string with coordinates), and a SubClass that contains additional, temporary info that I don't need (weather data for the SuperClass coordinates).
In Swift 3 I used to save data like this:
func saveUserDefaults() {
var superClassArray = [SuperClass]()
// subClassArray is of type [SubClass] and contains more data per element.
superClassArray = subClassArray
let superClassData = NSKeyedArchiver.archivedData(withRootObject: superClassArray)
UserDefaults.standard.set(superClassData, forKey: " superClassData")
}
SuperClass conformed to NSObject & NSCoding
It also included the required init decoder & the encode function.
It all worked fine.
In trying to switch to Swift 4 & codable I've modified SuperClass to conform to Codable.
SuperClass now only has one basic initializer and none of the encoder/decoder stuff from Swift 3. There is no KeyedArchiving happening with this new approach (below). SubClass remains unchanged. Unfortunately I crash on the line where I try? encoder.encode [giving a Thread 1: EXC_BAD_ACCESS (code=1, address=0x10)]. My assumption is that the encoder is getting confused with identical reference types where one is SuperClass and one SubClass (subClassArray[0] === superClassArray[0] is true).
I thought this might work:
func saveUserDefaults() {
var superClassArray = [SuperClass]()
superClassArray = subClassArray
// assumption was that the subclass would only contain parts of the superclass & wouldn't produce an error when being encoded
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(superClassArray){
UserDefaults.standard.set(encoded, forKey: " superClassArray ")
} else {
print("Save didn't work!")
}
}
Then, instead of creating an empty superClassArray, then using:
superClassArray = subClassArray, as shown above, I replace this with the single line:
let superClassArray: [SuperClass] = subClassArray.map{SuperClass(name: $0.name, coordinates: $0.coordinates)}
This works. Again, assumption is because I'm passing in the values inside of the class reference type & haven't made the superClassArray = subClassArray. Also, as expected, subClassArray[0] === superClassArray[0] is false
So why did the "old stuff" in Swift 3 work, even though I used the line superClassArray = subClassArray before the let superClassData = NSKeyedArchiver.archivedData(withRootObject: superClassArray)
? Am I essentially achieving the same result by creating the array in Swift 4 that was happening with the old Swift 3 encoder/decoder? Is the looping / recreation
Thanks!
Polymorphic persistence appears to be broken by design.
The bug report SR-5331 quotes the response they got on their Radar.
Unlike the existing NSCoding API (NSKeyedArchiver), the new Swift 4 Codable implementations do not write out type information about encoded types into generated archives, for both flexibility and security. As such, at decode time, the API can only use the concrete type your provide in order to decode the values (in your case, the superclass type).
This is by design — if you need the dynamism required to do this, we recommend that you adopt NSSecureCoding and use NSKeyedArchiver/NSKeyedUnarchiver
I am unimpressed, having thought from all the glowing articles that Codable was the answer to some of my prayers. A parallel set of Codable structs that act as object factories is one workaround I'm considering, to preserve type information.
Update I have written a sample using a single struct that manages recreating polymorphic classes. Available on GitHub.
I was not able to get it to work easily with subclassing. However, classes that conform to a base protocol can apply Codable for default encoding. The repo contains both keyed and unkeyed approaches. The simpler is unkeyed, copied below
// Demo of a polymorphic hierarchy of different classes implementing a protocol
// and still being Codable
// This variant uses unkeyed containers so less data is pushed into the encoded form.
import Foundation
protocol BaseBeast {
func move() -> String
func type() -> Int
var name: String { get }
}
class DumbBeast : BaseBeast, Codable {
static let polyType = 0
func type() -> Int { return DumbBeast.polyType }
var name:String
init(name:String) { self.name = name }
func move() -> String { return "\(name) Sits there looking stupid" }
}
class Flyer : BaseBeast, Codable {
static let polyType = 1
func type() -> Int { return Flyer.polyType }
var name:String
let maxAltitude:Int
init(name:String, maxAltitude:Int) {
self.maxAltitude = maxAltitude
self.name = name
}
func move() -> String { return "\(name) Flies up to \(maxAltitude)"}
}
class Walker : BaseBeast, Codable {
static let polyType = 2
func type() -> Int { return Walker.polyType }
var name:String
let numLegs: Int
let hasTail: Bool
init(name:String, legs:Int=4, hasTail:Bool=true) {
self.numLegs = legs
self.hasTail = hasTail
self.name = name
}
func move() -> String {
if numLegs == 0 {
return "\(name) Wriggles on its belly"
}
let maybeWaggle = hasTail ? "wagging its tail" : ""
return "\(name) Runs on \(numLegs) legs \(maybeWaggle)"
}
}
// Uses an explicit index we decode first, to select factory function used to decode polymorphic type
// This is in contrast to the current "traditional" method where decoding is attempted and fails for each type
// This pattern of "leading type code" can be used in more general encoding situations, not just with Codable
//: **WARNING** there is one vulnerable practice here - we rely on the BaseBeast types having a typeCode which
//: is a valid index into the arrays `encoders` and `factories`
struct CodableRef : Codable {
let refTo:BaseBeast //In C++ would use an operator to transparently cast CodableRef to BaseBeast
typealias EncContainer = UnkeyedEncodingContainer
typealias DecContainer = UnkeyedDecodingContainer
typealias BeastEnc = (inout EncContainer, BaseBeast) throws -> ()
typealias BeastDec = (inout DecContainer) throws -> BaseBeast
static var encoders:[BeastEnc] = [
{(e, b) in try e.encode(b as! DumbBeast)},
{(e, b) in try e.encode(b as! Flyer)},
{(e, b) in try e.encode(b as! Walker)}
]
static var factories:[BeastDec] = [
{(d) in try d.decode(DumbBeast.self)},
{(d) in try d.decode(Flyer.self)},
{(d) in try d.decode(Walker.self)}
]
init(refTo:BaseBeast) {
self.refTo = refTo
}
init(from decoder: Decoder) throws {
var container = try decoder.unkeyedContainer()
let typeCode = try container.decode(Int.self)
self.refTo = try CodableRef.factories[typeCode](&container)
}
func encode(to encoder: Encoder) throws {
var container = encoder.unkeyedContainer()
let typeCode = self.refTo.type()
try container.encode(typeCode)
try CodableRef.encoders[typeCode](&container, refTo)
}
}
struct Zoo : Codable {
var creatures = [CodableRef]()
init(creatures:[BaseBeast]) {
self.creatures = creatures.map {CodableRef(refTo:$0)}
}
func dump() {
creatures.forEach { print($0.refTo.move()) }
}
}
//: ---- Demo of encoding and decoding working ----
let startZoo = Zoo(creatures: [
DumbBeast(name:"Rock"),
Flyer(name:"Kookaburra", maxAltitude:5000),
Walker(name:"Snake", legs:0),
Walker(name:"Doggie", legs:4),
Walker(name:"Geek", legs:2, hasTail:false)
])
startZoo.dump()
print("---------\ntesting JSON\n")
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let encData = try encoder.encode(startZoo)
print(String(data:encData, encoding:.utf8)!)
let decodedZoo = try JSONDecoder().decode(Zoo.self, from: encData)
print ("\n------------\nAfter decoding")
decodedZoo.dump()
Update 2020-04 experience
This approach continues to be more flexible and superior to using Codable, at the cost of a bit more programmer time. It is used very heavily in the Touchgram app which provides rich, interactive documents inside iMessage.
There, I need to encode multiple polymorphic hierarchies, including different Sensors and Actions. By storing signatures of decoders, it not only provides with subclassing but also allows me to keep older decoders in the code base so old messages are still compatible.
Is there anyway to use conversion using a variable? I am using object stacking using type of "AnyObject" and I've been able to take the class type and populate a variable. Now I need to populate an array using conversion.
var myString = "Hello World"
var objectStack = [AnyObject]()
objectStack.append(myString)
let currentObject = String(describing: objectStack.last!)
var objectType = String()
let range: Range<String.Index> = currentObject.range(of: ":")!
objectType = currentObject.substring(to: range.lowerBound)
let range2: Range<String.Index> = objectType.range(of: ".")!
objectType = objectType.substring(from: range2.upperBound)
The code above will evaluate the class and set the value of "objectType" to "String". Now I'm trying to go the other way. Something like this:
for obj in objectStack{
obj = newObject as! objectType //this doesn't work
}
Is something like this possible?
There is a simpler, safer way to get the type:
let type = type(of: objectStack.last!) // String.Type
let typeString = String(describing: type) // "String"
The other way around is not possible because the type of the object is not known at compile time. Do you have a number of known types you want to try to cast to? In that case, use optional binding to check if the cast is successful:
let object = objectStack.last!
if let string = object as? String {
// do String stuff
}
else if let i = object as? Int {
// do Int stuff
}
// and so on
If you have a large number of possible types that share some common functionality: Use Protocols. See Swift Documentation for a nice introduction.
You define a protocol for some common functionality that different types can implement:
protocol Stackable {
func doStuff()
// (more methods or properties if necessary)
}
The protocol provides a contract that all types conforming to this protocol have to fulfill by providing implementations for all declared methods and properties. Let's create a struct that conforms to Stackable:
struct Foo: Stackable {
func doStuff() {
print("Foo is doing stuff.")
}
}
You can also extend existing types to make them conform to a protocol. Let's make String Stackable:
extension String: Stackable {
func doStuff() {
print("'\(self)' is pretending to do stuff.")
}
}
Let's try it out:
let stack: [Stackable] = [Foo(), "Cat"]
for item in stack {
item.doStuff()
}
/*
prints the following:
Foo is doing stuff.
'Cat' is pretending to do stuff.
*/
This worked for me:
var instance: AnyObject! = nil
let classInst = NSClassFromString(objectType) as! NSObject.Type
instance = classInst.init()
I have this class named Meal
class Meal {
var name : String = ""
var cnt : Int = 0
var price : String = ""
var img : String = ""
var id : String = ""
init(name:String , cnt : Int, price : String, img : String, id : String) {
self.name = name
self.cnt = cnt
self.price = price
self.img = img
self.id = id
}
}
and I have an array of Meal :
var ordered = [Meal]()
I want to duplicate that array and then do some changes to the Meal instances in one of them without changing the Meal instances in the second one, how would I make a deep copy of it?
This search result didn't help me
How do I make a exact duplicate copy of an array?
Since ordered is a swift array, the statement
var orderedCopy = ordered
will effectively make a copy of the original array.
However, since Meal is a class, the new array will contain references
to the same meals referred in the original one.
If you want to copy the meals content too, so that changing a meal in one array will not change a meal in the other array, then you must define Meal as a struct, not as a class:
struct Meal {
...
From the Apple book:
Use struct to create a structure. Structures support many of the same behaviors as classes, including methods and initializers. One of the most important differences between structures and classes is that structures are always copied when they are passed around in your code, but classes are passed by reference.
To improve on #Kametrixom answer check this:
For normal objects what can be done is to implement a protocol that supports copying, and make the object class implements this protocol like this:
protocol Copying {
init(original: Self)
}
extension Copying {
func copy() -> Self {
return Self.init(original: self)
}
}
And then the Array extension for cloning:
extension Array where Element: Copying {
func clone() -> Array {
var copiedArray = Array<Element>()
for element in self {
copiedArray.append(element.copy())
}
return copiedArray
}
}
and that is pretty much it, to view code and a sample check this gist
You either have to, as #MarioZannone mentioned, make it a struct, because structs get copied automatically, or you may not want a struct and need a class. For this you have to define how to copy your class. There is the NSCopying protocol which unifies that on the ObjC world, but that makes your Swift code "unpure" in that you have to inherit from NSObject. I suggest however to define your own copying protocol like this:
protocol Copying {
init(original: Self)
}
extension Copying {
func copy() -> Self {
return Self.init(original: self)
}
}
which you can implement like this:
class Test : Copying {
var x : Int
init() {
x = 0
}
// required initializer for the Copying protocol
required init(original: Test) {
x = original.x
}
}
Within the initializer you have to copy all the state from the passed original Test on to self. Now that you implemented the protocol correctly, you can do something like this:
let original = Test()
let stillOriginal = original
let copyOriginal = original.copy()
original.x = 10
original.x // 10
stillOriginal.x // 10
copyOriginal.x // 0
This is basically the same as NSCopying just without ObjC
EDIT: Sadly this yet so beautiful protocol works very poorly with subclassing...
A simple and quick way is to map the original array into the new copy:
let copyOfPersons: [Person] = allPersons.map({(originalPerson) -> Person in
let newPerson = Person(name: originalPerson.name, age: originalPerson.age)
return newPerson
})
The new Persons will have different pointers but same values.
Based on previous answer here
If you have nested objects, i.e. subclasses to a class then what you want is True Deep Copy.
//Example
var dogsForAdoption: Array<Dog>
class Dog{
var breed: String
var owner: Person
}
So this means implementing NSCopying in every class(Dog, Person etc).
Would you do that for say 20 of your classes? what about 30..50..100? You get it right? We need native "it just works!" way. But nope we don't have one. Yet.
As of now, Feb 2021, there is no proper solution of this issue. We have many workarounds though.
Here is the one I have been using, and one with less limitations in my opinion.
Make your class conforms to codable
class Dog: Codable{
var breed : String = "JustAnyDog"
var owner: Person
}
Create this helper class
class DeepCopier {
//Used to expose generic
static func Copy<T:Codable>(of object:T) -> T?{
do{
let json = try JSONEncoder().encode(object)
return try JSONDecoder().decode(T.self, from: json)
}
catch let error{
print(error)
return nil
}
}
}
Call this method whenever you need true deep copy of your object, like this:
//Now suppose
let dog = Dog()
guard let clonedDog = DeepCopier.Copy(of: dog) else{
print("Could not detach Dog")
return
}
//Change/mutate object properties as you want
clonedDog.breed = "rottweiler"
//Also clonedDog.owner != dog.owner, as both the owner : Person have dfferent memory allocations
As you can see we are piggy backing on Swift's JSONEncoder and JSONDecoder, using power of Codable, making true deep copy no matter how many nested objects are there under our object. Just make sure all your Classes conform to Codable.
Though its NOT an ideal solution, but its one of the most effective workaround.