What I want:
To initialise a struct before use. But I really don't know how, even after a lot of trying and searching.
Example of the struct:
struct Person: Decodable {
let name: String
let age: Int
}
I want to use this struct global in my script. The way I think it would have been is like this (but it does not work, because of line 3):
import UIKit
class ViewController: UIViewController {
let personExample: Person()
func configure(person: Person) {
self.personExample = person
}
override func viewDidLoad() {
super.viewDidLoad()
print(self.personExample)
}
}
The syntax here is simply incorrect:
let personExample: Person()
What you meant was this:
let personExample: Person
Person() is the result of calling init(), which is a value, not a type, and isn't a valid initializer for this type anyway. Person is the type, which is what you want for a declaration.
However, what you've written here doesn't quite line up with the description. If you want a "global," this is actually an instance variable. What you may have meant here is the following:
var personExample = Person(name: "Bob", age: 21)
That would assign personExample to that value (and through type inference, assign the type to Person). I've used var here because configure(person:) tries to modify it.
You also may be confused about initialization of instance variables in view controllers when using storyboards. If that's your issue (for example, you're seeing errors about personExample not being set during initialization), you'll need to explain a little more about your situation and we can help you with that.
You need
var personExample: Person?
func configure(person: Person) {
self.personExample = person
}
first this let personExample: Person() is an incorrect syntax and if you changed to
var personExample: Person
it'll raise an error as it must be initated in the vc init methods , also if you don't implement coding , remove : Decodable {
Related
For a project I am currently working on, it would be very useful to get the KVC-String from a KeyPath instance my method is receiving. Short example:
struct Person {
var name: String
}
let propertyCache = ["name": "something"]
func method<T>(_ keypath: KeyPath<Person, T>) -> T? {
let kvcName = keypath.kvc
return propertyCache[kvcName]
}
This might seem not very useful, but in my project it is :) I found a property on KeyPath called _kvcKeyPathString which is also public, but it returns nil every time I tried.
Or is their maybe a possibility to use reflection there? Thanks in advance for ideas/solutions!
I don't know of a pure Swift way to get the name of the property as a string yet.
But, if you add the #objc attribute to the property then _kvcKeyPathString will actually have a value instead of always being nil. Also, since Swift structs can't be represented in Objective-C, this method only works for classes.
A minimal working example usage:
class SomeClass {
#objc var someProperty = 5
}
let keyPath = \SomeClass.someProperty
print(keyPath._kvcKeyPathString)
I have a struct like,
struct LoginConstants {
struct Selectors {
let testa = "test1234"
}
}
and a class like,
class Login: XCTestCase {
override class func setUp () {
// below constant will have a value like "LoginConstants"
let localConstants = "\(String(describing: self))Constants"
}
}
... so here I have a struct-name as a string format in localConstants.
My Question is how I can access the LoginConstants properties from the localConstants string?
NOTE:
I know I can access the LoginConstants() directly.
But I am planning to create a parent class where I can access this ***Constants struct dynamically.
Thanks for the help!
Objective-C has the ability to do this, but Swift does not. If you give a class an Objective-C name via the #objc attribute, you can use the Objective-C runtime functions to access it by name. However, this is not possible with a struct.
It's probably not the best way to go anyway. A better solution is to rethink what you are trying to do, and access the struct type directly rather than by name.
I am trying to use a class func to set a title for a book, however it's not working. Please see my code below:
import Foundation
class Book: NSObject {
var bookTitle: String = ""
var bookPage: String = ""
override init(){
print("Book object has been created")
}
class func setPageTitle(title: String)
{
bookTitle = title //I get the error here
}
}
I want to make it mandatory to set a pageTitle when a Book object is created.
Can someone please help me ?
The best way to set any required property/attribute is when you initialize it. So try coding your class this way:
class Book: NSObject {
var bookTitle: String = ""
var bookPage: String = ""
init(title:String) {
bookTitle = title
print("Book object has been created")
}
}
Several notes:
Your error is because you declare class in your setPageTitle function. That makes no sense.
There are better (and other) ways to set the bookTitle, including after initialization. But you specifically wanted to make sure you have a title when initializing, so there you go.
There are definitely better ways to maintain the bookTitle attribute. (Most languages teach you to hold a price variable to start with.) I'm mostly trying to give you a way to initialize it with the class.
You probably don't need (or have) any superclass call to make, but you also may not need to make your Book class a NSObject either.
import Foundation
class Student: NSObject
{
var name: String
var year: Int
var major: String
var gpa : String
init(name:String, year:Int, major:String, gpa:String)
{
self.name = name
self.year = year
self.major = major
self.gpa = gpa
}
convenience init()
{
//calls longer init method written above
}
}
--
The error shows itself atthe line of the convenience init
Overriding declaration requires an 'override' keyword
I've tried Googling this and reading guides on initializers in Swift, but it seems like they were able to make their initializers just fine without overriding anything.
init is a designated initializer for NSObject. If you override it, you must mark this as override and call a superclass designated initializer. This follows the normal rules for initializer inheritance.
it looks like your convenience initializer is empty which is why you get the error. For example if you change it to:
convenience init() {
self.init(name: "", year: 0, major: "nothing", gpa: "4.0")
}
the error would go away.
I wonder why you set up Student to inherit from NSObject. It doesn't look necessary for your class.
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.