I am trying to create a singleton class in swift but I am getting an error
"cannot create a single-element tuple with an element label"
i am not getting it.
class GroupObject {
// we want the group object to be a singleton
var name: String
var id: Int
var groupJsonObject: JSON
init(groupJsonObject: JSON){
self.groupJsonObject = groupJsonObject
self.id = groupJsonObject["id"].int!
self.name = groupJsonObject["name"].string!
}
class var sharedInstance : GroupObject {
struct Static {
static let instance : GroupObject = GroupObject(groupJsonObject: JSON) // this is the problem line.
}
return Static.instance
}
}
The problem is that you cannot pass a parameter to the singleton. Your singleton implementation doesn't know to what JSON refers.
If you want this to be a singleton, you'd have to initialize the groupJsonObject separately from the initialization of the shared instance. For example:
class GroupObject {
var name: String!
var id: Int!
var groupJsonObject: JSON! {
didSet {
id = groupJsonObject["id"].int!
name = groupJsonObject["name"].string!
}
}
static let sharedInstance = GroupObject() // btw, this is a more concise syntax for declaring a singleton
}
And then, when you want to initialize those properties, you could do:
GroupObject.sharedInstance.groupJsonObject = json
If your "singleton" is supposed to hold some data passed to it on instantiation, how will it get that data? Where/when is it available?
I think you don't actually want a singleton at all; you want an instance created with your JSON data to be accessible from different points in your application. In that case, pick some "master controller", create it there, then pass it along to other controllers as needed.
Related
In the following example code, I create a struct and a class with similar members. With the struct I can initialize an instance by any number of the members into its constructor, and the rest will default. With a class, I have to specify every version of init I want to use. Seems like I must be missing some way to do it with a class though -- is there any way to do this? It looks like in 2016 there was not, but I know Swift has changed a ton since then. I'm hoping there is a way now.
import Foundation
struct FooStruct {
var id: UUID = UUID()
var title = ""
}
// these statements both work fine
let a = FooStruct(id: UUID())
let a2 = FooStruct(title: "bar")
class FooClass {
var id: UUID = UUID()
var title = ""
}
// these statements both give the same error:
// Argument passed to call that takes no arguments
let b = FooClass(id: UUID())
let b2 = FooClass(title: "bar")
What you are seeing with Structure types is what is called a memberwise initializer. Swift does not provide one of these to Class types because of the more complex way Classes are initialized, due to their inheritance model.
Swift provides a default initializer—different than a memberwise initializer—for any structure or class that provides default values for all of its properties and doesn’t provide at least one initializer itself. The default initializer simply creates a new instance with all of its properties set to their default values.
you could just use this:
class FooClass {
var id: UUID = UUID()
var title = ""
init(id: UUID = UUID(), title: String = ""){
self.id = id
self.title = title
}
}
and this will work:
let b = FooClass(id: UUID())
let b2 = FooClass(title: "bar")
I am initializing Realm object and I am getting these errors when I try to initialize the class:
After I add the initializer for realm, the errors are still there. Is this a bug with xcode?
Looking at the sample code for swift-realm ios project here, it looks like I don't need to call the required realm. Maybe the sample code is outdated.
EDIT ---
Here's a paste of the code:
class AgencyR: Object {
#objc dynamic var agency_id: String = ""
#objc dynamic var agency_name: String = ""
#objc dynamic var agency_timezone: String = ""
#objc dynamic var agency_url: String = ""
#objc dynamic var agency_lang: String = ""
#objc dynamic var agency_phone: String = ""
#objc dynamic var agency_fare_url: String = ""
required init(realm:Realm, agency_id: String, agency_name: String, agency_timezone: String, agency_url: String, agency_lang: String, agency_phone: String, agency_fare_url: String) {
self.init()
self.agency_id = agency_id
self.agency_name = agency_name
self.agency_timezone = agency_timezone
self.agency_url = agency_url
self.agency_lang = agency_lang
self.agency_phone = agency_phone
self.agency_fare_url = agency_fare_url
}
override static func primaryKey() -> String? {
return self.agency_id
}
}
If you wanted to create a designated initializer for an Object subclass, you'd need to implement all required initializers of Object since the compiler will not be able to synthetise those for you anymore.
You can get around this issue by making your custom initializer a convenience initializer rather than a designated one, which will allow you to call a designated initializer of the class rather than having to call the superclass initializer. You can still mark the convenience initializer as required.
You also have an issue in your primaryKey function. Since the function is a type function, you don't have access to the instance from inside the function, so you cannot call self to access an instance property. However, there's no need to do that anyways, since you simply need to return the variable name as a String that you want to use as a primary key.
class A: Object {
#objc dynamic var a = 1
required convenience init(_ a:Int) {
self.init()
self.a = a
}
override static func primaryKey()->String?{
return "a"
}
}
It should be -> Object, instead of ->String? as agency_id is of type Object.Try this.
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
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.
I have a model class written in Objective-C that I'm converting to Swift. It contains an NSMutableArray internally, but the method signature for the getter, as well as the actual return value, are NSArray. When called, it creates an immutable copy to return.
Essentially, I want callers to be able to iterate/inspect the container, but not modify it. I have this test snippet:
class Container {
internal var myItems = [String]()
func sayHello() {
"I have: \(myItems)"
}
}
let cont = Container()
cont.myItems.append("Neat") // ["Neat"]
cont.sayHello() // This causes sayHello() to print: "I have: [Neat]"
var isThisACopy = cont.myItems
isThisACopy.append("Huh") // ["Neat", "Huh"]
cont.sayHello() // This ALSO causes sayHello() to print: "I have: [Neat]"
I've been trying to find a way to override the getter for myItems so that it returns an immutable copy, but can't seem to determine how.
Attempt #1
This produces a compiler error: Function produces expected type '_ArrayBuffer<(String)>'; did you mean to call it with '()'?
internal var myItems = [String]() {
var copy = [String]()
for item in ... { // What to use in the ...?
copy.append(item)
}
return copy
}
Attempt #2
This also produces a compiler error, because I'm (understandably) redefining the generated getter Invalid redeclaration of 'myItems()':
internal func myItems() -> [String] {
var copy = [String]()
for item in myItems {
copy.append(item)
}
return copy
}
Try this:
class Container {
private var _myItems: [String] = ["hello"]
internal var myItems: [String] {
return _myItems
}
}
let cont = Container()
cont.myItems.append("Neat") //not allowed
It uses a private stored property and a computed property that returns an immutable copy. It's not possible for a stored property to use custom getters.
A better way to expose mutable properties as immutable:
class Container {
private (set) internal var myItems: [String]
}