Address of Array & Remove(at :) IOS - arrays

Question

I'm passing an array of a specific model by reference between ViewControllers.
If I change any value of a specific element in the array it reflects well in all ViewControllers but when I remove an element from that array it doesn't reflect to the other controllers.
Does the remove(at: ) function create new array and refer to another address?
And if so how to delete an element without changing the address of array so it can reflect this change on the other view controllers?

Answer 1

Swift Arrays are value types (specifically, an array is a struct), not reference types, so you are mistaken when you say that you are "passing an array of a specific model by reference between view controllers". You can only ever pass a Swift array as a value.
Arrays, like other structs, have copy-on-modify semantics. As soon as you change the array itself a copy is made and the change is made to the copy.
Now, in your case the array contains references to model objects; When you update the model object you change the object itself, not the reference held in the array, so you see the change reflected in all of your view controllers.
An analogy might be the difference between adding a house to a street (which changes the street itself) versus changing the occupants of an existing house on the street.
I would suggest you implement a model object that provides abstraction from the underlying array so that you have better code and avoid the issue with array references.
One approach could be something like:
struct MyModel {
let name: String
let size: Int
}
class MyData {
private var _models = [MyModel]()
var models: [MyModel] {
return _models
}
func insert(model: MyModel) {
self._models.append(model)
}
func removeModel(at: Int) {
guard at >= 0 && at < _models.count else {
return
}
self._models.remove(at: at)
}
}
Although this isn't ideal as it still requires model consumers to know indices in the underlying array. I would prefer something like this:
struct MyModel: Hashable {
let name: String
let size: Int
}
class MyData {
private var _models = [MyModel]()
var models: [MyModel] {
return _models
}
func insert(model: MyModel) {
self._models.append(model)
}
func remove(model: MyModel) -> Bool {
if let index = self._models.index(of: model) {
_models.remove(at: index)
return true
} else {
return false
}
}
}
Now I don't need to know what internal collection MyData uses to store the models.

Answer 2

If you need to pass an array (or any other value type) by reference, you could go through an intermediate structure that manages the indirection for you.
[EDIT] changed to use KeyPaths available in Swift 4.
// Generic class to hold a "weak" reference to a property from an object
// including properties that are valued types such as arrays, structs, etc.
// This is merely an encapsulation of Swift's native KeyPath feature
// to make the code a bit more readable and simpler to use
//
class ReferenceTo<ValueType> { var value:ValueType! { get { return nil} set {} } }
class Reference<OwnerType:AnyObject,ValueType>:ReferenceTo<ValueType>
{
internal weak var owner:OwnerType!
internal var property:ReferenceWritableKeyPath<OwnerType,ValueType>! = nil
internal var valueRef:KeyPath<OwnerType,ValueType>! = nil
init(_ owner:OwnerType, _ property:ReferenceWritableKeyPath<OwnerType,ValueType>)
{ (self.owner,self.property) = (owner,property) }
init(_ owner:OwnerType, get valueRef:KeyPath<OwnerType,ValueType>)
{ (self.owner,self.valueRef) = (owner,valueRef) }
override var value:ValueType!
{
get { return valueRef != nil ? owner?[keyPath:valueRef] : owner?[keyPath:property] }
set { owner?[keyPath:property] = newValue }
}
}
With this generic class you can create references to valued type properties of object instances and manipulate them anywhere in your code as if the valued type property was a reference type.
// Example class with a read/write and a read-only property:
class MyObject
{
var myArray = [1,2,3,4]
var total:Int { return myArray.reduce(0,+) }
}
var instance:MyObject! = MyObject()
// create a reference to the array (valued type)
// that can be used anywhere and passed around as a parameter
let arrayRef = Reference(instance, \.myArray)
// the value is accessed and manipulated using the
// "value" property of the reference
arrayRef.value.remove(at:2)
arrayRef.value.append(5)
print(instance.myArray) // [1,2,4,5]
// Read-only properties can also be manipulated as
// references
let valueRef = Reference(instance, get:\.total)
print(valueRef.value) // 12
The Reference class allows passing the value as a reference to function parameters
// a function that expects a reference to an array
// would be declared as follows
func changeArray(_ array:ReferenceTo<[Int]>)
{ array.value.insert(9, at: 1) }
// the reference can also be used as an inout parameter
func shift(_ array:inout [Int])
{ array = Array(array.dropFirst()) + Array(array.prefix(1)) }
changeArray(arrayRef)
shift(&arrayRef.value!)
print(instance.myArray) // [9,2,4,5,1]
...
// the reference uses a weak link to the owner
// of the referenced property or value
// so there will be no strong reference cycle issues even
// if the reference is used in an object held strongly
// by the owner itself
instance = nil
print(arrayRef.value) // none ... no more value after the owner is gone