I have encountered the problem "Cannot assign to property: 'self' is an immutable capture". How can i call API to check status and update "getUpdateSuccess" parameter
struct HomepageViewModel {
var getUpdateSuccess: Bool = false
init() {
getStatusUpdated.execute().done {
[self] isUpdated in
// Cannot assign to property: 'self' is an immutable capture
self.getUpdateSuccess = isUpdated
}
}
}
If you want to “reference” self later (e.g., update the same instance later), self should be a reference type, not a value type.
So you can solve this by making the view model a class (a reference type) rather than a struct (a value type).
“Model” objects are good candidates for value types, but “view models” are generally reference types. See Value and Reference Types for a discussion of when you might use a value type and when you might use a reference type. For a general discussion, see The Swift Programming Language: Structures and Classes.
By the way, the closure’s capture list syntax (the stuff in between the [ and the ]) has different practical implications if the captured variable is a reference type vs when it is a value type.
With a value type, the capture list makes sure you have a copy of the value:
struct Person {…}
var person = Person(…)
foo { [person] in
// this code will be dealing with a safe copy of the original `Person`
}
// maybe you’ll mutate the original `Person` instance here,
// but the above closure will have a copy of the original one
Contrast that with a reference type, where the capture list makes sure you have a strong reference to the reference type:
class ViewModel {
func bar() {
baz { [self] in
// this has a strong reference to this view model instance, not a copy of the instance
}
}
}
let viewModel = ViewModel(…)
viewModel.bar() // any updates that bar does asynchronously, will update this instance of the view model, not a copy of it
As an aside, with reference types, you will often see a capture list with the weak keyword, e.g., [weak self], if you want a “weak” reference to the class rather than a strong reference. This is common if you need to prevent a “strong reference cycle” (a topic beyond the scope of this conversation). This is discussed in The Swift Programming Language: Strong Reference Cycles for Closures.
But, whether a strong reference or weak reference, you are not dealing with a copy of the object, but rather a “reference” to the same original instance.
Related
I'm implementing an in-game store using Swift and SpriteKit. There is a class called Store which has a method setupItems() inside of which we declare and instantiate instances of a class StoreItem and also add each store item instance as a child of Store. Each StoreItem has an optional closure property called updateInventory which is set inside of setupItems() as well. This is an optional closure because some items don't have a limited inventory.
Store also has an unowned instance property storeDelegate which is responsible for determining how funds are deducted and how storeItems are applied once purchased. storeDelegate is unowned as it has an equal or greater lifetime than Store.
Now, here is where things get interesting - the closure updateInventory references a computed string variable called itemText which makes a calculation based on a property of storeDelegate. If itemText is declared and instantiated as a variable inside of setupItems() we have a reference cycle and store is not deallocated. If instead, itemText is declared and instantiated as an instance property of Store (right under the property unowned storeDelegate that it references) then there is no reference cycle and everything deallocates when it should.
This seems to imply that referencing storeDelegate from a computed variable inside an instance method of a class doesn't respect the unowned qualifier. Code examples follow:
Current Scenario
protocol StoreDelegate: AnyObject {
func subtractFunds(byValue value: Int)
func addInventoryItem(item: InventoryItem) throws
var player: Player! { get }
}
class Store: SKSpriteNode, StoreItemDelegate {
unowned var storeDelegate: StoreDelegate
init(storeDelegate: StoreDelegate) {
self.storeDelegate = storeDelegate
setupItems()
...
}
func setupItems() {
var itemText: String {
return "item info goes here \(storeDelegate.player.health)"
}
let storeItem = StoreItem(name: "coolItem")
storeItem.updateInventory = {
[unowned storeItem, unowned self] in
// some logic to check if the update is valid
guard self.storeDelegate.player.canUpdate() else {
return
}
storeItem.label.text = itemText
}
}
The above leads to a reference cycle, interestingly if we move
var itemText: String {
return "item info goes here \(storeDelegate.player.health)"
}
outside of updateItems and make it an instance variable of Store right below unowned var storeDelegate: StoreDelegate, then there is no reference cycle.
I have no idea why this would be the case and don't see mention of it in the docs. Any suggestions would be appreciated and let me know if you'd like any additional details.
storeItem.updateInventory now keeps strong reference to itemText.
I think the issue is that itemText holds a strong reference to self implicitly in order to access storeDelegate, instead of keeping a reference to storeDelegate. Anothe option is that even though self is keeping the delegate as unowned, once you pass ot to itemText for keeping, it is managed (ie, strong reference again).
Either way, you can guarantee not keeping strong reference changing itemText to a function and pass the delegate directly:
func setupItems() {
func itemText(with delegate: StoreDelegate) -> String
return "item info goes here \(storeDelegate.player.health)"
}
let storeItem...
storeItem.updateInventory = {
// ...
storeItem.label.text = itemText(with self.storeDelegate)
}
}
You can copy this playground verbatim:
var closures=[() -> Void]()
class Thing {
let name: String
init(_ name: String) { self.name=name }
}
class RefThingWrapper {
let thing: Thing
init(_ t: Thing) {
self.thing=t
closures.append { [thing, weak self] in // Even though `thing` captured strongly, `self` could still get deallocated.
print((self == nil) ? "`self` deallocated" : "`self` not deallocated")
print(thing.name) // Decided to use `thing` within closure to eliminate any chance of optimizations affecting the test results — which I found when I captured `self` strongly without using it.
}
}
}
struct ValThingWrapper {
let thing: Thing
init(_ t: Thing) {
self.thing=t
closures.append { [weak thing] in
print((thing == nil) ? "`thing` deallocated" : "`thing` not deallocated")
}
}
}
var wrapper=Optional(RefThingWrapper(Thing("Thing 1"))) // Test with reference type.
//var wrapper=Optional(ValThingWrapper(Thing("Thing 1"))) // Test with value type.
closures[0]()
wrapper=nil
closures[0]()
It demonstrates how a property of self — whether self is a reference or value type — can be captured within a closure independently of self. Running the program as is demonstrates a captured property existing after self has been deallocated. Testing with the value type wrapper demonstrates that, if weakly captured, the instance will be deallocated once the referencing value instance is deallocated.
I wasn't sure this was possible because when creating the closure at first, I forgot to initialize the property I was capturing. So the compiler complained — in the capture list — 'self' used before all stored properties are initialized. So I figured self was being captured implicitly, and only after digging deeper discovered otherwise.
Is this documented somewhere? I found this post by Joe Groff where he proposes:
For 'let' properties of classes, it'd be reasonable to propose having
closures capture the property directly by default in this way
instead of capturing 'self' (and possibly allowing referencing them
without 'self.', since 'self' wouldn't be involved in any cycle formed
this way).
This was back in 2015, and I didn't find any implemented proposals that arose from the discussion. Is there any authoritative source that communicates this behavior?
If you’re just asking for documentation on capture lists and reference types, see The Swift Programming Language Resolving Strong Reference Cycles for Closures. Also see the Language Reference: Capture Lists
If your capture list includes value type, you’re getting copy of the value.
var foo = 1
let closure = { [foo] in
print(foo)
}
foo = 42
closure() // 1; capturing value of `foo` as it was when the closure was declared
If your capture list includes a reference type, you’re getting a copy of the reference to that current instance.
class Bar {
var value: Int
init(value: Int) { self.value = value }
}
var bar = Bar(value: 1)
let closure = { [bar] in
print(bar.value)
}
bar.value = 2
bar = Bar(value: 3)
closure() // 2; capturing reference to first instance that was subsequently updated
These captured references are strong by default, but can optionally be marked as weak or unowned, as needed, too.
That Capture Lists document outlines the way that you can capture a property without capturing self:
You can also bind an arbitrary expression to a named value in a capture list. The expression is evaluated when the closure is created, and the value is captured with the specified strength. For example:
// Weak capture of "self.parent" as "parent"
myFunction { [weak parent = self.parent] in print(parent!.title) }
I’m not crazy about their code sample, but it illustrates the capture of a property without capturing self, nonetheless.
I am confused of how memory is managed here, lets say there is a scenario:
import Foundation
class SomeObject {
deinit {
print("deinitCalled")
}
}
struct SomeStruct {
let object = SomeObject()
var closure: (() -> Void)?
}
func someFunction() {
var someStruct = SomeStruct()
someStruct.closure = {
print(someStruct)
}
someStruct.closure?()
}
someFunction()
print("the end")
What I would expect here is:
Optional(test.SomeStruct(object: test.SomeObject, closure: Optional((Function))))
deinitCalled
the end
However what I get is this:
SomeStruct(object: test.SomeObject, closure: Optional((Function)))
the end
And if I look at memory map:
retain cycle
How do I manage memory in this case
First, you should be very, very careful about putting reference types inside of value types, and especially a mutable reference type that is visible to the outside world. Structs are always value types, but you also want them to have value semantics, and it's challenging to do that while containing reference types. (It's very possible, lots of stdlib types do it in order to implement copy-on-write; it's just challenging.)
So the short version is "you almost certainly don't want to do what you're doing here."
But if you have maintained value semantics in SomeStruct, then the answer is to just make a copy. It's always fine to make a copy of a value type.
someStruct.closure = { [someStruct] in
print(someStruct)
}
This gives the closure it's own immutable value that is a copy of someStruct. Future changes to someStruct won't impact this closure.
If you mean for future changes to someStruct to impact this closure, then you may be violating value semantics, and you should redesign (likely by making SomeStruct a class, if you mean it to have reference semantics).
class HTMLElement {
let name : String
let text: String?
//Declaring a lazy variable that has a strong reference to this closure
lazy var asHTML: Void -> String = {
//Optional binding here
if let text = self.text {
return "<\(self.name)>\(text)<\(self.name)>"
} else {
return "<\(self.name) >"
}
}
init(name: String, text: String? = nil){
self.name = name
self.text = text
}
deinit {
print("\(name) is being deinitialized")
}
}
My Question is: Why is the closure declared Lazy, I know it has something to do with self not being known in the closure, but isn't that the same case for the init method where self hasn't been created?
Secondly,Where exactly is the strong reference cycle in this code example, is it self that strongly references to asHTML, if so where is the second part of the strong referencing that causes the cycle?
Third, Why is the constant text property an optional when constants cannot change value(from nil to a value and back to nil)?
Lastly, What does it mean to have the parameter text: String? = nil in the init method when the init method is used to accept parameters sent by the user?
Sorry for this long question, I'm just confused on the closure strong reference cycle....although I do understand strong reference cycles between class properties and class instances.
1
lazy is used for attributes that are only created when called upon. So before you call myClass.myLazyAttribute it will not take up any space. This also means that it will init after the class has initialised, which can be very useful.
In this case lazy is used to get access to self, like you stated, because self is not available until the instance has been initialised.
2
The apple doc from where code is.
Closures capture the values used in them. In this case it captures self.
It does not create a Strong Reference Cycle between class A and class B, but between itself and a closure. It makes a lot more sense if you imagine that the operation inside the closure takes a very long time. During the execution something else has happened and you want to deinit the instance. But the closure has captured self and it will keep the instance alive until it is done.
By using [unowned self] in you can again deinit the instance while the closure is running. Although this will crash your app.
good info on this specific use : link
In the specific case of a closure, you just need to realize that any variable that is referenced inside of it, gets "owned" by the closure. As long as the closure is around, those objects are guaranteed to be around. The only way to stop that ownership, is to do the [unowned self] or [weak self].
What a Strong Reference Cycle in essence is:
you have an instance of a class
the instance has a reference count higher than 0
there is no longer a reference to the instance available to your program.
Or even shorter: the reference count of the instance is higher than the number of accessible references.
In this case the reference count of self goes up by 1 because it is captured by the closure. We can not access that reference because we can not say something like: closure.selfAttribute, so we can not set that to nil. Only when the closure is finished will the reference count go down by 1 again.
3
It is an optional constant, but it's initial value is set in the init method of the class. So it can receive a value in the init method, but it will be immutable. This is called a late init.
4
This is a function parameter with a default value.
func someFunction(myParamWithDefaultValue : Int = 10) {
print(myParamWithDefaultValue)
}
someFunction() // 10
someFunction(5) // 5
I have the following code to create an observable property for data binding. It's in the works so I'm not sure what the final implementation is going to be and I'm still pretty new to Swift.
class Observable<T> {
typealias Observer = T -> Void
var value: T {
didSet {
for observer in self.observers {
observer?(self.value)
}
}
}
var observers: [Observer?] = []
init(_ val: T) {
self.value = val
}
}
I would like to keep weak references to the Observer closures. I don't want to rely on the client to ensure that the closure is weak/unowned before passing it in, via the capture list. Especially because there can be many observable properties on a given class.
Is it possible to make the closure references weak in my Observable class?
UPDATE:
I found a couple of resources that I think will help me accomplish what I want:
Make self weak in methods in Swift
specifically,
func methodPointer<T: AnyObject>(obj: T, method: (T) -> () -> Void) -> (() -> Void) {
return { [unowned obj] in method(obj)() }
}
The following link refers to the above stackoverflow answer and goes into some more detail:
http://blog.xebia.com/2014/10/09/function-references-in-swift-and-retain-cycles/
and this is a two-way binding example:
http://five.agency/solving-the-binding-problem-with-swift/
specifically,
class BondBox<T> {
weak var bond: Bond<T>?
init(_ b: Bond<T>) { bond = b }
}
where the listener is wrapped in a class called Bond, which is weakly referenced in the BondBox.
Is it possible to make the closure references weak in my Observable class
No. Only class instances can be referred to via weak references in Swift, and a function is not a class instance. (And not only must they be class instances, they must be an Optional wrapping a class instance.)
There are some pretty obvious ways around this, or course - the simplest being a wrapper class. But I do not actually recommend that in this situation, because you have not convinced me that weak references to functions are needed here in the first place. Remember, a weak reference to an object to which there is no strong reference will instantly lose the reference and will be pointing at nil. I can't believe that is what you want. I think you're barking up a wrong tree here.
Weak/strong are for memory management of objects, so only apply for variables of reference types (i.e. types which point to objects). Function types in Swift are not reference types, and therefore it does not make sense to talk about weak/strong for their variables.
Plus, you do not actually have variables of function type in your code (except in the middle of the iteration). You simply have a variable of array type. Even in Objective-C, you can only mark variables as weak or strong, not values that are stored inside other things.
And if you were to write the thing you are writing in Objective-C, you would want the "Observable" to have strong references to the closures. Otherwise, who else would have a strong reference to the closure?