Swift Struct Memory Leak

43
votes

We're trying to use Swift structs where we can. We are also using RxSwift which has methods which take closures. When we have a struct that creates a closure that refers to self, that creates a strong reference cycle.

import Foundation
import RxSwift

struct DoesItLeak {

    var someState: String = "initial value"
    var someVariable: Variable<String> = Variable("some stuff")

    let bag = DisposeBag()

    mutating func someFoo() {

        someVariable.subscribeNext { person in

            self.someState = "something"
        }
        .addDisposableTo(bag)
    }
}

How do I know this? If I create 100,000 DoesItLeak objects and call someFoo() on each of them, I believe I have 100,000 objects with strong reference cycles. In other words, when I get rid of the DoesItLeak array containing those objects, the objects stay in memory. If I do not call someFoo(), there is no problem.

Variable is a class. So, I can see this memory issue by using xcode's Instruments' Allocations and filtering in Variable< String >

Filtering By Variable

enter image description here

If I try to use [weak self] such as in the following, I get a compiler error:

someVariable.subscribeNext { [weak self] person in

The compiler error is "weak cannot be applied to non-class type"

In real/non-example code, we access methods and variables via self and it's a memory issue.

How can I resolve this memory issue while keeping the DoesItLeak a struct?

Thanks for your help.

4
swiftmemory-leaksrx-swift
It's possible that insisting on using a struct is a violation of this Choosing Between Classes and Structures guideline: "It is reasonable to expect that the encapsulated values will be copied rather than referenced when you assign or pass around an instance of that structure." -- developer.apple.com/library/ios/documentation/Swift/Conceptual/… – finneycanhelp
I don't think you have a memory leak--at least not one caused by the closure using self to refer to a struct. From what I can tell, when you use self to refer to a struct in a closure, that merely makes a copy of the struct-- it doesn't create a reference to the struct. A strong reference cycle would have to involve only classes and closures, not structs. – Marc Khadpe
Is the Variable type a class or struct? – Marc Khadpe
Okay. It would seem you would want a way to specify that the someVariable property of the struct is captured weakly, but I'm not aware of a way to do that. – Marc Khadpe
Can't be done. How would this be implemented in C or C++? We want DoesItLeak to be a value type which means it may exist only on a stack frame. So what do we pass as self to the closure? A pointer to an object on the stack frame, and then we hope like hell that the stack frame is still around when the closure executes. You can actually do that in Swift, but it's completely unsafe. We'd like DoesItLeak to be a value type, but as soon as it's passed to a closure on a reference-counted object then it must also be reference-counted value type. Therefore, DoesItLeak can't be a struct. – Darren

4 Answers

17
votes

As Darren put it in the comments: "DoesItLeak can't be a struct" We cannot have the DoesItLeak be a struct and safely resolve the strong reference cycle issue.

Value types like structs exist on the stack frame. Closures and classes are reference types.

As the Strong Reference Cycles for Closures section puts it:

This strong reference cycle occurs because closures, like classes, are reference types.

Since the struct has the Variable class and the closure referring to self is stored into the Variable class using subscribeNext, it creates the strong reference cycle. See "Resolving Strong Reference Cycles for Closures" in Automatic Reference Counting Apple documentation.

11
votes

For anyone still facing this issue.

  1. [weak self] is not possible because Struct is value type and not a Reference type, so no pointer as such.

  2. The main issue of leak here is you are trying to access the Struct property self.someState = something inside the completion block which will basically create a new copy of your structure on assignment.

You should not access Struct Property inside completion block.

5
votes

You can solve the problem by creating a weak reference to the object which is captured by the closure.

Here is your example without memory leak:

import Foundation
import RxSwift

struct WithoutLeak {

    var someState: String = "initial value"
    var someVariable: Variable<String> = Variable("some stuff")

    let bag = DisposeBag()

    mutating func someFoo() {

        weak let weakSomeState = someState // <-- create a weak reference outside the closure

        someVariable.subscribeNext { person in

            weakSomeState = "something" // <-- use it in the closure
        }
        .addDisposableTo(bag)
    }
}
0
votes

The pattern of capturing self by an escaping closure in a writable context is now disallowed. The swift compiler will emit an error "Closure cannot implicitly capture a mutating self parameter". If the context is read-only, the value of self could be copied or shared and in either case there wouldn't be a reference cycle.