Capturing Values In Swift Closures

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Capturing Values In Swift Closures

12 Jun 2017 1639 words 8 minutes

Contents

We use closures pretty much every day, and, some times, it may be not very clear what happens and how a closure works. Let’s find out how a closure captures a variable and how to prevent weird behaviours.

Introduction

In this article, we’ll see mainly two things: how a closure captures a variable and a specific scenario where the closure captures the self context.

You don’t have to be a Guru of Swift closures to understand this article, but I suggest you a good understanding of what’s a strong and weak reference before starting reading this article, otherwise something may be confusing since I don’t explain thoroughly how an object reference works in Swift.

Happy Reading!

Default Variable Capturing

When we use a closure, most of the time, we need to use variables from the surrounding context—like class properties or local variables. We are able to use these variables capturing them inside the closure. The capturing is transparent to us, we can use these variables without any efforts like in the following example:

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struct Calculator {
	var a: Int
	var b: Int
	
	var sum: Int {
		return a + b
	}
}

let calculator = Calculator(a: 3, b: 5)

let closure = {
	print("The result is \(calculator.sum)")
}

closure() // Prints "The result is 8"

In this example, there is a struct Calculator which provides the sum of two properties. Then, we have a Calculator instance—calculator— which is captured inside closure to print sum.

Note

When we capture a variable and change its value inside the closure, we affect its value also outside the closure scope once the closure is called:

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var calculator = Calculator(a: 3, b: 5) // 0x618000220400
    
let closure = {
    calculator = Calculator(a: 33, b: 55) // 0x610000221be0
}

// calculator has address 0x618000220400

closure()

// calculator has address 0x610000221be0

In the example above, we instantiate a Calculator object which has the memory address 0x618000220400. Then, inside the closure, we assign a new value to calculator and the address becomes 0x610000221be0. Finally, after calling the closure, calculator remains the one set inside the closure, as we can see with the memory address.

Capture List

Unfortunately, the implementation used in “Default Variable Capture” has a problem. If we change the value of some calculator properties before calling the closure, the sum inside the closure is no longer 8 but it’s the sum of the new properties values:

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var calculator = Calculator(a: 3, b: 5)

let closure = {
	print("The result is \(calculator.sum)")
}

calculator.b = 20

closure() // Prints "The result is 23"

If we want to prevent this behaviour and print 8 even if the properties change after their capturing inside the closure, we can explicitly capture the variable with a capture list like this:

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let closure = { [calculator] in
	print("The result is \(calculator.sum)")
}

In this way, we keep an immutable copy of the variable calculator. Thanks to this copy, further changes to calculator, outside the closure, will not affected the closure.

We can capture several variables in the same closure, separating them with a comma:

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let closure = { [variable1, variable2, variable3] in
	print(variable1)
	print(variable2)
	print(variable3)
}

Note

Since the variables inside the capture list are immutable, they are just read-only. It means that we cannot change the value of calculator inside the closure:

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let closure = { [calculator] in
	calculator = Calculator(a: 1, b: 2) // Throws compile error
	print("The result is \(calculator.sum)")
}

Make Aliases

When we use a capture list, we can also use aliases to use a different name for a specific variable with the syntax alias = variable_name:

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let closure = { [alias1 = variable1, variable2, alias3 = variable3] in
	print(alias1)
	print(variable2)
	print(alias3)
}

Therefore, we can change the closure used in the previous examples like this:

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let closure = { [calc = calculator] in
	print("The result is \(calc.sum)")
}

Capture Self Context

Explicit `self`

If we want to capture some properties or methods of the object where we set the closure, like in the following example:

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class MyClass {
	var p = 0

	func method() {
		let closure = {
			print("P is \(p)")
		}

		closure()
	}
}

we have the following compile error: Reference to property 'p' in closure requires explicit 'self.' to make capture semantics explicit.

This means that, when we want to capture some properties or methods in the self context, we must add explicitly self to tell the compiler that the variable comes from the object self—MyClass in our example.

We can fix the compile error like this:

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class MyClass {
	var p = 0

	func method() {
		let closure = {
			print("P is \(self.p)")
		}

		closure()
	}
}

Strong Reference Cycles

A strong reference cycle occurs when two objects keep a strong reference of each other. Because of this cycle, the two objects won’t be deallocated, since their reference count doesn’t drop to zero.

Let’s see an example:

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class ClassA {
	var refB: ClassB?
}

class ClassB {
	var refA: ClassA?
}

let classA = ClassA()
let classB = ClassB()

classA.refB = classB
classB.refA = classA

This is a very plain example where we can see that both classA and classB are keeping a strong reference of each other creating a strong reference cycle.

This is just a plan example and the cycle is very easy to catch. It may become more difficult when we capture self inside a closure, if we don’t understand well how the closure works.

By default, a closure keeps a strong reference of the variable captured. This means that in the following example:

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class MyClass {

	func doSomethig() { }
}

class Handler {
	var closure: (() -> Void)?

	func setupClosure() {
		let obj = MyClass()

		closure = {
			obj.doSomethig()
		}
	}
}

let handler = Handler()
handler.setupClosure()

obj won’t be deallocated at the end of setupClosure scope since closure is keeping a strong reference of it. It will be deallocated once we destroy closure.

It means that if we capture self inside a closure—for example to use a property or a method—we keep a strong reference of self.

Let’s see an example where we use self inside a closure creating a strong reference cycle:

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class MyClass {

	func doSomethig() { }
}

class Handler {
	var closure: (() -> Void)?
	let obj = MyClass()

	func setupClosure() {
		closure = {
			self.obj.doSomethig()
		}
	}
}

let handler = Handler()
handler.setupClosure()

In this example, handler keeps a strong reference of its properties closure and obj. Then, in setupClosure, closure keeps an additional strong reference of self to use its obj property. In this way, we are creating a strong reference cycle, since closure and self are keeping a strong reference of each other.

We can break this strong reference cycle using self in the capture list with either a weak or an unowned reference:

Weak

A weak reference is a reference that does not keep a strong hold on the instance it refers to, and so does not stop ARC from disposing of the referenced instance. This behaviors prevents the reference from becoming part of a strong reference cycle.

Apple ARC Documentation

Since a weak reference may be nil, the variable captured becomes an optional. Therefore, we should use a guard to safety unwrapped it:

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closure = { [weak self] in
	guard let strongSelf = self else { return }
	strongSelf.obj.doSomethig()
}

Unowned

Like a weak reference, an unowned reference does not keep a strong hold on the instance it refers to. Unlike a weak reference, however, an unowned reference is used when the other instance has the same lifetime or a longer lifetime.

Apple ARC Documentation

This means that we should use unowned just when we are sure that the reference will never be nil inside the closure, otherwise the app would crash.

We can use an unowned reference like this:

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closure = { [unowned self] in
	self.obj.doSomethig()
}

We don’t need the guard like for weak since an unowned reference is not optional since it cannot be nil.

Note

We can use weak and unowned with any variable in the capture list and we can also combine with the aliases:

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let obj = MyClass()

let closure = { [unowned aliasObj = obj] in
	aliasObj.doSomethig()
}

We don’t need to use weak/unowned every time we use self. There are situation where we don’t need it—like with the UIView animation method:
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UIView.animate(withDuration: 5, animations: { self.obj.doSomethig() })
``
Since we are using a static UIView method and self doesn’t keep any strong reference of UIView.

Conclusion

The closure is a very powerful Swift feature which allows us to read a clean and readable Swifty codebase. Moreover, we have to use closures in a lot of Apple API—like in Foundation, UIKit and so on. To take advantage of closures we must be able to master it before to hurt ourself. For this reason, with a good understanding of how a closure captures the variables, we may be able to avoid troubles like strong reference cycles.

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