What is @escaping in Swift closures

19 Oct 2020 ⋅ 5 min read ⋅ Swift Closure

Table of Contents

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Escaping Closures

Escaping closures (@escaping) is a keyword used to indicate the life cycle of a closure that passes as an argument to the function. By prefix any closure argument with @escaping, you convey the message to the caller of a function that this closure can outlive (escape) the function call scope. Without escaping, a closure is non-escaping by default, and its lifecycle end along with function scope.

The following is an example of a non-escaping closure. A function that benchmark an execution time of a passing closure. A passing closure end when a function end. No closure escape from this function scope.

func benchmark(_ closure: () -> Void) {
    let startTime = Date()
    closure()
    let endTime = Date()
    
    let timeElapsed = endTime.timeIntervalSince(startTime)
    print("Time elapsed: \(timeElapsed) s.")
}

How can a closure escape?

The term @escaping might sound alienate to you, but an actual implementation to make a closure survive a calling function's scope is very simple. To make a passing closure survive when a function ended, you have to store it in a variable that's defined outside the function.

For example, I create a wrapper around CLLocationManager that exposes a new method to get a current location in the form of a callback. The getCurrentLocation function returns after it calls locationManager.requestLocation(), but the closure isn't called until we get back a user location from delegate callback. So, we store it in the completionHandler variable.

import Foundation
import CoreLocation

class MyLocationManager: NSObject, CLLocationManagerDelegate {
    let locationManager: CLLocationManager
    
    private var completionHandler: ((_ location: CLLocation) -> Void)? // <1>
    
    override init() {
        locationManager = CLLocationManager()
        super.init()
        locationManager.delegate = self
    }
    
    func getCurrentLocation(_ completion: @escaping (_ location: CLLocation) -> Void) { // <2>
        completionHandler = completion // <3>
        locationManager.requestLocation()
    }
    
    // MARK: - CLLocationManagerDelegate
    func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        if let location = locations.first {
            completionHandler?(location) // <4>
            completionHandler = nil // <5>
        }
    }
    
    func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) {}
}

<1> A variable to store closure.
<3> We help a closure to survive a function scope by storing it outside the function scope.
<2> We need to put @escaping here to indicate our intention. Failing to do so would result in the following compile error.

Assigning non-escaping parameter 'completion' to an @escaping closure

<4> After we get location data back, we call the closure with that information.
<5> And then we release it from duty.

Nested escape

Another way that closure can escape is by using that closure inside another escaping closure. In the following example, we pass a closure inside a dispatch queue.

func delay(_ closure: @escaping () -> Void) { // <1>
    DispatchQueue.main.asyncAfter(wallDeadline: .now() + 3) {
        closure() // <2>
    }
}

<1> You need to mark a closure as @escaping since <2> asyncAfter is a @escaping function.

public func asyncAfter(wallDeadline: DispatchWallTime, qos: DispatchQoS = .unspecified, flags: DispatchWorkItemFlags = [], execute work: @escaping @convention(block) () -> Void)

You will get the following compile error message if you forget to escape your closure.

Escaping closure captures non-escaping parameter 'closure'

In this case, we don't need to know the underlying implementation of asyncAfter. All we need to know is DispatchQueue holds a reference to a passing closure and may outlive a call to DispatchQueue.main.asyncAfter. Anything passes into that closure also gets captured and retained by a dispatch queue.

Why we need to know whether it is @escaping?

The fact that @escaping closure is stored (retain) somewhere else makes it possible to accidentally create a strong reference cycle. So, @escaping is like a precaution sign for a caller to stay alert when using them.

Let's take our previous MyLocationManager class as an example.

class DetailViewController: UIViewController {
    let locationManager = MyLocationManager() // <1>
    
    override func viewDidLoad() {
        super.viewDidLoad()

        locationManager.getCurrentLocation { (location) in
            print("Get location: \(location)")
            self.title = location.description // <2>
        }
    }
}

<1> DetailViewController own a locationManager.
<2> We reference a self (DetailViewController) in a passing closure, which is capture(retain) by a closure. And an escaping closure is own by MyLocationManager.

This results in a strong reference cycle.

The cycle will only break if we get a location update and set completionHandler to nil. If we failed to get a location, nobody would get a release, which leads to a memory leak.

// MARK: - CLLocationManagerDelegate
func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
    if let location = locations.first {
        completionHandler?(location) 
        completionHandler = nil // <1>
    }
}

func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) {} // <2>

<1> The strong reference cycle will break once we get a location.
<2> For fail case, the strong reference cycle remains (since we do nothing here).

Conclusion

@escping is a way to inform those who consume our function that the closure parameter is stored somewhere and might outlive the function scope. If you see any @escaping keyword, you have to be cautious about what you passed into that closure since it may cause a strong reference cycle.

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Closures - Swift.org

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