What is @escaping in Swift closures

What is @escaping in Swift

Escaping closures (@escaping) is a keyword that provides information about the life cycle of a closure that passes as an argument to the function.

By prefixing 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.

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Non-escaping closure

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 benchmarks an execution time of a passing closure.

• A passing closure end when a function end.
• No closure escaped 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.")
}

Escaping closure

The term @escaping might sound alienating to you, but an actual implementation to make a closure survive a calling function's scope is very simple.

We can either.

1. Store a closure as a variable.
2. Pass the closure to another escaping closure.

Store a closure as a variable

To make a passing closure outlive the scope of the function, you have to store it in a variable outside the scope of 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 end after it calls locationManager.requestLocation().
• But we get the result later in the delegate callback.
• So, we need to store completion closure in the instance variable, completionHandler.

import Foundation
import CoreLocation

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

1 A variable to store closure.
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

3 We store a completion closure in a class variable. This makes it survive the function scope.
4 After we get the location data back, we call the closure with that information.
5 And then we release it from duty.

Since CLLocationManager uses a delegate pattern, we will get the current location later in another delegate method, locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]). So, we need to mark a closure escaped in this case.

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.

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

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)

If you forget to escape your closure, you will get the following compile error message.

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 that passes into that closure also gets captured and retained by a dispatch queue.

Why do we need to know whether it is @escaping?

The fact that @escaping closure is stored (retained) 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 {
    // 1
    let locationManager = MyLocationManager()
    
    override func viewDidLoad() {
        super.viewDidLoad()

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

1 DetailViewController own a locationManager.
2 We reference a self (DetailViewController) in a passing closure, which is captured (retained) by a closure. And an escaping closure is owned 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, leading to a memory leak.

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

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

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

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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.