Swift Combine: What is AnyCancellable? And how to avoid Memory Leak?

Good at development Or a high-level engineer. Developers who are evaluated in the market in such a statement are thorough in memory management. None of the apps should have any memory leaks. In fact, it is not an exaggeration to say that the development of a developer’s skill is the ability to implement what they want while efficiently managing memory. Combine is convenient but you need to consider memory leak.

Problem Statement

Whenever you are calling publisher.sink(), it is going to return a particular subscription.

Now, if you don’t really cancel the subscription, then it can start leaking memory or it will start getting the event from the publisher when you don’t need it.

So, maybe you only needed it for once or twice, but if you don’t cancel your subscription, you will keep receiving the vents which are being originated from the publisher.

Cancel

// Publisher
let publisher = CurrentValueSubject<String, Never>("")

// Subscription
let subscription: AnyCancellable = publisher.sink { value in
    print(value)
}

publisher.send("hi") // hi 
publisher.send("hello world") // hello world

subscription.cancel()

publisher.send("Change Value!!") // Doesn't work
// Subscription
let subscription = publisher.sink { value in
    print(value)
}

subscription.cancel()

The simplest way is to store your subscription to a variable. By doing this, you can use subscription.cancel() method. It cancels the subscription.

Set your subscription to nil

// Publisher
let publisher = CurrentValueSubject<String, Never>("")

// Subscriptoin
var subscription: AnyCancellable? = publisher.sink { value in
    print(value)
}

publisher.send("hi") // hi
publisher.send("hello world") // hello world

// Cancel Subscription 
subscription = nil 

publisher.send("Change Value!!") // Doesn't work

// Cancel Subscription 
subscription = nil

You can cancel your subscription just by making it optional and setting its value to nil.

What is AnyCancellable?

var subscription: AnyCancellable

Your guess is right. AnyCancellable is a data type that can hold any subscription. Nothing more to explain.

Why usually declaring AnyCancellable on Global Scope?

import UIKit
import Combine

class ViewController: UIViewController {
    
   // Publisher
    let publisher = CurrentValueSubject<String, Never>("")
    var subscription: AnyCancellable?

    override func viewDidLoad() {
        super.viewDidLoad()
       
        // Subscriptoin
        subscription = publisher.sink { value in
            print(value)
        }
        
        publisher.send("hi") // hi
        publisher.send("hello world") // hello world
        
        subscription = nil
        
        publisher.send("Change Value!!") // Doesn't work
        
    }

}

class ViewController: UIViewController {
    
    var subscription: AnyCancellable?

    override func viewDidLoad() {
        super.viewDidLoad()
				...
		}
		
}

You probably have seen this pattern multiple times. It’s just simple. This is because when function execution is once finished, it’s gone from memory forever. That’s why you need to declare AnyCancellable or your subscriptions in your global scope.

Save Multiple Subscriptions

import UIKit
import Combine

class ViewController: UIViewController {
    
    var subscriptions: Set<AnyCancellable> = []

    // Publishers
	  let publisher = CurrentValueSubject<String, Never>("")
	  let publisher2 = CurrentValueSubject<String, Never>("")	

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Subscription
        publisher.sink { value in
            print(value)
        }
        .store(in: &subscriptions)

        // Subscription2
        publisher2.sink { value in
            print(value)
        }
        .store(in: &subscriptions)
        
        publisher.send("hi") // hi
        publisher.send("hello world") // hello world
        
        subscriptions.removeAll()
        
        publisher.send("Change Value!!") // Doesn't work
        
    }

}

.store(in: &subscriptions)

subscriptions.removeAll()

In order to save your subscriptions, you need to use this method .store(in: &set).

To cancel all the subscriptions, just call subscriptions.removeAll().

Conclusion

• Save the subscription to AnyCancellable variable and call cancel()
• When you have multiple publishers, remove them from your Set<AnyCancellable>