‘self’ captured by a closure before all members were initialized

Query: ‘self’ captured by a closure before all members were initialized

In Swift, when declaring a closure, it can capture and store references to variables and constants from the surrounding context in which it is defined. One common scenario is when a closure captures the value of ‘self’ to reference properties or methods of the enclosing class or structure.

However, if the closure captures ‘self’ and is executed before all the members of ‘self’ are initialized, it can lead to unexpected behavior or even crashes, known as a “capture list reference cycle” or “retain cycle”.

Let’s consider an example to illustrate this scenario. Suppose we have a class called ‘Person’ with a property ‘name’ and a closure ‘sayHello’ that captures ‘self’ and prints a greeting message:

class Person {
    var name: String
    var sayHello: (() -> Void)!
    
    init(name: String) {
        self.name = name
        
        sayHello = {
            print("Hello, \(self.name)!")
        }
    }
    
    func greet() {
        sayHello()
    }
}

// Usage
let person = Person(name: "John")
person.greet()
  

In this example, the ‘sayHello’ closure captures ‘self’ and tries to access ‘name’ property. However, if we call the ‘greet’ method immediately after initializing ‘person’, we may encounter a crash or incorrect output.

To avoid this capture list reference cycle, we can use a capture list in the closure definition to specify the capture semantics. The capture list allows us to define how ‘self’ should be captured before the closure is executed. For example, we can capture ‘self’ as a weak reference to prevent retain cycles:

init(name: String) {
    self.name = name
    
    sayHello = { [weak self] in
        guard let self = self else {
            return
        }
        print("Hello, \(self.name)!")
    }
}
  

With the capture list and using a weak reference, the closure will first check if ‘self’ still exists before accessing any of its members. If ‘self’ no longer exists, it gracefully returns from the closure.

By understanding this concept and using capture lists appropriately, we can prevent unwanted retain cycles and ensure the correct behavior of closures capturing ‘self’ in Swift.