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Hao's Keeper

A blog to place memos

• on swift

Escaping Closures

Escaping Closures

A closure is said to escape a function when the closure is passed as an argument to the function, but is called after the function returns.

As an example, many functions that start an asynchronous operation take a closure argument as a completion handler.

var completionHandlers: [() -> Void] = []
func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) {
	completionHandlers.append(completionHandler)
}

Avoid Strong Reference Cycle

If you want to capture self, write self explicityly when you use it, or include self in the closure’s capture list. Writing self explicitly let you express your intent, and reminds you to confirm that there isn’t a reference cycle.

func someFunctionWithNonescapingClosure(closure: () -> Void) {
	closure()
}

class SomeClass {
	var x = 10
	func doSomething() {
		someFunctionWithEscapingClosure { self.x = 100 }
		someFunctionWithNonescapingClosure { x = 200 }
	}
}

let instance = SomeClass()
instance.doSomething()
print(instance.x)
// Prints "200"

completionHandlers.first?()
print(instance.x)
// Prints "100"

Escaping closure can’t capture a mutable reference to self when self is an instance of a structure or an enumeration.

Structures and enumerations don’t allow shared mutability.

struct SomeStruct {
	var x = 10
	mutating func doSomething() {
		someFunctionWithNonescapingClosure { x = 200 }	// Ok
		someFunctionWithEscapingClosure { x = 100 }		// Error
	}
}