Defer.md

defer

With the guard and new throw statement, Swift encourages a style of early return programming over nested if statements. This poses a challenge however. How does one clean up initialized resources after returning from the block?

The defer keyword helps us here by declaring a block that will only be executed when the executions leaves the current scope.

Here is an example that creates a boolean that every path in the method can set, and then only calls the actual defer block and the completion block when everyone exists. This has the advantage of only offering one path our of the method despite there being multiple paths (due to the guard clauses).

func fetchActivationStatus(completion: @escaping (Bool) -> Void ) {

        OrderDetailsService.sharedInstance.fetchOrderDetails { (orderDetails, error) in

            let success: Bool

            defer {
                completion(success)
            }

            guard error == nil else {
                success = false
                assertionFailure(String(describing: error))
                return
            }

            guard let internet = orderDetails?.internet else {
                success = false
                assertionFailure("Error unwrapping orderDetails")
                return
            }

            success = internet.isSelfActivatable
        }
    }

Use defer whenever an API requires calls to be balanced. For example

• allocate() / deallocate()
• wait() / signal()
• open() / close()

Links that help

• NSHipster