What does defer do?

Sources & Resources

Main Source: 🔗 Ace the iOS Interview

Additional Sources:

Further Reading:

TL/DR

The defer keyword allows us to specify code that should be executed only when we are leaving the current function’s scope.

It's commonly used for releasing a shared resource, closing a connection, or performing any last-minute cleanup.

In this case, we're using it to close the connection to a database:

class DatabaseManager {
    func writeLineToDatabase(entry: String) {
        let database = Database()
        defer {
            database.disconnect()
        }
        database.connect()
        do {
            try database.write(entry: "Hello world!")
        } catch {
            print("An error occurred!")
        }
    }
}

You’ll see that regardless of whether the:

try database.write(entry: "Hello world!")

call succeeds or fails, we will always close out the database connection without having to duplicate the disconnect() call. Lastly, we can use defer to write setup and cleanup code next to each other, even though they need to be executed at different times.