Swift World: Design Patterns — Command

From today, we will talk about behavioral patterns which focus on the communication between objects. It includes command, mediator, observer, state, strategy, etc.

In this article, we will introduce command pattern. As in real world, a command is a request sent from its invoker (aka caller) to its receiver. By isolating and encapsulating the request, this pattern separates the invoker and the receiver as the following figure depicts.

from Command pattern — Wikipedia

Then what advantage does it give us? Let’s get started with our stories on the car.

protocol Car {
    func move()
}
class Sedan: Car {
    func move() {
        print("Sedan is moving.")
    }
}
class SUV: Car {
    func move() {
        print("SUV is moving.")
    }
}

Now, you, as an iOS app developer, have a sedan and drive it daily.

class Developer {
    let sedan = Sedan()
    func driveCar() {
        sedan.move()
    }
}

One day, you make so much money from App Store that you buy another SUV. If you want to drive it, you need to add an instance in the class.

class Developer {
    let sedan = Sedan()
    let suv = SUV()
    func driveSedan() {
        sedan.move()
    }
    func driveSUV() {
        suv.move()
    }
}

But what if your app becomes popular to make you a millionaire one day? You have so many nice cars that your class becomes bloated. It’s command pattern’s showtime.

protocol Command {
    func execute()
}
class MovementCommand: Command {
    var car: Car
    init(car: Car) {
        self.car = car
    }
    func execute() {
        car.move()
    }
}

In the command, we put in the car which is the receiver and the request to get the car to move. Then all you need to do is to replace all the cars with a command instance.

class Developer {
    var command: Command
    init(command: Command) {
        self.command = command
    }
    func drive() {
        command.execute()
    }
}

Let’s see how to use it.

let sedan = Sedan()
let sedanMovementCommand = MovementCommand(car: sedan)
let suv = SUV()
let suvMovementCommand = MovementCommand(car: suv)
let developer = Developer(command: sedanMovementCommand)
developer.drive()

The command pattern is so popular in iOS app development that there is a specific class NSInvocation to help create command. You can refer to its document for detailed explanation. Even it is not available in Swift, we can take it as example of command pattern.

Thanks for your time. Please clap to get this article seen by more people. Please follow me by clicking Follow. As a passionate iOS developer, blogger and open source contributor, I’m also active on Twitter and GitHub.