30 April 2019, 15:00

the command pattern

Name: Command

Type: Behavioural

Use when you want to:

Encapsulate actions in objects. The key idea behind this pattern is to provide the means to decouple client from receiver.

From Wikipedia:

In object-oriented programming, the command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time. This information includes the method name, the object that owns the method and values for the method parameters.

Real World Example:

client -> invoker -> command -> reciever

You (the client) are switching (the command) on a light (the reciever)

via a remote control (the invoker).

Typescript Code Example:

The receiver has the implementation of every action that could be performed:

// Receiver
class Bulb { 
    public turnOn() { 
        console.log('bulb on!')
    }

    public turnOff() { 
        console.log('bulb off!')
    }
}

There is also an iterface for each command and an implementation of that command:

interface ICommand { 
    execute(): void;
    undo(): void;
    redo(): void;
}

// Command
class TurnOn implements ICommand { 
    constructor(protected bulb: Bulb) { }
    public execute() {
        this.bulb.turnOn();
     }
    public undo() {
        this.bulb.turnOff();
     }
    public redo() { 
        this.execute();
    }
}

class TurnOff implements ICommand { 
    constructor(protected bulb: Bulb) { }
    public execute() {
        this.bulb.turnOff();
     }
    public undo() {
        this.bulb.turnOn();
     }
    public redo() { 
        this.execute();
    }
}

Then an invoker with whom the client will interact with to process the command:

class RemoteControl { 
    submit(command: ICommand) { 
        command.execute();
    }
}

And now we can tie it all together:

// receiver
var bulb = new Bulb();

// commands
var turnOn = new TurnOn(bulb);
var turnOff = new TurnOff(bulb);

// invoker
var remote = new RemoteControl();

// client
remote.submit(turnOn);
remote.submit(turnOff);