Command in Go

Conceptual Example

Let’s look at the Command pattern with the case of a TV. A TV can be turned ON by either:

• ON Button on the remote;
• ON Button on the actual TV.

We can start by implementing the ON command object with the TV as a receiver. When the execution method is called on this command, it, in turn, calls the TV.on function. The last part is defining an invoker. We’ll actually have two invokers: the remote and the TV itself. Both will embed the ON command object.

Notice how we have wrapped the same request into multiple invokers. The same way we can do with other commands. The benefit of creating a separate command object is that we decouple the UI logic from underlying business logic. There’s no need to develop different handlers for each of the invokers. The command object contains all the information it needs to execute. Hence it can also be used for delayed execution.

button.go: Invoker

package main

type Button struct {
	command Command
}

func (b *Button) press() {
	b.command.execute()
}

command.go: Command interface

package main

type Command interface {
	execute()
}

onCommand.go: Concrete command

package main

type OnCommand struct {
	device Device
}

func (c *OnCommand) execute() {
	c.device.on()
}

offCommand.go: Concrete command

package main

type OffCommand struct {
	device Device
}

func (c *OffCommand) execute() {
	c.device.off()
}

device.go: Receiver interface

package main

type Device interface {
	on()
	off()
}

tv.go: Concrete receiver

package main

import "fmt"

type Tv struct {
	isRunning bool
}

func (t *Tv) on() {
	t.isRunning = true
	fmt.Println("Turning tv on")
}

func (t *Tv) off() {
	t.isRunning = false
	fmt.Println("Turning tv off")
}

main.go: Client code

package main

func main() {
	tv := &Tv{}

	onCommand := &OnCommand{
		device: tv,
	}

	offCommand := &OffCommand{
		device: tv,
	}

	onButton := &Button{
		command: onCommand,
	}
	onButton.press()

	offButton := &Button{
		command: offCommand,
	}
	offButton.press()
}

output.txt: Execution result

Turning tv on
Turning tv off