command

Behavioral » Command

Elements

• Command - an interface for executing operation;
• ConcreteCommand - implement the Execute method;
• Client - creates a ConcreteCommand;
• Invoker - asks the command to carry out the request;

Description

While strategy pattern is focused on changing algorithm, in Command pattern the focus is on invocation of something or on the abstraction of some type. A command pattern is commonly seen as a container.

Implementation

This pattern requires an interface for all commands that could be executed in a certain moment.

type Command interface {
	Execute()
}

Here first example of command:

type SomeCommand struct {
	message string
}

func (s *SomeCommand) Execute() {
	fmt.Println(s.message)
}

And here a second one:

type SomeSpecialCommand struct {
	message string
}

func (s *SomeSpecialCommand) Execute() {
	message := "@" + s.message
	fmt.Println(message)
}

In the SomeCommand there is a simple Println output. Instead, using another SomeSpecialCommand we also add a @ symbol at the beginning of the string.

Finally we have an invoker, a component of this pattern that:

• append commands into queue;
• process entire queue;

type CommandInvoker struct {
	queue []Command
}

func (c *CommandInvoker) processQueue() {
	for i := range c.queue {
		c.queue[i].Execute()
	}
}

func (c *CommandInvoker) addToQueue(i Command) {
	fmt.Println("Appending command")
	c.queue = append(c.queue, i)
	if len(c.queue) == 3 {
		c.processQueue()
		c.queue = []Command{}
	}
}