Why do we need a "receiver" class in the Command design pattern

Question

I am learning command design pattern. As far as I know, four terms always associated with the command pattern are command, receiver, invoker and client.

A concrete command class has an execute() method and the invoker has a couple of commands. The invoker decides when to call the execute() method of a command.

When the execute() method is called, it calls a method of the receiver. Then, the receiver does the work.

I don't understand why do we need the receiver class? We can do the work inside execute() method, it seems that the receiver class is redundant.

Thank in advance.

Answer 1

Design patterns are used to solve software problems.

You have to understand the problem before trying to understand the solution (in this case Command pattern)

The problems which command pattern apply are in the context of an object A (client) invoking a method in an object B (receiver), so the Receiver is part of the problem, not part of the solution.

The solution or idea that command pattern offers is to encapsulate the method invocation from A to B in an object (Command), in fact this is close to the formal pattern definition. When you manage a request as an object you are able to solve some problems or to implement some features. (you also will need other pieces like the one called Invoker)

This list can give you some good examples of what kind of problems o features are suitable for command pattern.

note: Comamnd pattern is not necesary about decoupling, in fact the most common example pattern immplementation, the client needs to make a new instance of the receiver so we cannot talk about decoupling here.

Answer 2

Imagine a class that can do couple of things, like Duck, it can eat and quack. Duck is a receiver in this example. To apply command pattern here, you need to be able to wrap eating and quacking into a command. They should be separate classes that derive from Command base class with execute() method because Duck can have only single execute() method. So EatCommand.execute() calls Duck.eat() and QuackCommand.execute() calls Duck.quack().