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.
Four terms always associated with the command pattern are command, receiver, invoker and client. A command object knows about receiver and invokes a method of the receiver. Values for parameters of the receiver method are stored in the command. The receiver object to execute these methods is also stored in the command object by aggregation. The receiver then does the work when the execute() method in command is called. An invoker object knows how to execute a command, and optionally does bookkeeping about the command execution. The invoker does not know anything about a concrete command, it knows only about the command interface. Invoker object(s), command objects and receiver objects are held by a client object. The client decides which receiver objects it assigns to the command objects, and which commands it assigns to the invoker. The client decides which commands to execute at which points. To execute a command, it passes the command object to the invoker object.
execute()
Using command objects makes it easier to construct general components that need to delegate, sequence or execute method calls at a time of their choosing without the need to know the class of the method or the method parameters. Using an invoker object allows bookkeeping about command executions to be conveniently performed, as well as implementing different modes for commands, which are managed by the invoker object, without the need for the client to be aware of the existence of bookkeeping or modes.
The central ideas of this design pattern closely mirror the semantics of first-class functions and higher-order functions in functional programming languages. Specifically, the invoker object is a higher-order function of which the command object is a first-class argument.
The command[1] design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
Using the command design pattern can solve these problems:[2]
Implementing (hard-wiring) a request directly into a class is inflexible because it couples the class to a particular request at compile-time, which makes it impossible to specify a request at run-time.
Using the command design pattern describes the following solution:
This enables one to configure a class with a command object that is used to perform a request. The class is no longer coupled to a particular request and has no knowledge (is independent) of how the request is carried out.
See also the UML class and sequence diagram below.
In the above UML class diagram, the Invoker class doesn't implement a request directly. Instead, Invoker refers to the Command interface to perform a request (command.execute()), which makes the Invoker independent of how the request is performed. The Command1 class implements the Command interface by performing an action on a receiver (receiver1.action1()).
Invoker
Command
command.execute()
Command1
receiver1.action1()
The UML sequence diagram shows the run-time interactions: The Invoker object calls execute() on a Command1 object. Command1 calls action1() on a Receiver1 object, which performs the request.
action1()
Receiver1
Action
The terminology used to describe command pattern implementations is not consistent and can therefore be confusing. This is the result of ambiguity, the use of synonyms, and implementations that may obscure the original pattern by going well beyond it.
CopyCommand
moveUp
ExecutedRoutedEventHandler
This C++14 implementation is based on the pre C++98 implementation in the book.
#include <iostream> #include <memory> class Command { public: // declares an interface for executing an operation. virtual void execute() = 0; virtual ~Command() = default; protected: Command() = default; }; template <typename Receiver> class SimpleCommand : public Command { // ConcreteCommand public: typedef void (Receiver::* Action)(); // defines a binding between a Receiver object and an action. SimpleCommand(std::shared_ptr<Receiver> receiver_, Action action_) : receiver(receiver_.get()), action(action_) { } SimpleCommand(const SimpleCommand&) = delete; // rule of three const SimpleCommand& operator=(const SimpleCommand&) = delete; // implements execute by invoking the corresponding operation(s) on Receiver. virtual void execute() { (receiver->*action)(); } private: Receiver* receiver; Action action; }; class MyClass { // Receiver public: // knows how to perform the operations associated with carrying out a request. Any class may serve as a Receiver. void action() { std::cout << "MyClass::action\n"; } }; int main() { // The smart pointers prevent memory leaks. std::shared_ptr<MyClass> receiver = std::make_shared<MyClass>(); // ... std::unique_ptr<Command> command = std::make_unique<SimpleCommand<MyClass> >(receiver, &MyClass::action); // ... command->execute(); }
The program output is
MyClass::action
The first published mention of using a Command class to implement interactive systems seems to be a 1985 article by Henry Lieberman.[4] The first published description of a (multiple-level) undo-redo mechanism, using a Command class with execute and undo methods, and a history list, appears to be the first (1988) edition of Bertrand Meyer's book Object-oriented Software Construction,[5] section 12.2.