Command pattern

In object-oriented programming, the Command pattern is a design pattern in which objects are used to represent actions. A command object encapsulates an action and its parameters.

For example, a printing library might include a PrintJob class. A user would typically create a new PrintJob object, set its properties (the document to be printed, the number of copies, and so on), and finally call a method to send the job to the printer.

In this case, the same functionality could be exposed via a single SendJobToPrinter() procedure with many parameters. As it takes more code to write a command class than to write a procedure, there must be some reason to use a class. There are many possible reasons:

A command object is convenient temporary storage for procedure parameters. It can be used while assembling the parameters for a function call and allows the command to be set aside for later use.
A class is a convenient place to collect code and data related to a command. A command object can hold information about the command, such as its name or which user launched it; and answer questions about it, such as how long it will likely take.
Treating commands as objects enables data structures containing multiple commands. A complex process could be treated as a tree or graph of command objects. A thread pool could maintain a priority queue of command objects consumed by worker threads.
Treating commands as objects supports undo-able operations, provided that the command objects are stored (for example in a stack)
The command is a useful abstraction for building generic components, such as a thread pool, that can handle command objects of any type. If a new type of command object is created later, it can work with these generic components automatically. For example, in Java, a generic ThreadPool class could have a method addTask(Runnable task) that accepts any object that implements the java.lang.Runnable interface.

Uses for the Command pattern

Command objects are useful for implementing:

Multi-level undo: If all user actions in a program are implemented as command objects, the program can keep a stack of the most recently executed commands. When the user wants to undo a command, the program simply pops the most recent command object and executes its undo() method.
Transactional behavior: Undo is perhaps even more essential when it's called rollback and happens automatically when an operation fails partway through. Installers need this. So do databases. Command objects can also be used to implement two-phase commit.
Progress bars: Suppose a program has a sequence of commands that it executes in order. If each command object has a getEstimatedDuration() method, the program can easily estimate the total duration. It can show a progress bar that meaningfully reflects how close the program is to completing all the tasks.
Wizards: Often a wizard presents several pages of configuration for a single action that happens only when the user clicks the "Finish" button on the last page. In these cases, a natural way to separate user interface code from application code is to implement the wizard using a command object. The command object is created when the wizard is first displayed. Each wizard page stores its GUI changes in the command object, so the object is populated as the user progresses. "Finish" simply triggers a call to execute(). This way, the command class contains no user interface code.
GUI buttons and menu items: In Swing programming, an Action is a command object. In addition to the ability to perform the desired command, an Action may have an associated icon, keyboard shortcut, tooltip text, and so on. A toolbar button or menu item component may be completely initialized using only the Action object.
Thread pools: A typical, general-purpose thread pool class might have a public addTask() method that adds a work item to an internal queue of tasks waiting to be done. It maintains a pool of threads that execute commands from the queue. The items in the queue are command objects. Typically these objects implement a common interface such as java.lang.Runnable that allows the thread pool to execute the command even though the thread pool class itself was written without any knowledge of the specific tasks for which it would be used.
Macro recording: If all user actions are represented by command objects, a program can record a sequence of actions simply by keeping a list of the command objects as they are executed. It can then "play back" the same actions by executing the same command objects again in sequence. If the program embeds a scripting engine, each command object can implement a toScript() method, and user actions can then be easily recorded as scripts.
Networking: It is possible to send whole command objects across the network to be executed on the other machines, for example player actions in computer games.

Terminology

The terminology used to describe command pattern implementations is not consistent and can therefore be confusing.

Client, Source: the button, toolbar button, or menu item clicked, the shortcut key pressed by the user.
Invoker, Command Object, Routed Command Object, Action Object: a singleton object (e.g. there is only one CopyCommand object), which knows about shortcut keys, button images, command text, etc. related to the command. A client/source object calls the Command/Action object's execute/performAction method. The Command/Action object notifies the appropriate client/source objects when the availability of a command/action has changed. This allows buttons and menu items to become inactive (grayed out) when a command/action cannot be executed/performed.
Receiver, Target Object: the object that is about to be copied, pasted, moved, etc.
Command Object, routed event args, event object: the object that is passed from the source to the Command/Action object, to the Target object to the code that does the work. Each button click or shortcut key results in a new command/event object. Some implementations add more information to the command/event object as it is being passed from one object (e.g. CopyCommand) to another (e.g. document section). Other implementations put command/event objects in other event objects (like a box inside a bigger box) as they move along the line, to avoid naming conflicts.
Handler, ExecutedRoutedEventHandler, method, function: the actual code that does the copying, pasting, moving, etc. In some implementations the handler code is part of the command/action object. In other implementations the code is part of the Receiver/Target Object, and in yet other implementations the handler code is kept separate from the other objects.
Command Manager, Undo Manager, Scheduler, Queue, Dispatcher, Invoker: an object that puts command/event objects on an undo stack or redo stack, or that holds on to command/event objects until other objects are ready to act on them, or that routes the command/event objects to the appropriate receiver/target object or handler code.

Examples

This Python program defines a command class for sending a simple SMTP mail message. The test() function demonstrates how the command class can be used to send a message.

import email.MIMEText, smtplib

class SendMailCommand:
    """ A simple command object for sending SMTP mail. """

    def __init__(self):
        self.server = 'mail'
        self.port = smtplib.SMTP_PORT
        self.sender = None
        self.recipient = None
        self.subject = ''
        self.message = ''

    def execute(self):
        """ Send the message via SMTP. """
        msg = email.MIMEText.MIMEText(self.message)
        msg['Subject'] = self.subject
        msg['From'] = self.sender
        msg['To'] = self.recipient

        s = smtplib.SMTP(self.server, self.port)
        s.sendmail(self.sender, [self.recipient], msg.as_string())
        s.close()

def test():
    cmd = SendMailCommand()
    cmd.sender = 'harold@example.com'
    cmd.recipient = 'maude@example.com'
    cmd.subject = 'Design patterns'
    cmd.message = open('dp.txt').read()
    cmd.execute()

The next example, also in Python, defines a class, ThreadPool, for executing commands asynchronously.

import threading, Queue

class _WorkerThread(threading.Thread):
    def __init__(self, queue):
        threading.Thread.__init__(self)
        self.queue = queue

    def run(self):
        while True:
            cmd = self.queue.get()
            cmd.execute()

class ThreadPool:
    """ A simple thread pool for executing commands asynchronously. """

    def __init__(self, N=4):
        self.queue = Queue.Queue()
        self.threads = []
        for i in range(N):
            t = _WorkerThread(self.queue)
            t.start()
            self.threads.append(t)

    def addCommand(self, cmd):
        """ Add a command to the queue of tasks.

        The command will be executed in a worker thread.  Commands are
        executed in first-come-first-served order.

        cmd - Any object with an .execute() method that accepts 0 arguments.
        """
        self.queue.put(cmd)

This generic thread pool works with any command object. It could be passed SendMailCommand objects, for example. Generic code that operates on command objects can be powerful and flexible while providing a simple API.

(Note: For brevity, this example omits many features that a real-world thread pool should have, such as exception handling.)

Here is a C++ example:

// Imaginary example of a command object used with a text editor

class EditorCommandInterface {
  public:
    virtual void execute(TextEditor& target) = 0;
    virtual ~EditorCommandInterface() { }
};
  
class CutCommand : public EditorCommandInterface {
  public:
    virtual void execute(TextEditor& target) {
      string s = target.GetSelectedText();
      target.DeleteRange(target.GetSelectStart(), target.GetSelectEnd());
      Clipboard.Copy(s);
    }
};

Here is a C# example:

namespace DesignPattern.Command {

 using System;
 abstract class Command
 {
   protected Receiver receiver;
   public Command(Receiver receiver)
   {
     this.receiver = receiver;
   }
   abstract public void Execute();
 }
 class ConcreteCommand : Command
 {
   public ConcreteCommand(Receiver receiver) : base (receiver) { }
   public override void Execute()
   {
     receiver.Action();
   }
 }
 class Receiver
 {
   public void Action()
   {
     Console.WriteLine("Called Receiver.Action()");
   }
 }
 class Invoker
 {
   private Command command;
   public void SetCommand(Command command)
   {
     this.command = command;
   }
   public void ExecuteCommand()
   {
     command.Execute();
   }
 }
 public class Client
 {
   public static void Main(string[] args)
   {
     Receiver r = new Receiver();
     Command c = new ConcreteCommand(r);
     Invoker i = new Invoker();
     i.SetCommand(c);
     i.ExecuteCommand();
   }
 }

}

References

Freeman, E; Sierra, K; Bates, B (2004). Head First Design Patterns. O'Reilly.

External links

Uses for the Command pattern

Terminology

Examples

References

See also

External links