The Chain-of-Command Pattern: OOP Techniques in PHP

Will Fitch — Tue, 10 Mar 2009 12:07:43 +0000

The chain-of-command pattern, like most others, assists with maintaining a loose coupling within your classes. By providing a series of classes that implement the ICommand interface and do a specific bit of processing, the developer doesn’t have to care which method to execute.

The ICommand Interface

interface ICommand
{
    /**
     * Run a command
     * @param string $command_type
     * @param args $args
     */
    public function onCommandCall($command_type, $args);
}

This simple interface provides a method for executing a call to initiate a bit of functionality. Along with the classes that execute the functionality, you’ll need a class to handle the chain. We’ll provide this class with another interface requiring the use of a method to add chains and execute commands:

interface ICommandBuilder
{
	/**
	 * Run a command
	 *
	 * @param string $command_type
	 * @param mixed $args
	 */
	public function runCommand($command_type, $args);
 
	/**
	 * Add a new command class
	 *
	 * @param mixed $commanding_class
	 */
	public function addCommand($commanding_class);
}

The two methods here are used for executing commands and adding new command classes. Let’s build the chain-of-command class.

class ChainBuilder implements ICommandBuilder
{
 
	private $_commands=array();
 
	public function runCommand($command_type, $args)
	{
		if (count($this->_commands) > 0)
		{
			foreach ($this->_commands as $class)
			{
				if ($class->onCommandCall($command_type, $args))
				{
					return;
				}
			}
		}
	}
 
	public function addCommand($commanding_class)
	{
		$this->_commands[] = $commanding_class;
		return $this;
	}
}

The class above provides a mechanism for chaining your requests. You add the necessary commands to execute, then pass requests to runCommand, which will loop through all of the commands until one successfully completes the action, or it exhausts all commands. Here you could add interface checks and throw the necessary exceptions for commands not found, etc.

Let’s add a couple of command implementations: email and stream.

class EmailCommand implements ICommand
{
	public function onCommandCall($command, $args)
	{
		if ($command == 'email')
		{
			// send an email with $args
			return true;
		}
		return false;
	}
}
 
class StreamCommand implements ICommand
{
	public function onCommandCall($command, $args)
	{
		if ($command == 'writeStream')
		{
			// Write the stream
			return true;
		}
		return false;
	}
}

These commands email a user and write to a stream. Now let’s use them!

$command = new ChainBuilder();
$command->addCommand(new StreamCommand())->addCommand(new EmailCommand());
$command->runCommand('email',array('email'=>'me@you.com'));
$command->runCommand('writeStream',array('content'=>'Hi, stream!'));

Conclusion

At first glance, the chain-of-command pattern may look like overhead. But on a large code base, this code be useful when making a lot of modifications, etc.

Will Fitch's Blog » chain of command

The Chain-of-Command Pattern: OOP Techniques in PHP