ActionScript 3.0 Design Patterns

Minimalist Example of a Command Pattern | 251

The client assigns eachConcreteCommandinstance to specific triggers ininvokers.

Invokers are where the commands are called from. They hold on to the

ConcreteCommandobjects and call theirexecute( )methods when it’s time to execute

the command. You’ll clearly see how this is implemented in ActionScript 3.0 in the

minimalist application.

Minimalist Example of a Command Pattern

This example implements the command pattern class diagram in Figure 7-3. The

command pattern consists of thecommand interface,concrete commandsthat imple-

ment the command interface,invokersthat call theexecute( )method in concrete

commands,receiversthat implement the behavior required of commands, andclients

that create concrete commands and pass them on to invokers.

Code examples Examples 7-1 through 7-5 show the minimalist implementation of

the command pattern.

The Command Interface

Example 7-1 shows theICommandclass that defines the interface for commands. It

defines a single method calledexecute( ).

The Concrete Command

Example 7-2 shows theConcreteCommandclass that implements theICommandinter-

face. The parameterized constructor takes aReceiverclass instance and assigns it to

thereceiverproperty. Theexecute( )command is implemented by delegating to the

receiverinstance by calling itsaction( )method. Note that, because the receiver

Figure 7-3. Command pattern class diagram

Example 7-1. ICommand.as

package
{
public interface ICommand {
function execute( ):void;
}
}

Client

Receiver receiver

action()

ConcreteCommand

execute() receiver->action()

Command

execute()

Invoker