Command
- Encapsulate a request as an object, thereby letting you parameterize
clients with different requests, queue or log requests, and support undoable
operations.

- Command : declares an interface for executing an operation.
- ConcreteCommand : defines a binding between a Receiver object and
an action. implements Execute by invoking the corresponding operation(s) on
Receiver.
- Client : creates a ConcreteCommand object and sets its receiver.
- Invoker : asks the command to carry out the request.
- Receiver : knows how to perform the operations associated with
carrying out a request. Any class may serve as a Receiver.

- The client creates a ConcreteCommand object and specifies its receiver.
- An Invoker object stores the ConcreteCommand object.
- The invoker issues a request by calling Execute on the command. When
commands are undoable, ConcreteCommand stores state for undoing the command
prior to invoking Execute.
- The ConcreteCommand object invokes operations on its receiver to carry out
the request.
- parameterize objects by an action to perform, as MenuItem objects did
above. You can express such parameterization in a procedural language with a
callback function, that is, a function that's registered somewhere to be
called at a later point. Commands are an object-oriented replacement for
callbacks.
- specify, queue, and execute requests at different times. A Command object
can have a lifetime independent of the original request. If the receiver of
a request can be represented in an address space-independent way, then you
can transfer a command object for the request to a different process and
fulfill the request there.
- support undo. The Command's Execute operation can store state for
reversing its effects in the command itself. The Command interface must have
an added Unexecute operation that reverses the effects of a previous call to
Execute. Executed commands are stored in a history list. Unlimited-level
undo and redo is achieved by traversing this list backwards and forwards
calling Unexecute and Execute, respectively.
- support logging changes so that they can be reapplied in case of a
system crash. By augmenting the Command interface with load and store
operations, you can keep a persistent log of changes. Recovering from a
crash involves reloading logged commands from disk and reexecuting them with
the Execute operation.
- Command decouples the object that invokes the operation from the one that
knows how to perform it.
- Commands are first-class objects. They can be manipulated and extended
like any other object.
- You can assemble commands into a composite command. An example is the
MacroCommand class described earlier.
- It's easy to add new Commands, because you don't have to change existing
classes.