Command

DCS – SWC 2

Executing a command

• Executing a command appears simple at first, but many details to consider:– Who creates a command?– Who invokes a command?– Who executes a command?– Who knows the details of the command?

DCS – SWC 3

Executing a command

• Details to consider (continued):– When should the command be executed?– Should the command return a value?– Should we be able to undo a command?– What about sets of commands?

DCS – SWC 4

Executing a command

• Parties involved in a command– Client: formulates the command– Invoker: sets the ”wheels in motion” for

getting the command executed– Manager: receives the command from the

invoker– Worker: Actually executes the command

DCS – SWC 5

Simple commands

• Simple execution of a command:– Client and Invoker are the same– Manager and Worker are the same– Everybody knows all details of the command– Command executed immediately by Worker– Command does not return a value– Command cannot be undone– Command stands alone

DCS – SWC 6

Simple commandsGet a shave,

now!!Sir, right

away, sir!!

Client and

Invoker

Manager and

Worker

DCS – SWC 7

Simple commands

• A simple command is similar to a method call

• Not really any reason – or opportunity – for reducing coupling

• For more advanced systems, this is not flexible enough

DCS – SWC 8

Complex commands

• Consider how an order is processed in a restaurant– Customer: decides which menu item to order– Waiter: picks up orders from Customers, and

brings them to the Kitchen manager– Kitchen manager: keeps track of incoming

orders, and distributes them to Chefs– Chef: Prepares the order

DCS – SWC 9

Complex commands

Customer

Customer

Customer

Customer

Waiter

KitchenManager

Chef

Chef

Chef

DCS – SWC 10

Complex commands

• If the scheme for simple commands was used, customers should yell orders directly to chefs…

• System would break down quickly

• By turning commands into objects, we gain much more flexibility

DCS – SWC 11

Complex commands

• Using objects for commands enables– Queueing of commands – we can postpone

execution of a command– Undoing of commands– Logging of commands– Better separation of knowledge and

responsibility – decoupling!

DCS – SWC 12

Complex commands

• A Customer in a restaurant decides which order to prepare – but does not care about the details of preparation

• The Waiter and Kitchen Manager do not really care about details either

• The Chef, however, must know all details

• Encapsulate what varies…

DCS – SWC 13

Complex commands

• In OO terms, we therefore need an interface for commands

• Interface for all commands is the same, but implemen-tation is of course specific for each command

Commandexecute()

DCS – SWC 14

Complex commands

Commandexecute()

MakePancakesexecute()

MakeSteakexecute()

Contains all details about making pancakes

DCS – SWC 15

Complex commands

• Customer wants pancakes, so he creates a MakePancakes object

• Waiter takes order to Kitchen Manager, but only knows that it is a Command object (think numbers for an order…)

DCS – SWC 16

Complex commands

• Kitchen Manager can manage the object (queue it, etc.) but also only knows the interface

• When appropriate, the Kitchen Manager calls execute on the Command object

• Code in Command object is then executed (by a Chef)

DCS – SWC 17

Complex commands

Customer

Waiter

KitchenManager

Chef

Command order = new MakePancakes(…);

manager.send(order);

order.execute();

// Do execute() for MakePancakes

DCS – SWC 18

The Command pattern

Client

Invoker Manager

Worker

Command

Concrete-Command

DCS – SWC 19

The Command pattern

• The Client can create specific commands

• The Invoker and Manager only knows the Command interface

• The Worker carries out the actual work defined in the command

• Further flexibility can be introduced by making a Worker interface

DCS – SWC 20

The Command pattern

Workeraction(Command c)

ConcreteWorkeraction(Command c)

CommandsetWorker(Worker w)execute()

ConcreteCommandsetWorker(Worker w)execute()

DCS – SWC 21

The Command pattern

// Client code

Worker w = new MergeSortWorker();

Command c = new SortBankAccounts();

c.setWorker(w);

theInvoker.invokeCommand(c);

...

// Code from Command

public void execute()

{

theWorker.action(this);

}

DCS – SWC 22

Queueing of commands

• Queueing is used when immediate execution of a command cannot be guaranteed

• Command is executed when needed resources are available

• Queue of command is managed by the Manager

DCS – SWC 23

Queueing of commands

• Typical example of a ”producer/consumer” scenario, using threads:

Command

Command

Command

Command

Command

…

Producer

thread

Consumer

thread Consumer

thread Consumer

Thread(s)

add remove

DCS – SWC 24

Undo of commands

• Is – in principle – very easy; just let concrete Command classes implement an undo method, which has the reverse effect of the execute method:

public void execute() { light.On(); }

public void undo() { light.Off(); }

DCS – SWC 25

Undo of commands

• Simple or complex command – the specific Command class always knows how to undo an action.

• Somebody (Manager?) must keep track of which action to undo…

DCS – SWC 26

Logging of commands

• Primary application is for crash recovery

• Add two new methods store and load to the Command interface.

• store: Save the state of the command to disk

• load: Load the state on the command into the command. Calling execute() will now execute the reestablished command.

DCS – SWC 27

Logging of commands

• Could call store as part of execute, or at certain externally defined ”checkpoints”.

• Important use is for transactional proces-sing, where e.g. system crash must not change the execution of the command