Command Pattern When Actions Speak Louder Than Words

Command Pattern When Actions Speak Louder Than Words

Programmable Remote § Example in book § On/off for seven devices § Single “Undo” button referring to the last button pushed § Can control a wide variety of devices from lights to TVs to other appliances § Difference devices support different operations § Many are on/off § set. Temperature(), circulate(), select. Channel(), etc.

Goals of a Good design For This Situation § Separate the remote’s ability to execute a command from the variety of operations and devices that can be supported. § What varies? § Devices and operations § What Stays the same? § The structure of the remote control

Four Parts of our Design § The actions that we want each button to cause (Command) § Someone to set up those actions (Client) § The remote control to execute the actions (Invoker) § The device affected by the action (Receiver)

Command Pattern Client Invoker set. Command() <<interface>> Command execute() undo() Receiver Concrete. Command action() execute() undo() public void execute() { receiver. action() }

Command § There a variety of commands, but we want the remote to treat them all the same § Need Command interface (with an execute() method) § Need Concrete Commands for each operation we will support § Concrete Commands pair the operation with the device it will affect - “light On” paired with a specific light in the house.

Receiver § The object the command should affect

Invoker § Executes the Commands § Possibly at a later time

Let’s Look at the Code § Command. Simple. Remote § Command. Remote § Find § § Devices Concrete Commands Remote Control Client

What about that Undo Button? § Add undo() to the Command interface § Each Concrete. Command must know how to undo the action it executed § Make Remote remember the last command executed and call it’s undo method for the undo button.

Let’s Look at the code § How does remote remember previously executed command? § Undo for § Lights - simple (does light off need the hardcoded previous value? ) § Dimmer - ask device for state before execution § Ceiling fan - see any duplication? How could we eliminate it?

Macro Commands § Can make a Concrete Command (subclass of Command) that contains a list of commands § Commands can be given at construction, or set up via method calls § Execute method calls the execute method on each of the commands in the list § How could we use this to give a good progress indicator?

Other uses of command § Queuing requests § Can allocate commands to various threads for processing to load balance between threads/processors § Logging requests (audit trail) § Just have to save the command objects as they execute. § If something goes wrong, we can read the log and re-create the sequence of commands (so no data is lost). § Can also back out changes that cause troubles

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