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  setTemperature(), circulate(), selectChannel(), etc.  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  setTemperature(), circulate(), selectChannel(), 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  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)  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 ClientInvoker setCommand() > Command execute() undo() Receiver action() ConcreteCommand execute() undo() public void execute() { receiver.action() }

Command  There are 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.  There are 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  Executes the Commands  Possibly at a later time

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

What about that Undo Button?  Add undo() to the Command interface  Each ConcreteCommand 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.  Add undo() to the Command interface  Each ConcreteCommand 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?  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?  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  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

Starting Lab TODAY!