Commands are going to replace the controllers of Joomla! < 4.0. Each command implements a single task, that can be addressed from arbitrary channels.

A generic Controller will gather the available commands for a given domain (usually a component), identify the requested command, and execute it. The structure is illustrated by the following diagram:

When the Controller is instantiated, it creates the Command objects for the current component. When it is asked to run, it creates a CommandProcessor, which will handle the execution of the command. Just before the command is executed, the processor triggers the beforeExecute event.

On the beforeExecute event, an ACL plugin could check, whether or not the current user is permitted to execute the command. The Command and the component, it belongs to, can (and should) be completely ACL agnostic. However, the Command is covered by the ACL plugin due to the CommandProcessor, as any other command is.

Since the Command knows everything it need to execute, an arbitrary class can be used as a model. There is no limitation. For example, JImage functions could get accessible directly using commands. For Models with a well defined common interface, generic Commands can be used.

The Command gets its data from the Input object. It may or may not trigger its own events and call arbitrary library methods, which in turn may trigger standardized events. The results of the operation are collected in an Output object.

When the Command returns to the CommandProcessor, the latter triggers the afterExecute event, and gives control back to the Controller.

The following sequence diagram shows the control flow: