What Is “Command-Based” Programming?¶
WPILib supports a robot programming methodology called “command-based” programming. In general, “command-based” can refer both the general programming paradigm, and to the set of WPILib library resources included to facilitate it.
“Command-based” programming is an example of what is known as a design pattern. It is a general way of organizing one’s robot code that is well-suited to a particular problem-space. It is not the only way to write a robot program, but it is a very effective one; command-based robot code tends to be clean, extensible, and (with some tricks) easy to re-use from year to year.
The command-based paradigm is also an example of what is known as declarative programming. In declarative programming, the emphasis is placed on what the program ought to do, rather than how the program ought to do it. Thus, the command-based libraries allow users to define desired robot behaviors while minimizing the amount of iteration-by-iteration robot logic that they must write. For example, in a command-based program, a user can specify that “the robot should perform an action when a button is pressed” (note the use of a lambda):
In contrast, in an ordinary imperative program, the user would need to check the button state every iteration, and perform the appropriate action based on the state of the button.
Subsystems and Commands¶
The command-based pattern is based around two core abstractions: commands, and subsystems.
Subsystems are the basic unit of robot organization in the design-based paradigm. Subsystems encapsulate lower-level robot hardware (such as motor controllers, sensors, and/or pneumatic actuators), and define the interfaces through which that hardware can be accessed by the rest of the robot code. Subsystems allow users to “hide” the internal complexity of their actual hardware from the rest of their code - this both simplifies the rest of the robot code, and allows changes to the internal details of a subsystem without also changing the rest of the robot code. Subsystems implement the
Commands define high-level robot actions or behaviors that utilize the methods defined by the subsystems. A command is a simple state machine that is either initializing, executing, ending, or idle. Users write code specifying which action should be taken in each state. Simple commands can be composed into “command groups” to accomplish more-complicated tasks. Commands, including command groups, implement the
How Commands Are Run¶
For a more detailed explanation, see The Command Scheduler.
Commands are run by the
CommandScheduler, a singleton class that is at the core of the command-based library. The
CommandScheduler is in charge of polling buttons for new commands to schedule, checking the resources required by those commands to avoid conflicts, executing currently-scheduled commands, and removing commands that have finished or been interrupted. The scheduler’s
run() method may be called from any place in the user’s code; it is generally recommended to call it from the
robotPeriodic() method of the
Robot class, which is run at a default frequency of 50Hz (once every 20ms).
Multiple commands can run concurrently, as long as they do not require the same resources on the robot. Resource management is handled on a per-subsystem basis: commands may specify which subsystems they interact with, and the scheduler will never schedule more than one command requiring a given subsystem at a time. This ensures that, for example, users will not end up with two different pieces of code attempting to set the same motor controller to different output values. If a new command is scheduled that requires a subsystem that is already in use, it will either interrupt the currently-running command that requires that subsystem (if the command has been scheduled as interruptible), or else it will not be scheduled.
Subsystems also can be associated with “default commands” that will be automatically scheduled when no other command is currently using the subsystem. This is useful for continuous “background” actions such as controlling the robot drive, or keeping an arm held at a setpoint.
When a command is scheduled, its
initialize() method is called once. Its
execute() method is then called once per call to
CommandScheduler.getInstance().run(). A command is un-scheduled and has its
end(boolean interrupted) method called when either its
isFinished() method returns true, or else it is interrupted (either by another command with which it shares a required subsystem, or by being canceled).
It is often desirable to build complex commands from simple pieces. This is achievable by composing commands into “command groups.” A command group is a command that contains multiple commands within it, which run either in parallel or in sequence. The command-based library provides several types of command groups for teams to use, and users are encouraged to write their own, if desired. As command groups themselves implement the
Command interface, they are recursively composable - one can include command groups within other command groups. This provides an extremely powerful way of building complex robot actions with a simple library.
Creating a Robot Project¶
Creating a project is detailed in Creating a Robot Program. Select “Template” then your programming language then “New Command Robot” to create a basic Command-Based Robot program.
When you create a New Command Robot project, the new command based vendor library is automatically imported. If you imported a 2019 project or created a different type of project, the old command library is imported, and it is necessary to import the new command based vendor library per 3rd Party Libraries and remove the old command library.