Subsystem requirements: "Why can't it just be static?"

[GalexY727]

What even is a subsystem?

Subsystems are the mechanical parts of our robot. They contain motors that run certain components, and it's important to be able to handle them throughout your code while also keeping in mind how many subsystems are being told to move at any specific moment. They commonly have Command methods inside of them, which give other files easy access to complete tasks using the subsystem, such as intake.getIntakeCommand().

Requirements:

Here's how WPILib explains it:

Subsystems also serve as the backbone of the CommandScheduler’s resource management system. Commands may declare resource requirements by specifying which subsystems they interact with; the scheduler will never concurrently schedule more than one command that requires a given subsystem. An attempt to schedule a command that requires a subsystem that is already-in-use will either interrupt the currently-running command or be ignored, based on the running command’s Interruption Behavior.

Subsystems can be associated with “default commands” that will be automatically scheduled when no other command is currently using the subsystem. This is useful for “background” actions such as controlling the robot drive, keeping an arm held at a setpoint, or stopping motors when the subsystem isn’t used. Similar functionality can be achieved in the subsystem’s periodic() method, which is run once per run of the scheduler; teams should try to be consistent within their codebase about which functionality is achieved through either of these methods. Subsystems are represented in the command-based library by the Subsystem interface (Java, C++).

Interpretation/translation:

The best to keep track of our currently used subsystems is through requirements, which is an optional additional parameter that can be applied to almost every Command, such as Commands.run() and Commands.runOnce(). Additionally, since our subsystem extends SubsystemBase, we are given the alternative (and cleaner) solution to adding requirements by calling super.runOnce() which can also be written as just runOnce(). By removing the Commands. prefix to our Commands.runOnce()s, we automatically add a requirement of this to our command, which means that additional commands requesting access to the subsystem will either be interrupted and stopped (if it is a default command) or ignored since the subsystem in question is already occupied with a task.