Command scheduler

WPILib Description Link

The CommandScheduler is the class responsible for actually running commands

Each iteration - ordinarily once per 20ms - the scheduler polls all registered buttons - schedules commands for execution accordingly - runs the command bodies of all scheduled commands - ends those commands that have finished or are interrupted

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The CommandScheduler also runs the periodic() method of each registered Subsystem

The CommandScheduler is a singleton, meaning that it has only one instance - To access the scheduler, users must call the CommandScheduler.getInstance() command. - To run scheduler, users must call CommandScheduler.getInstance().run() from the robotPeriodic() method of their Robot class

To schedule a command, users call the schedule() method - This method takes a command, and attempts to add it to list of currently-running commands - If it is added, its initialize() method is called 1. Verifies that the command isn’t in a composition. 2. No-op if scheduler is disabled, command is already scheduled, or robot is disabled and command doesn’t runsWhenDisabled. 3. If requirements are in use: - If all conflicting commands are interruptible, cancel them. - If not, don’t schedule the new command. 4. Call initialize()

The initialize() method of each Command is called when the command is scheduled, which is not necessarily when the scheduler runs (unless that command is bound to a button).

What does a single iteration of the scheduler’s run() method actually do? 1. the scheduler runs the periodic() method of each registered Subsystem 2. the scheduler polls the state of all registered triggers to see if any new commands that have been bound to those triggers should be scheduled. Then runs its Initialize() method 3. the scheduler calls the execute() method of each currently-scheduled command, and then checks whether the command has finished by calling the isFinished() method. If the command has finished, the end() method is also called, and the command is de-scheduled and its required subsystems are freed 4. Finally, any registered Subsystem has its default command scheduled (if it has one). Note that the initialize() method of the default command will be called at this time.

The scheduler can be disabled by calling CommandScheduler.getInstance().disable(). When disabled, the scheduler’s schedule() and run() commands will not do anything.