Lesson 5: Scheduling Commands
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The CommandScheduler is the class responsible for actually running commands. Each iteration (ordinarily once per 20ms, or 50 times a second), the scheduler polls all registered buttons, schedules commands for execution accordingly, runs the command bodies of all scheduled commands, and ends those commands that have finished or are interrupted.
The CommandScheduler is a singleton like our subsystems, meaning that it is a globally-accessible class with only one instance. Accordingly, in order to access the scheduler, you must call the CommandScheduler.getInstance() command.
For the most part, you don't have to call scheduler methods directly - almost all important scheduler methods have convenience wrappers elsewhere (e.g. in the Command and Subsystem interfaces).
However, there is one exception: you must call CommandScheduler.getInstance().run() from the robotPeriodic() method of their Robot class. If this is not done, the scheduler will never run, and the command framework will not work.
To schedule a command, you call the schedule() method. This method takes a command (and, optionally, a specification as to whether that command is interruptible), and attempts to add it to list of currently-running commands, pending whether it is already running or whether its requirements are available. If it is added, its initialize() method is called.
What does a single iteration of the scheduler’s run() method actually do? The following section walks through the logic of a scheduler iteration.
First, the scheduler polls the state of all registered buttons to see if any new commands that have been bound to those buttons should be scheduled. If the conditions for scheduling a bound command are met, the command is scheduled and its initialize() method is run.
Secondly, 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.
Note that this sequence of calls is done in order for each command - thus, one command may have its end() method called before another has its execute() method called. Commands are handled in the order they were scheduled.
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.
Occasionally, it is desirable to have the scheduler execute a custom action whenever a certain command event (initialization, execution, or ending) occurs. This can be done with the following three methods:
The onCommandInitialize method runs a specified action whenever a command is initialized.
The onCommandExecute method runs a specified action whenever a command is executed.
The onCommandFinish method runs a specified action whenever a command finishes normally (i.e. the isFinished() method returned true).
The onCommandInterrupt method runs a specified action whenever a command is interrupted (i.e. by being explicitly canceled or by another command that shares one of its requirements).
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