2.4.1 - Command structure

WPILib offers a Command class that you can subclass (inherit from) to create new commands. These commands have four important methods that can be overridden to change the behavior of the command.

Initialization

Commands need to be initialized, meaning that there is some setup necessary to run them. The initialization is just a series of steps that commands run to ensure that they begin in a known state or configuration. For example, these might involve setting the desired state for a subsystem or setting variables to the correct values.

Initialization occurs once when the command is initially scheduled, and is the first thing the command will do.

When subclassing Command, this behavior is in the initialize() method.

Execution

The next phase of a command’s life is execution, which is where the command repeats the same steps periodically. On a robot, this occurs by default at 50 hertz (meaning 50 times per second, or every 20ms).

Execution should involve steps such as applying a new output based on a sensor’s readings, or something that needs to be re-ran every cycle.

This behavior is found in the execute() method of the Command class.

End conditions

Often, commands have a specific way to check whether they’ve completed. If a command is completed, it should stop executing and be unscheduled.

To specify when a command should naturally end, we can override the isFinished() method in Command, which returns a boolean value - true if the command is finished and false otherwise.

Note

Be aware that commands can be interrupted which causes them to terminate and be unscheduled even if isFinished() returns false. In a moment, we’ll discuss reasons that commands may be canceled like this.

Ending

When a command ends, there may be some behavior that needs to run to wrap up the command’s behavior.

This could involve stopping a subsystem, or doing some other “instant” cleanup.

The method here to override in Command is end(). Unlike the previously mentioned methods, however, this method accepts a boolean parameter. This parameter refers to whether the command was cancelled.

If the argument is false, then the command was not interrupted and therefore completed naturally. However, if the command was interrupted, then the value of the argument must be true to signify this.