Common Command Functions

Here we have listed some of the most basic and common functions we use when defining commands to control the robot. This is not a comprehensive list. You can read more about this API and discover more functions at the links below.

• The full API documentation for the Command class
• The full API documentation for the Commands utility class
• The full API documentation for the Trigger class
• WPILib docs on more command compositions
• WPILib docs on structuring command based projects

Base command functions

These functions work great as a base for a sequence of actions you want to define.

•  run(action)
  • Repeatedly runs an action until interrupted
• runEnd(runAction, endAction)
  • Repeatedly runs an action until interrupted, then runs a second action
• runOnce(action)
  • Runs an action once. that's it
• startEnd(startAction, endAction)
  • Runs an action once and another action when interrupted

Examples

A function that takes in a number representing the power to run the arm motor at. When the command ends, the motor will stop. Note that there is nothing actually specifying when the command ends, only what happens when it does.

// ArmSubsystem.java
public Command moveAndStop(double power) {
    return runEnd(() -> armKraken.setControl(new DutyCycleOut(power)), armKraken::stopMotor);
}

Modifier command functions

These functions work great to enhance a command's functionality. They can refine when a command ends and join a command with others.

• andThen(command)
  • called on a command to run another command when the first one finishes
• alongWith(command)
  • called on a command to run another command in parallel with the first one
• until(condition)
  • called on a command to end it when the condition is true
• withTimeout(seconds)
  • called on a command to end it after a specified number of seconds

Example

We have modified the function from the previous example with a timeout. Now we have specified when the command should end. In this case, after two seconds.

public Command moveAndStop(double power) {
    return runEnd(() -> armKraken.setControl(new DutyCycleOut(power)), armKraken::stopMotor).withTimeout(2);
}

Binding commands to buttons

These functions use the Trigger api to start commands when some condition is true. The most common use case is starting a command when a button is pressed. You can read more about triggers here.

• onTrue(condition)
  • Starts a command when the condition changes from false to true
• whileTrue(condition)
  • Starts a command when the condition changes from false to true, and then cancels the command when the condition changes back to false. You can use this when you want a command to run only while holding a button.
• onFalse(condition)
  • Starts a command when the condition changes from true to false
• whileFalse(condition)
  • Starts a command when the condition changes from true to false, and then cancels the command when the condition changes back to true

Example

Line 3 of this example shows how to run the command we made in the previous example when button 1 on the joystick is pressed. Since we pass in 0.5 into the moveAndStop method, the arm will move at 50% power. The command will then stop after 2 seconds just as we defined it. Note that the configureBindings method is one that is preexisting in RobotContainer. We do not make this for every binding.

// RobotContainer.java
private void configureBindings() {
    joystick.button(1).onTrue(arm.moveAndStop(0.5));
}