binding-commands-to-triggers

(WPI Explanation) If your command isn’t an autonomous command or a default command, then you need to bind your command to a trigger for the driver to be able to run the command.

Trigger documentation ← Useful for viewing Trigger functions, like OnTrue() and WhileFalse().

Declare instance of trigger, then binding command to trigger instance:

Declare trigger using CommonGenericHID factories (docs) in configureBindings():

CommandXboxController exampleCommandController = new CommandXboxController(1); // Creates a CommandXboxController on port 1.
Trigger xButton = exampleCommandController.x(); // Creates a new Trigger object for the `X` button on exampleCommandController

Then you can bind the command:

xButton.onTrue(new ExampleCommand(m_exampleSubsystem))

Alternatively, declare and bind trigger instance in-line:

Creates trigger in the configureBindings() function (in RobotContainer), then declare an onTrue() binding.

import edu.wpi.first.wpilibj.templates.commandbased.commands.ExampleCommand;
 
public class RobotContainer {
	private final HatchSubsystem m_hatchSubsystem = new HatchSubsystem();
	
	private void configureBindings() {
		// Schedule `ExampleCommand` when `exampleCondition` changes to `true`
	    new Trigger(m_hatchSubsystem::exampleCondition)
			.onTrue(new ExampleCommand(m_exampleSubsystem));
  }
}

(In this case, exampleCondition is a method in HatchSubsystem):

public boolean exampleCondition() {
	// Query some boolean state, such as a digital sensor.
	return false;
}

Because you can declare a command in-line AND bind it to a trigger in-line, you can do both in-line. So when the command to run is simple, you can do all that in-line. This is actually what we did in the beginning of the command-based structure note (here).

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More fun with triggers

Chaining Calls: It is useful to note that the command binding methods all return the trigger that they were called on, and thus can be chained to bind multiple commands to different states of the same trigger. For example:

xButton
    // Binds a CommandA to be scheduled when the button is pressed
    .onTrue(new CommandA())
    // Binds a CommandB to be scheduled when that same button is released
    .onFalse(new CommandB());

Composing Triggers: The Trigger class can be composed to create composite triggers through the and(), or(), and negate() methods (or, in C++, the &&, ||, and ! operators). For example:

// Binds an ExampleCommand to be scheduled when both the 'X' and 'Y' buttons of the driver gamepad are pressed
exampleCommandController.x()
    .and(exampleCommandController.y())
    .onTrue(new ExampleCommand());

Debouncing: aka coding for the “rising edge” — don’t worry about this unless some commands are running twice

// debounces exampleButton with a 0.1s debounce time, rising edges only
exampleButton.debounce(0.1).onTrue(new ExampleCommand());
 
// debounces exampleButton with a 0.1s debounce time, both rising and falling edges
exampleButton.debounce(0.1, Debouncer.DebounceType.kBoth).onTrue(new ExampleCommand());