Binding Commands to Triggers

Apart from autonomous commands, which are scheduled at the start of the autonomous period, and default commands, which are automatically scheduled whenever their subsystem is not currently in-use, the most common way to run a command is by binding it to a triggering event, such as a button being pressed by a human operator. The command-based paradigm makes this extremely easy to do.

As mentioned earlier, command-based is a declarative paradigm. Accordingly, binding buttons to commands is done declaratively; the association of a button and a command is “declared” once, during robot initialization. The library then does all the hard work of checking the button state and scheduling (or cancelling) the command as needed, behind-the-scenes. Users only need to worry about designing their desired UI setup - not about implementing it!

Command binding is done through the Trigger class (Java, C++) and its various Button subclasses (Java, C++).

Trigger/Button Bindings

Note

The C++ command-based library offers two overloads of each button binding method - one that takes a forwarding reference, and one that takes a raw pointer. The forwarding reference overload transfers ownership (by either moving or copying depending on if the command is an rvalue or and lvalue) to the scheduler, while the raw pointer overload leaves the user responsible for the lifespan of the command object. It is recommended that users preferentially use the forwarding reference overload unless there is a specific need to retain a handle to the command in the calling code.

There are a number of bindings available for the Trigger class. All of these bindings will automatically schedule a command when a certain trigger activation event occurs - however, each binding has different specific behavior. Button and its subclasses have bindings with identical behaviors, but slightly different names that better-match a button rather than an arbitrary triggering event.

whenActive/whenPressed

This binding schedules a command when a trigger changes from inactive to active (or, accordingly, when a button changes is initially pressed). The command will be scheduled on the iteration when the state changes, and will not be scheduled again unless the trigger becomes inactive and then active again (or the button is released and then re-pressed).

whileActiveContinuous/whileHeld

This binding schedules a command repeatedly while a trigger is active (or, accordingly, while a button is held), and cancels it when the trigger becomes inactive (or when the button is released). Note that scheduling an already-running command has no effect; but if the command finishes while the trigger is still active, it will be re-scheduled.

whileActiveOnce/whenHeld

This binding schedules a command when a trigger changes from inactive to active (or, accordingly, when a button is initially pressed) and cancels it when the trigger becomes inactive again (or the button is released). The command will not be re-scheduled if it finishes while the trigger is still active.

whenInactive/whenReleased

This binding schedules a command when a trigger changes from active to inactive (or, accordingly, when a button is initially released). The command will be scheduled on the iteration when the state changes, and will not be re-scheduled unless the trigger becomes active and then inactive again (or the button is pressed and then re-released).

toggleWhenActive/toggleWhenPressed

This binding toggles a command, scheduling it when a trigger changes from inactive to active (or a button is initially pressed), and cancelling it under the same condition if the command is currently running. Note that while this functionality is supported, toggles are not a highly-recommended option for user control, as they require the driver to mentally keep track of the robot state.

cancelWhenActive/cancelWhenPressed

This binding cancels a command when a trigger changes from inactive to active (or, accordingly, when a button is initially pressed). the command is canceled on the iteration when the state changes, and will not be canceled again unless the trigger becomes inactive and then active again (or the button is released and re-pressed). Note that cancelling a command that is not currently running has no effect.

Binding a command to a joystick button

The most-common way to trigger a command is to bind a command to a button on a joystick or other HID (human interface device). To do this, users should use the JoystickButton class.

Creating a JoystickButton

In order to create a JoystickButton, we first need a Joystick. All types of joysticks (including gamepads) are represented in code by the GenericHID class (Java, C++), or one of its subclasses:

Java

C++

Joystick exampleStick = new Joystick(1); // Creates a joystick on port 1
XboxController exampleController = new XboxController(2); // Creates an XboxController on port 2.

frc::Joystick exampleStick{1}; // Creates a joystick on port 1
frc::XBoxController exampleController{2} // Creates an XboxController on port 2

Note

When creating a JoystickButton with an XboxController, it is usually a good idea to use the button enum (Java, C++) to get the button number corresponding to a given button.

After the joystick is instantiated, users can then pass it to a JoystickButton object (Java, C++):

Warning

There is an issue with using JoystickButtons and POVButtons in C++. Take a look at the known issues page for more information.

Java

C++

JoystickButton exampleButton = new JoystickButton(exampleStick, 1); // Creates a new JoystickButton object for button 1 on exampleStick

frc2::JoystickButton exampleButton(&exampleStick, 1); // Creates a new JoystickButton object for button 1 on exampleStick

Binding a Command to a JoystickButton

Note

In the C++ command-based library, button objects do not need to survive past the call to a binding method, and so the binding methods may be simply called on a temp.

Putting it all together, it is very simple to bind a button to a JoystickButton:

Java

C++

// Binds an ExampleCommand to be scheduled when the trigger of the example joystick is pressed
exampleButton.whenPressed(new ExampleCommand());

// Binds an ExampleCommand to be scheduled when the trigger of the example joystick is pressed
exampleButton.WhenPressed(ExampleCommand());

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

Java

C++

exampleButton
    // Binds a FooCommand to be scheduled when the `X` button of the driver gamepad is pressed
    .whenPressed(new FooCommand())
    // Binds a BarCommand to be scheduled when that same button is released
    .whenReleased(new BarCommand());

exampleButton
    // Binds a FooCommand to be scheduled when the `X` button of the driver gamepad is pressed
    .WhenPressed(FooCommand())
    // Binds a BarCommand to be scheduled when that same button is released
    .WhenReleased(BarCommand());

Remember that button binding is declarative: bindings only need to be declared once, ideally some time during robot initialization. The library handles everything else.

Composing Triggers

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

Java

C++

// Binds an ExampleCommand to be scheduled when both the 'X' and 'Y' buttons of the driver gamepad are pressed
new JoystickButton(exampleController, XBoxController.Button.kX.value)
    .and(new JoystickButton(exampleController, XboxController.Button.kY.value))
    .whenActive(new ExampleCommand());

// Binds an ExampleCommand to be scheduled when both the 'X' and 'Y' buttons of the driver gamepad are pressed
(frc2::JoystickButton(&exampleController, frc::XBoxController::Button::kX)
    && JoystickButton(&exampleController, frc::XboxController::Button::kY))
    .WhenActive(new ExampleCommand());

Note that these methods return a Trigger, not a Button, so the Trigger binding method names must be used even when buttons are composed.

Creating Your Own Custom Trigger

While binding to HID buttons is by far the most common use case, advanced users may occasionally want to bind commands to arbitrary triggering events. This can be easily done by simply writing your own subclass of trigger or button:

Java

C++

public class ExampleTrigger extends Trigger {
  @Override
  public boolean get() {
    // This returns whether the trigger is active
  }
}

class ExampleTrigger : public frc2::Trigger {
 public:
  bool get() override {
    // This returns whether the trigger is active
  }
}

Alternatively, this can also be done inline by passing a lambda to the constructor of trigger or button:

Java

C++

// Here it is assumed that "condition" is an object with a method "get" that returns whether the trigger should be active
Trigger exampleTrigger = new Trigger(condition::get);

// Here it is assumed that "condition" is a boolean that determines whether the trigger should be active
frc2::Trigger exampleTrigger([&condition] { return condition; });