Lesson 2.4: Using start() and stop() for One-Time Robot Actions

Technical Context

start() and stop() improve robot reliability by handling transitions. start() is often used to reset the getRuntime() timer exactly when the match begins, ensuring autonomous timing is accurate. stop() acts as a fail-safe, allowing you to force motors to zero power or close files when the match ends, preventing hardware damage or data corruption.

start() and stop() Functions

While init() and loop() are required, start() and stop() are optional lifecycle methods.

start() is called once the instant the "Play" button is pressed, making it more precise than init() for resetting counters.

stop() is called once when the "Stop" button is hit or the match timer expires. This is a reliable way to handle cleanup logic, such as ensuring a high-speed flywheel is commanded to setPower(0) before the program terminates.

Annotated Code

@TeleOp(name="Transition_Demo")
public class TransitionDemo extends OpMode {

    @Override
    public void init() {
        telemetry.addData("Hub", "Standby");
    }

    @Override
    public void start() {
        // Resetting the internal clock the moment the match begins
        resetRuntime();
    }

    @Override
    public void loop() {
        telemetry.addData("Match Time", getRuntime());
    }

    @Override
    public void stop() {
        // Cleanup: Ensures no logic persists after the program ends
        telemetry.addData("Match", "Complete");
    }
}

How an FTC Team Separates Lifecycle Work

Titan Robotics Club keeps initialization, enable-time work, and cleanup in separate callbacks. This shortened excerpt leaves out logging and vision setup but preserves the calls that hand each phase to the robot object:

@Override
public void robotInit() {
    robot = new Robot(TrcRobot.getRunMode());
    // ... gamepad and drive setup ...
}

@Override
public void startMode(TrcRobot.RunMode prevMode, TrcRobot.RunMode nextMode) {
    robot.startMode(nextMode);
}

@Override
public void stopMode(TrcRobot.RunMode prevMode, TrcRobot.RunMode nextMode) {
    robot.stopMode(prevMode);
}

The important design choice is the boundary: construction belongs to initialization, enabling sensors belongs to startMode(), and cleanup belongs to stopMode(). Adapt the idea to the SDK callbacks in this lesson rather than copying the team's framework types. Excerpted from Team 3543 Titan Robotics Club's FtcTeleOp.java at a pinned commit under the MIT License.

Fill-in-the-Blank Practice

The __________ method is called exactly once when the driver presses the "Play" button.
To ensure all motors are deactivated at the end of a match, logic should be placed in the __________ method.
The method that runs repeatedly between the "Init" and "Play" buttons being pressed is __________.

Show answers

start()
stop()
init_loop()

Simulator Challenge

Your team needs to reset the robot's timer at the start of the match and display a "Robot Shutdown" message when the program is stopped.

Telemark Unit 2 Simulator

Supports `init()`, `init_loop()`, `start()`, `loop()`, and `stop()`.

Reads `@TeleOp`, `@Autonomous`, `@Disabled`, telemetry, and `gamepad1`.

Handles local variables, simple math, `resetRuntime()`, and `getRuntime()`.

Does not execute hardware, `gamepad2`, `else` blocks, or loop structures.

Show answer

@Override
public void start() {
    resetRuntime();
}

@Override
public void stop() {
    telemetry.addData("Status", "Robot Shutdown");
}

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Technical Context​

start() and stop() Functions​

Annotated Code​

How an FTC Team Separates Lifecycle Work​

Fill-in-the-Blank Practice​

Simulator Challenge​