Gamepad Edge Detection

In the servo control lesson, you used if (gamepad1.a) to control a servo. That works for simple cases, but it has a subtle problem: in a real TeleOp loop running hundreds of times per second, holding a button for just a fraction of a second means the condition is true for dozens of loop iterations. For a toggle mechanism, this is a disaster. In this lesson, you will learn how to detect the moment a button is pressed.

The Problem with Raw Button Checks

Consider this code that tries to toggle a servo between two positions:

boolean isOpen = false;
while (opModeIsActive()) {
    if (gamepad1.a) {
        isOpen = !isOpen;  // Toggle the state
    }
if (isOpen) {
        claw.setPosition(1.0);
    } else {
        claw.setPosition(0.0);
    }
}

This looks reasonable, but it does not work. Here is why: the loop runs roughly 100-300 times per second. Even a quick button tap takes about 100-200 milliseconds. That means gamepad1.a will be true for 10-60 consecutive loop iterations.

On the first iteration: isOpen flips from false to true. On the second iteration: isOpen flips back from true to false. On the third: flips again. And so on.

By the time the driver releases the button, isOpen has toggled back and forth dozens of times, and you have no idea what state it ended up in. The servo will appear to do nothing, or it will flicker.

Rising Edge Detection

The solution is to detect the rising edge of the button press -- the exact moment the button transitions from "not pressed" to "pressed." You do this by comparing the current button state to the previous button state:

boolean aPressed = gamepad1.a && !previousGamepad1.a;

This expression is only true for one loop iteration: the first iteration where the button is down. After that, previousGamepad1.a catches up and the expression becomes false even though the button is still held.

The State-Copy Pattern

To implement edge detection, you need to save the previous gamepad state before reading the new one. The FTC SDK provides a copy() method for this:

Gamepad previousGamepad1 = new Gamepad();
Gamepad currentGamepad1 = new Gamepad();
while (opModeIsActive()) {
    // Save previous state, then capture current state
    previousGamepad1.copy(currentGamepad1);
    currentGamepad1.copy(gamepad1);
// Rising edge: button just pressed this iteration
    if (currentGamepad1.a && !previousGamepad1.a) {
        // This runs exactly ONCE per button press
        isOpen = !isOpen;
    }
claw.setPosition(isOpen ? 1.0 : 0.0);
}

The order matters: you must copy the old "current" into "previous" before copying the new gamepad data into "current."

The Toggle Pattern

Putting it all together, here is the complete toggle pattern:

@TeleOp(name = "Toggle Demo")
public class ToggleDemo extends LinearOpMode {
    @Override
    public void runOpMode() {
        Servo claw = hardwareMap.get(Servo.class, "clawServo");
boolean clawOpen = false;
        Gamepad previousGamepad1 = new Gamepad();
        Gamepad currentGamepad1 = new Gamepad();
waitForStart();
while (opModeIsActive()) {
            previousGamepad1.copy(currentGamepad1);
            currentGamepad1.copy(gamepad1);
// Toggle claw on A button press
            if (currentGamepad1.a && !previousGamepad1.a) {
                clawOpen = !clawOpen;
            }
claw.setPosition(clawOpen ? 1.0 : 0.0);
telemetry.addData("Claw", clawOpen ? "Open" : "Closed");
            telemetry.update();
        }
    }
}

This is one of the most useful patterns in FTC TeleOp programming. You can use it for any mechanism that needs to toggle between states.

Simplification for This Exercise

In our tutorial sandbox, the code runs for one iteration (not in a continuous loop). Since there is no "previous" state, we will simplify the exercise: gamepad1.a starts as true, and since the servo starts at position 0.0, pressing A should set it to 1.0.

In real FTC code, you would use the full edge detection pattern shown above. But the core concept is the same: check a condition and set the servo position based on a state variable.

Your Exercise

You have a servo named "clawServo". The gamepad's A button is pressed (gamepad1.a is true).

Your task:

1. Get the servo from the hardware map.
2. Call waitForStart().
3. Create a boolean variable clawOpen starting as false.
4. Check if gamepad1.a is pressed. If so, toggle clawOpen to true.
5. If clawOpen is true, set the servo to position 1.0. Otherwise, set it to 0.0.

Give it a try!