Accessing Gamepads
Reading the Gamepad
To perform in TeleOp, your code needs to be able to read inputs from the driver's handheld controllers in real time. This is accomplished by accessing the fields of two instances of theGamepad class in the TeleOp class: gamepad1 and gamepad2. Both of these fields will have been initialized by the FTC Robot Controller app while the program is initializing, so you do not have to worry about their initialization and may program under the assumption that they are non-null instances of the Gamepad class.
How do you know which controller is gamepad1 and which is gamepad2? These fields correspond to the User 1 and User 2 registrations on the Driver Station, which may be set up by the drivers on the FTC Driver Station app before the program is initialized; see FTC's wiki page#running-your-op-mode-with-a-gamepad-connected) on how these registrations work.
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Notice how each control column on a standard Gamepad is reflected with a field in Gamepad. The values for these fields are updated in real time as your OpMode executes, so accessing them at any time will yield the most up-to-date data from the controller itself. For example, the following expression is true whenever the "X" button of Gamepad 1 is pressed:
Different inputs on controllers will correspond to different data types. Knowing which field of the Gamepad class will result in which data type is essential to a functioning TeleOp.
The controllers' buttons-X, A, B, Y, Back, and Stop-are booleans. Thus, gamepad1.x, gamepad1.a, gamepad1.b, gamepad1.y, gamepad1.back, andgamepad1.start, respectively, will be true if the button is pressed and false otherwise.
The left and right bumpers, too, are booleans. Thus, gamepad1.left_bumper and gamepad1.right_bumper will be true if the bumper is pressed and false otherwise.
The x and y axes values of each joystick is a double, ranging between -1 and 1. The center of an axis is at the value 0. The x and y values of the left and right sticks may be read from the following fields: gamepad1.left_stick_x, gamepad1.left_stick_y, gamepad1.right_stick_x, andgamepad1.right_stick_y. If the joysticks were in the positions shown in the image above, all four of these expressions would evaluate to 0.
The values of the left and right triggers, too, are doubles, ranging between 0 and 1. gamepad1.left_trigger and gamepad1.right_trigger would give 0 if the triggers were unpressed, 1 if they were pressed fully to the ground, and a decimal value in between the two if the trigger were not fully pressed.
Finally, the D-pad buttons give boolean values. gamepad1.dpad_up, gamepad1.dpad_right, gamepad1.dpad_down, and gamepad1.dpad_left will each give true if the corresponding region of the dpad is pressed and false otherwise.
Relaying Inputs in an iterative OpMode
Recall that in an iterative OpMode, the loop() method is executed repeatedly when the OpMode is running. Given that the values in gamepad1 and gamepad2 are constantly updated, we can conveniently relay the state of input columns to output devices. Consider the following example of a simple tank-drive control scheme implementation:
public void loop() {
leftMotor.setPower(-gamepad1.left_stick_y);
rightMotor.setPower(-gamepad1.right_stick_y);
}
In this example, we simply pass the values of the fields of gamepad1 to the two drivetrain motors repeatedly. Whenever a driver pushes the left stick forward, leftMotor receives a positive power value; whenever a driver pushes the right stick forward, rightMotor receives a positive power value.
Warning
Notice how the \(y\) axes of the gamepad sticks are inverted: pushing the stick forward will cause the corresponding field to have a negative value. We typically need to negate the field in code whenever we wish to access these control axes.
Examples from FTC and the Club
Examine the following examples from both FIRST and the club's previous programs. Try to understand each control scheme and imagine explaining it to a potential driver—the control scheme of your robot is something you will need to negotiate!