Controls

In this section, we'll make the ball that we displayed in the last section move by pressing the D-Pad.

The GBA controls

The GBA has 10 buttons we can read the state of, and this is the only way a player can directly control the game. They are the 4 directions on the D-Pad, A, B, Start, Select, and the L and R triggers.

Reading the button state

There are two ways to capture the button state in agb, interrupts and polling. In most games, you will want to use polling, so that is what we will use now. Interrupts will be covered in a later chapter.

To add button control to our game, we will need a ButtonController. Add this near the top of your main function:


#![allow(unused)]
fn main() {
let mut input = agb::input::ButtonController::new();
}

The button controller is not part of the Gba struct because it only allows for reading and not writing so does not need to be controlled by the borrow checker.

Replace the inner loop with the following:


#![allow(unused)]
fn main() {
let mut ball_x = 50;
let mut ball_y = 50;

// now we initialise the x and y velocities to 0 rather than 1
let mut x_velocity = 0;
let mut y_velocity = 0;

loop {
    ball_x = (ball_x + x_velocity).clamp(0, agb::display::WIDTH - 16);
    ball_y = (ball_y + y_velocity).clamp(0, agb::display::HEIGHT - 16);

    // x_tri and y_tri describe with -1, 0 and 1 which way the d-pad
    // buttons are being pressed
    x_velocity = input.x_tri() as i32;
    y_velocity = input.y_tri() as i32;
    
    ball.set_x(ball_x as u16).set_y(ball_y as u16);

    agb::display::busy_wait_for_vblank();
    object.commit();

    // We must call input.update() every frame otherwise it won't update based
    // on the actual button press state.
    input.update();
}
}

Here we use the x_tri() and y_tri() methods. They return instances of the Tri enum which describes which buttons are being pressed, and are very helpful in situations like these where you want to move something in a cardinal direction based on which buttons are pressed.

Detecting individual button presses

If you want to detect if any button is pressed, you can use the is_pressed method on ButtonController. For example, we can do the following:


#![allow(unused)]
fn main() {
use agb::input::Button;

if input.is_pressed(Button::A) {
    // the A button is pressed
}
}

ButtonController also provides the is_just_pressed method. This will return true for 1 frame, the one where the player actually pressed the button. From that point on, it'll return false again until the player presses it again.

What we did

We added very basic button control to our bouncing ball example. In the next step, we'll cover meta-sprites and actually add a bat to our game of pong.

Exercise

Make it so the ball moves twice as fast if you're pressing the A button while moving it around.

The agb book