Add a second half to the controls section

book/src/pong/04_controls.md

@@ -21,3 +21,62 @@ Add this near the top of your main function:
 ```
 
 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:
+
+```rust
+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_try() 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`](https://docs.rs/agb/latest/agb/input/enum.Tri.html) 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:
+
+```rust
+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 arond.