2) Let’s Move! — Events & Input

Learning Targets

• I can read keyboard input (Arrow keys and WASD).
• I can change an object’s position using x and y (or motion actions).
• I can keep a player inside the room bounds using conditionals.
• I can extend movement to diagonal and variable speed.

Teacher Setup (≈5 min)

1. Ensure students have spr_player and obj_player using that sprite.
2. Suggested room size: 1024 × 768 (or your site standard). Place obj_player near center.
3. Remind the class: support both Arrow keys and WASD.

Student Build (Choose DnD or GML)

A. Drag-and-Drop (DnD) Path

1. Create Event (obj_player)
   • Add Set Variable: moveSpeed = 4
   • (Optional) Ensure the sprite is set: sprite_index = spr_player
2. Step Event (continuous input)
   • Add Set Variable: hInput = 0, vInput = 0
   • If Key Down (Right or D) → hInput = 1
   • If Key Down (Left or A) → hInput = -1
   • If Key Down (Down or S) → vInput = 1
   • If Key Down (Up or W) → vInput = -1
   • (Optional diagonal normalize) If both hInput and vInput ≠ 0, multiply each by 0.707
   • Move:
     • Set X: x = x + hInput * moveSpeed
     • Set Y: y = y + vInput * moveSpeed
   • Clamp to room bounds (use half sprite size):
     • If x < sprite_width/2 → set to sprite_width/2
     • If x > room_width - sprite_width/2 → set to that value
     • If y < sprite_height/2 → set to sprite_height/2
     • If y > room_height - sprite_height/2 → set to that value

B. GML (Code) Path

Create Event (obj_player)

moveSpeed = 4;

Step Event (obj_player)

// 1) Read input
var h = 0;
var v = 0;

if (keyboard_check(vk_right) || keyboard_check(ord("D"))) h += 1;
if (keyboard_check(vk_left)  || keyboard_check(ord("A"))) h -= 1;
if (keyboard_check(vk_down)  || keyboard_check(ord("S"))) v += 1;
if (keyboard_check(vk_up)    || keyboard_check(ord("W"))) v -= 1;

// 2) Optional: normalize diagonals so diagonals aren't faster
if (h != 0 && v != 0) {
    var diag = 1 / sqrt(2); // ≈ 0.707
    h *= diag;
    v *= diag;
}

// 3) Move
x += h * moveSpeed;
y += v * moveSpeed;

// 4) Clamp to room bounds using sprite size
var halfW = sprite_width  * 0.5;
var halfH = sprite_height * 0.5;

x = clamp(x, halfW, room_width  - halfW);
y = clamp(y, halfH, room_height - halfH);

Checks for Understanding

• CFU 1: Why normalize diagonal movement?
• CFU 2: Predict where the player stops if x = -10 before clamping.
• CFU 3: If the sprite “sticks” on a wall, which bound might be wrong?

Troubleshooting

• No movement? Is logic in the Step event (not Key Press) and attached to obj_player?
• Too fast/slow? Adjust moveSpeed.
• Leaves the room? Use room_width/room_height and sprite_width/sprite_height correctly.
• Faster on diagonals? Add normalization (divide by √2).

Extensions

1. Variable speed (Sprint with Shift)

   var base = 4;
   moveSpeed = keyboard_check(vk_shift) ? 6 : base;

2. Smooth acceleration / friction

   // Create
   vx = 0; vy = 0; accel = 0.5; maxSpeed = 4; fric = 0.85;
   
   // Step (after computing h and v)
   vx = (h != 0) ? clamp(vx + h * accel, -maxSpeed, maxSpeed) : vx * fric;
   vy = (v != 0) ? clamp(vy + v * accel, -maxSpeed, maxSpeed) : vy * fric;
   
   x += vx; y += vy;
   
   var halfW = sprite_width * 0.5, halfH = sprite_height * 0.5;
   x = clamp(x, halfW, room_width - halfW);
   y = clamp(y, halfH, room_height - halfH);

3. Screen wrap (alternative to clamping)

   if (x < -sprite_width) x = room_width + sprite_width;
   if (x > room_width + sprite_width) x = -sprite_width;
   if (y < -sprite_height) y = room_height + sprite_height;
   if (y > room_height + sprite_height) y = -sprite_height;

4. Controller support (map gamepad left stick with gamepad_axis_value()).

Assessment & Rubric

Success Criteria

• Player moves in 4 directions with Arrow and WASD.
• Player remains fully inside the visible room.
• Logic is clear and commented (or blocks labeled).

Pseudocode (AP CSP Tie-In)

SET moveSpeed ← 4
EVERY FRAME:
  h ← 0; v ← 0
  IF Right OR D pressed: h ← h + 1
  IF Left  OR A pressed: h ← h - 1
  IF Down  OR S pressed: v ← v + 1
  IF Up    OR W pressed: v ← v - 1

  IF h ≠ 0 AND v ≠ 0:
     h ← h / √2
     v ← v / √2

  x ← x + h * moveSpeed
  y ← y + v * moveSpeed

  x ← CLAMP(x, sprite_width/2, room_width - sprite_width/2)
  y ← CLAMP(y, sprite_height/2, room_height - sprite_height/2)

Standards Bridge (Quick)

• AP CSP: Program design & development; algorithms & control; abstraction; testing & debugging.
• AP CSA (optional): Variables, conditionals, and state.
• Security+ tie-in (optional): Deterministic behavior and boundary checking.

Submission Requirements

Submit the following to Google Classroom and drop it off into google drive:

• PX_lastname_G_Move.gml — Game Maker source code.


• PX_lastname_G_Move.png — Screenshot of the Game Maker code.


• PX_lastname_G_Move.mp4 — Short screen recording of running your program.

Exit Ticket (1–2 min)

1. In one sentence, explain how clamping prevents out-of-bounds movement.
2. Circle which is true for continuous movement: Key Down or Key Press?