Scratch Code Patterns

Every game in this class is built from a small set of patterns. Each one below has the code, what it does, and where it shows up. Lesson pages link here instead of repeating the code.

Arrow-Key Movement

The simplest way to move a sprite. One event block per key.

when [up arrow v] key pressed
change y by (10)

when [down arrow v] key pressed
change y by (-10)

when [left arrow v] key pressed
change x by (-10)

when [right arrow v] key pressed
change x by (10)

Each press moves the sprite one step. Holding a key repeats the press, but with a short pause first, so movement feels jerky. Good enough for a maze. Used in the Maze Game.

Smooth Movement

Check the keyboard every frame inside a forever loop. Holding a key moves the sprite continuously.

when green flag clicked
forever
  if <key [left arrow v] pressed?> then
    change x by (-7)
  end
  if <key [right arrow v] pressed?> then
    change x by (7)
  end
end

This is the game loop pattern: a loop that never stops, with if blocks inside that check what is happening every frame. Almost every game you have played runs on it. Used in the Falling Objects Game and the Platformer.

Wall Reset

Send the sprite back to the start when it touches a wall color.

when green flag clicked
forever
  if <touching color (#000000)?> then
    go to x: (-200) y: (150)
  end
end

Click the color swatch and use the eyedropper to pick the exact wall color. Set the go to coordinates to your start position — drag the sprite there and read the x and y below the stage. If the walls are their own sprite, touching [Walls v]? works the same way. Used in the Maze Game.

Score and Collect

A variable that goes up when the player touches a collectable, which then moves somewhere new.

when green flag clicked
set [score v] to (0)
forever
  if <touching [Coin v]?> then
    change [score v] by (1)
    go to [random position v]
  end
end

Make the variable with Variables → Make a Variable, named score, For all sprites. set replaces the value; change adds to it. Put the set block right after the green flag so the score resets every game. Used in the Maze Game and the Platformer.

Falling Object

An object that starts at a random spot along the top, falls, and resets when it reaches the bottom.

when green flag clicked
go to x: (pick random (-200) to (200)) y: (180)
forever
  change y by (-3)
  if <touching [Player v]?> then
    change [score v] by (1)
    go to x: (pick random (-200) to (200)) y: (180)
  end
  if <(y position) < (-170)> then
    go to x: (pick random (-200) to (200)) y: (180)
  end
end

Replace -3 with a speed variable to make the game get harder over time. Used in the Falling Objects Game.

Gravity and Velocity

Real objects accelerate as they fall. A velocity variable tracks vertical speed and gravity changes it a little every frame.

when green flag clicked
set [velocity v] to (0)
forever
  if <not <touching [ground v]?>> then
    change [velocity v] by (-1)
  else
    set [velocity v] to (0)
  end
  change y by (velocity)
end

• In the air, velocity drops by 1 each frame: -1, -2, -3… so the fall speeds up.
• On the ground, velocity resets to 0 so the sprite stops.
• change y by (velocity) moves the sprite by the current speed.

Compare this to the fixed-speed version, change y by (-5), which falls at the same speed forever. Used in the Platformer.

Jumping

Set velocity to a positive number. Gravity does the rest: the sprite rises, slows, and falls back.

when [space v] key pressed
if <touching [ground v]?> then
  set [velocity v] to (10)
end

The if touching ground check stops the player from jumping in midair. Used in the Platformer.

Landing on Platforms

Treat platforms like the ground with or. Move first, then check, and bounce gently on landing so the sprite does not sink into the surface.

when green flag clicked
set [gravity v] to (-1)
set [velocity v] to (0)
forever
  change y by (velocity)
  if <not <<touching [ground v]?> or <touching [platform v]?>>> then
    change [velocity v] by (gravity)
  else
    if <(velocity) < (0)> then
      set [velocity v] to ((-0.5) * (velocity))
    else
      set [velocity v] to (0)
    end
  end
end

If velocity is -8 when the player lands, -0.5 * -8 = 4: a small upward bounce that settles the sprite on top of the surface. Used in the Platformer.

Wall Collision

Stop the player from walking through the side of a platform. The trick is the step-up test: move sideways, and if you are touching something, step up 5 pixels. If you are still touching, it was a wall, so undo the move.

when green flag clicked
forever
  if <key [a v] pressed?> then
    change x by (-5)
    if <<touching [ground v]?> or <touching [platform v]?>> then
      change y by (5)
      if <<touching [ground v]?> or <touching [platform v]?>> then
        change x by (5)
      end
      change y by (-5)
    end
  end
  if <key [d v] pressed?> then
    change x by (5)
    if <<touching [ground v]?> or <touching [platform v]?>> then
      change y by (5)
      if <<touching [ground v]?> or <touching [platform v]?>> then
        change x by (-5)
      end
      change y by (-5)
    end
  end
end

Without the step-up, standing on flat ground would count as a wall hit every frame, because the player is already touching the ground. Used in the Platformer.

Clone Factory

One hidden sprite creates copies of itself. Each clone runs its own script and deletes itself when it is done.

when green flag clicked
hide
forever
  wait (pick random (0.5) to (1.5)) seconds
  create clone of [myself v]
end

when I start as a clone
go to x: (pick random (-200) to (200)) y: (180)
show
repeat until <(y position) < (-170)>
  change y by (-3)
  if <touching [Player v]?> then
    change [score v] by (1)
    delete this clone
  end
end
delete this clone

• The original sprite is the factory. It stays hidden.
• when I start as a clone runs once for each new clone.
• Always end with delete this clone, or dead clones pile up and slow the project down.

Tune the wait time to spawn more or fewer objects. Used in the Falling Objects Game.

Broadcasts and Game States

A broadcast is a message every sprite can hear. Use it to switch between a start screen, playing, and game over.

Start screen sprite:

when green flag clicked
go to x: (0) y: (0)
show

when this sprite clicked
broadcast [start game v]
hide

Player sprite — hidden until the game starts:

when green flag clicked
hide

when I receive [start game v]
set [score v] to (0)
set [lives v] to (3)
go to x: (0) y: (-140)
show
forever
  if <key [left arrow v] pressed?> then
    change x by (-7)
  end
  if <key [right arrow v] pressed?> then
    change x by (7)
  end
end

Game over sprite:

when green flag clicked
hide

when I receive [game over v]
go to x: (0) y: (0)
show
stop [other scripts in sprite v]

Any sprite can send broadcast [game over v] when the player runs out of lives. Every clone factory should start on when I receive [start game v] instead of the green flag, and should stop on when I receive [game over v]. Used in the Falling Objects Game.

Sound on Touch

when green flag clicked
forever
  if <touching [Star v]?> then
    start sound [Pop v]
    wait (0.5) seconds
  end
end

Pick any sound from the Sounds tab. The short wait stops the sound from retriggering every frame while the sprites overlap. start sound keeps the script moving; play sound until done pauses the script until the sound finishes.