Storm Flight I

Review — the Bubble Popper arc 5 min

• Clones are copies that run their own script, and every one needs a delete.
• For this sprite only gives each clone its own speed, size and worth.
• A round can end itself with repeat until <(lives) = (0)>, and a guard stops lives going negative.

Quick-fire

Bubble Popper ended with a game-over card. But to play again you had to click the green flag. Why is that not good enough?

Today’s Topic 3 min

• A state variable, holding a word: title, play, over
• Every script asking the state before it acts
• broadcast [start v] to move from one state to the next
• One key that means different things depending on where you are

Learning outcome

By the end of this lesson you will be able to:

1. Say what situation your game is in, out loud, at any moment.
2. Build a title screen that hands over to the game.
3. Recognise the bug where one script never heard the news.

Learnt — a game is a set of situations 28 min

1 · Everything at once

Here is a game with a title screen and no state. Both scripts start under the green flag, so both run immediately:

when flag clicked
show
go to x: (-150) y: (0)
repeat (26)
  next costume
  change x by (9)
end
              

when flag clicked
wait (0.1) seconds
say [STORM FLIGHT - press the arrows to fly] for (2) seconds
              

2 · One variable names the situation

A state is the name of the situation your game is in right now. Almost every game has the same three:

• title — waiting for the player to start
• play — the game is happening
• over — it has finished, and the score is on screen

Make one variable called state, for all sprites, and put a word in it. Then every script begins by asking what it holds.

3 · Moving between states

Two blocks do all the work:

• set [state v] to [play] — the change itself
• broadcast [start v] — telling everybody it happened

You could do it with the variable alone, and small games often do. A broadcast is better once more than one sprite needs to react, because a when I receive [start v] runs the moment it is sent, instead of waiting for some loop to come round again.

when flag clicked
set [state v] to [title]
go to x: (0) y: (-30)
show
say [STORM FLIGHT - press space] for (1.6) seconds
wait until <key (space v) pressed?>
broadcast [start v]
            

4 · Every script asks

This is the part that gets forgotten. A game with four sprites has a dozen scripts, and each one needs to know whether it should be doing anything. A script that does not ask keeps running in every state — under the title screen, after the game is over, and (next lesson but one) while the game is paused.

Basic — the title screen 24 min

Start a new project. A stormy backdrop and a parrot to fly.

Step 1 — the sprites

1. Add the Parrot sprite and set its size to about 55. It has two costumes, so next costume flaps its wings.
2. Choose a stormy or cloudy backdrop.

Step 2 — the state

1. Make a variable state, for all sprites, and show it.
2. Under the green flag: set [state v] to [title].

Step 3 — the title

1. Move the parrot to the middle and say the game’s name.
2. wait until <key (space v) pressed?>.
3. Then broadcast [start v].

Step 4 — the game

1. Add when I receive [start v].
2. Set the state to play, move the parrot to the left of the Stage.
3. Add the flying: four if blocks, one per arrow key.

if <key (up arrow v) pressed?> then
  change y by (7)
end
if <key (down arrow v) pressed?> then
  change y by (-7)
end
if <key (right arrow v) pressed?> then
  change x by (7)
end
if <key (left arrow v) pressed?> then
  change x by (-7)
end
            

Step 5 — the ending

1. Wrap the flying in a repeat, so the round has a length.
2. Afterwards: set [state v] to [over] and say so.

The script that forgot to ask

Add a lightning bolt whose script says nothing about the state, and watch it misbehave.

• A bolt that flashes somewhere random, over and over, in a forever loop.
• A counter of how many times it has struck.
• Read the counter at the moment the game actually begins.

One if better

Fix it. The bolt should only strike while the game is being played.

• The bolt’s loop opens with if <(state) = [play]> then.
• It hides when it is not playing.
• The strikes counter reads 0 on the title screen, every run.

when flag clicked
hide
forever
  if <(state) = [play]> then
    go to x: (pick random (-160) to (190)) y: (pick random (-90) to (120))
    show
    change [strikes v] by (1)
    wait (0.25) seconds
    hide
  end
  wait (0.2) seconds
end
            

Never touch the green flag again

One key, three meanings. Space starts a round from the title, and takes you back to the title when the round is over.

• A single when [space v] key pressed script.
• Inside it, an if for each state it cares about.
• A rounds counter, to prove you played more than once without the flag.

when [space v] key pressed
if <(state) = [title]> then
  broadcast [start v]
end
if <(state) = [over]> then
  broadcast [to title v]
end
            

Words, not numbers

Build the same machine twice: once with state holding 1, 2 and 3, and once holding title, play and over. They behave identically. Argue for one of them.

• Both versions on the Stage at the same time, as two readouts.
• They change together, so you can see they are the same machine.
• Write one sentence saying which you would rather find in a broken project.

Summary 5 min

• A state is the name of the situation the game is in. Almost always title, play, over.
• state is shared — there is one game — and holds a word.
• Every script asks the state before it acts. The one that forgets is the commonest bug in this arc.
• broadcast announces a change; the variable records it.
• The green flag is the programmer’s button. Players get their own key.

State

The named situation a game is in right now.

State machine

The set of states plus the rules for moving between them.

Transition

Moving from one state to the next, usually on a broadcast.

Title screen

The state a game sits in before the player starts it.

Hand in

Save your Storm Flight. Next lesson you add the way to lose it — and find out why one lightning bolt can cost three lives in a fifth of a second.