Variable Watchers on the Stage

Learning Goals 3 min

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

1. Use show variable [score v] and hide variable [score v] to control the on-Stage watcher from your script (not just the checkbox in the palette).
2. Right-click a watcher on the Stage and switch between the three modes: normal readout, large readout, and slider.
3. Use a slider watcher as an input — the player drags it, and your script reads the new value with the round reporter (score).

Warm-Up — the box you've been ignoring 7 min

Open the project you saved as homework last lesson — the two-variable game with score and lives. Look at the top-left corner of the Stage. You should see two little grey boxes:

when flag clicked
set [score v] to (0)
set [lives v] to (3)
        

Those boxes are called variable watchers. They've been there the whole time. You probably ticked them once and forgot. But they're not just decoration — they have three modes, and one of them turns them into a slider the player can drag.

Today we'll stop ignoring those little boxes and start using them — to make the game look better, to surprise the player, and (the cool one) to let them change a number with the mouse.

New Concept — the watcher, three modes, two blocks 15 min

A variable watcher is the little box on the Stage that shows a variable's current value. Every variable you make gets one, and every watcher starts off ticked-on (visible). You can hide it with the checkbox in the Variables palette — or, much more usefully, from inside a script.

Show and hide from inside a script

Two orange blocks at the bottom of the Variables palette do this:

show variable [score v]
hide variable [score v]
        

Why script it instead of just clicking the checkbox? Because games have moments. The score should hide on the title screen and appear when the game starts. The lives counter should hide on the win screen. The "secret bonus" variable should stay hidden until the player finds it.

when flag clicked
hide variable [score v]
hide variable [lives v]
wait (2) seconds
show variable [score v]
show variable [lives v]
        

The three watcher modes

Right-click (or two-finger tap on a trackpad) on any watcher on the Stage. A small menu opens with three options at the top:

• normal readout — the default. A small grey box with the variable's name on the left and its value on the right. Good for most things.
• large readout — a big bright orange box with just the value, no name. Good for "the score" in an arcade game where you want the number to be huge.
• slider — adds a draggable handle under the value. The player can drag it to change the variable's value while the game runs.

You don't switch modes from a block. Modes are a property of the watcher itself, set by the right-click menu, and they stay set across runs and saves.

The slider is the magic one

Switch a watcher to slider mode. A horizontal bar with a draggable circle appears under the number. The player can grab that circle and slide it — and as they slide, the variable's value updates in real time.

That means your script can read what the player has chosen using the round reporter:

when flag clicked
forever
  set [size v] to (cat-size)
end
        

You can also set the slider's min and max. Right-click the slider again and pick change slider range — type a min (like 10) and a max (like 200). The slider won't go beyond those.

Worked Example — Priya's resize-the-cat slider 12 min

Priya wants a tiny project where the player can drag a slider to grow and shrink the cat in real time. We'll build it in eight steps using just one variable and the slider mode.

Step 1 — Start fresh

New Scratch project. Default cat in the middle of the Stage.

Step 2 — Make the variable

Variables palette → Make a Variable → type cat-size → OK. A watcher appears on the Stage showing cat-size 0.

Step 3 — Set a sensible starting value

On the cat, drop this stack:

when flag clicked
set [cat-size v] to (100)
        

Step 4 — Switch the watcher to slider mode

Right-click the cat-size watcher on the Stage. From the menu, pick slider. The watcher grows a draggable handle below the number.

Step 5 — Set the slider range

Right-click the slider again. Pick change slider range. Type minimum 10, maximum 200, OK. Now the slider can't go bigger than 200% or smaller than 10%.

Step 6 — Make the cat's size follow the variable

Add this second stack on the cat:

when flag clicked
forever
  set size to (cat-size) %
end
        

Step 7 — Click the flag and drag

Click the green flag. Drag the slider on the Stage. The cat grows and shrinks live. That's it — the player is now interacting with your game without touching a key or moving the mouse around the sprites.

Step 8 — Add a hide-and-show flourish

Make the slider disappear for the first second of each run, like a little dramatic reveal:

when flag clicked
hide variable [cat-size v]
set [cat-size v] to (100)
wait (1) seconds
show variable [cat-size v]
        

The full assembled cat-sprite scripts

when flag clicked
hide variable [cat-size v]
set [cat-size v] to (100)
wait (1) seconds
show variable [cat-size v]
        

when flag clicked
forever
  set size to (cat-size) %
end
        

What you just built: a Scratch project with a real input control — the player changes a number with the mouse and the sprite responds instantly. That's the same idea behind volume sliders, brightness sliders, and game-difficulty sliders in every app on your phone.

Try It Yourself — three watcher drills 15 min

when flag clicked
set [secret v] to (42)
hide variable [secret v]
forever
  if <key [space v] pressed?> then
    show variable [secret v]
    wait (1) seconds
    hide variable [secret v]
  end
end
          

when flag clicked
set [volume v] to (50)
forever
  set volume to (volume) %
  play drum (1 v) for (0.25) beats
  wait (0.75) seconds
end
          

when flag clicked
set [spin-speed v] to (5)
set [cat-colour v] to (0)
forever
  turn cw (spin-speed) degrees
  set [color v] effect to (cat-colour)
end
          

Mini-Challenge — the slider that does nothing 5 min

"Why doesn't Daniel's cat get bigger?"

Daniel made a variable called cat-size, switched its watcher to slider mode with range 10–200, and wrote this:

when flag clicked
set [cat-size v] to (100)
set size to (cat-size) %
        

Daniel drags the slider all the way to 200, but the cat stays at 100%. Why? What's missing?

when flag clicked
set [cat-size v] to (100)
forever
  set size to (cat-size) %
end
          

Recap 3 min

You met the variable watcher properly. Every variable in your project has a little box on the Stage that shows its current value. Tick or untick its checkbox in the palette to toggle it, or — more powerfully — use show variable and hide variable blocks to control it from a script. Right-click any watcher to switch between normal, large, and slider modes. The slider mode turns the watcher into a player input — they drag, the variable updates, and a forever loop in your script can read the new value and do something with it.

Variable watcher

The little box on the Stage that shows a variable's current value. Created automatically for every variable you make, and toggled by the checkbox in the Variables palette.

Show / hide variable

The two blocks show variable [ v] and hide variable [ v] at the bottom of the Variables palette. They control the watcher's visibility from inside scripts — useful for title screens, end screens, and surprises.

Slider mode

A watcher display mode (right-click → slider) that lets the player drag a handle to change the variable's value while the game runs. Set the allowed range with the change slider range menu item.

Read-the-variable loop

The pattern of putting (name) inside a block (like set size to ()) and wrapping the whole thing in a forever, so the sprite re-checks the variable many times a second.

Homework 2 min

The Music-Box Slider. Build a one-sprite project where the player controls a sound effect with two sliders.

1. Make two variables: pitch-shift (range −50 to 50) and beat-speed (range 0.1 to 2).
2. Right-click each watcher and switch both to slider mode. Set the ranges above.
3. Reset both variables at the flag: set [pitch-shift v] to (0) and set [beat-speed v] to (1).
4. Add a forever loop: set [pitch v] effect to (pitch-shift), then play drum (1 v) for (0.25) beats, then wait (beat-speed) seconds.
5. Add a third script: when the down arrow is pressed, hide variable [pitch-shift v] and hide variable [beat-speed v]. When up arrow, show both again.

Save as HW-L2-32-Music-Box.sb3. Click the flag and drag the sliders while the drum plays. Press down to hide them, up to show them.

Bring back next class:

• The .sb3 file.
• Your answer to: "What happens if you set the slider range backwards (max smaller than min)? Try it and write one sentence about what you see."

Heads up for next class: SCR-L02-33 is about naming your variables well. You'll see why cat-size is a great name and why x, var1, and score2 are terrible ones — and how a good name today saves you (and your teacher) ten minutes of head-scratching tomorrow.