Activity 1.13

Taking Control

Key Concepts

Asking Whether a Key Is Held

Being Told a Key Was Pressed

Where the Mouse Is

What You Are Building

Something that moves because you moved it.

Everything So Far Ran on Its Own

Twelve sessions of programs that do the same thing every time you press Run.

Today the program has a person in it.

Way One: Ask

if love.keyboard.isDown("right") then
    x = x + speed * dt
end

Way One: Ask

if love.keyboard.isDown("right") then
    x = x + speed * dt
end

love.keyboard.isDown is a question, and it produces true or false — so it goes straight into an if.

Asked in love.update, that is sixty times a second. Hold the key and it moves smoothly.

Way Two: Be Told

function love.keypressed(key)
    if key == "space" then
        big = not big
    end

    if key == "escape" then
        love.event.quit()
    end
end

Way Two: Be Told

function love.keypressed(key)
    if key == "space" then
        big = not big
    end

    if key == "escape" then
        love.event.quit()
    end
end

You do not call this. The engine calls it, once, at the moment a key goes down.

Same arrangement as love.draw and love.update. It has been in your template since session 2.

They Are Not Interchangeable

isDown, in love.update

For things that happen while a key is held. Moving.

love.keypressed

For things that happen once, at the moment of pressing. Jumping, pausing, firing.

Use the wrong one and the program still runs. It just feels broken.

The Mouse Answers Two Questions

local mx, my = love.mouse.getPosition()

One call, two answers, two names on the left separated by a comma.

First time you have seen that. It is the same idea as one question having an x and a y.

And Clicks Are Told, Not Asked

function love.mousepressed(mx, my, button)
    markerX = mx
    markerY = my
end

The engine hands you where the click happened, in the same coordinates you have been drawing in since session 2.

Today's Objectives

• Move a shape with the arrow keys
• Keep it inside the window
• Respond to a single key press
• Use the mouse position and mouse clicks

Key Terms

love.keyboard.isDown

Asks whether a key is held right now. Produces a boolean.

love.keypressed

Called by the engine once, when a key goes down.

Clamping

Forcing a value back inside a range when it goes past.

love.mousepressed

Called by the engine when a mouse button goes down, with the position.

'F' → Fullscreen

Build

Task 1: Move With the Arrow Keys

Copy template-game to 1-13-control. Give it x, y, size and speed at the top, draw a square, and add four questions:

1
function love.update(dt)
2
    if love.keyboard.isDown("right") then
3
        x = x + speed * dt
4
    end
5

6
    if love.keyboard.isDown("left") then
7
        x = x - speed * dt
8
    end
9

10
    if love.keyboard.isDown("up") then
11
        y = y - speed * dt
12
    end
13

14
    if love.keyboard.isDown("down") then
15
        y = y + speed * dt
16
    end
17
end

Run it and move around. Hold two keys at once and write down what happens — nothing in the code says diagonal.

Remember that up subtracts from y. That is session 2's coordinate system, and it will feel wrong for about thirty seconds.

Task 2: Keep It on the Screen

Right now the square leaves and does not come back. This is not a bounce — you do not want it to turn around, you want it to stop:

1
if x < 0 then
2
    x = 0
3
end
4

5
if x > 800 - size then
6
    x = 800 - size
7
end

Add the same two for y, using 600. Run it and push into every wall.

Task 3: Press, Do Not Hold

Add a variable local big = false at the top, and make the square draw at size or size * 2 depending on it.

1
function love.keypressed(key)
2
    if key == "space" then
3
        big = not big
4
    end
5

6
    if key == "escape" then
7
        love.event.quit()
8
    end
9
end

Press space. Hold space. It toggles once, no matter how long you hold it, because the engine only calls this at the moment the key goes down.

big = not big is last session's not doing something useful: whatever it was, make it the other one.

Task 4: The Mouse

Copy the template again to 1-13-mouse. Draw a ring wherever the mouse is:

1
function love.draw()
2
    local mx, my = love.mouse.getPosition()
3

4
    love.graphics.setColor(0.3, 0.7, 1)
5
    love.graphics.circle("line", mx, my, 26)
6

7
    love.graphics.print("Mouse at " .. mx .. ", " .. my, 40, 40)
8
end

Then drop a marker where you click:

1
function love.mousepressed(mx, my, button)
2
    markerX = mx
3
    markerY = my
4
end

Add markerX and markerY at the top, start them off screen at -100, and draw a filled circle there.

Starting the marker at -100 is a small trick worth knowing: it exists, it is drawn every frame, and nobody can see it until you move it.

Challenge (Optional): Break It on Purpose

Go back to 1-13-control and move one of the four movement questions into love.keypressed instead:

function love.keypressed(key)
    if key == "right" then
        x = x + 10
    end
end

Hold the right arrow down. Compare it with holding the left arrow, which still uses isDown. Write down exactly what the difference feels like, and which one you would want for a game.

No error. Both versions are correct Lua and both do what they say.

Challenge (Optional): Follow the Mouse

Make the square from 1-13-control chase the mouse instead of using the keyboard — each frame, move it a little toward wherever the pointer is.

Two decisions across, two down. If the square is left of the mouse, move right; if it is right of it, move left. Watch what happens when it arrives.

Checkpoint

• 1-13-control moves in four directions and cannot leave the window
• Space toggles the size once per press, not repeatedly
• 1-13-mouse tracks the pointer and drops a marker where you click
• You can say which of the two keyboard methods is for holding and which is for pressing

Reflection

Answer the following questions before submitting your work.

1. Nothing in your program says the word "diagonal", but holding two arrow keys moves the square diagonally. Explain how that happens.
2. Session 9's wall reversed a speed and today's wall overwrote a position. Describe a situation where you would want each one, and say what today's program would look like if you had used a bounce instead.
3. You never wrote a line that calls love.keypressed, and it ran anyway. Name two other things in your program that work the same way, and say what they all have in common.

Submit

Submit the required files to the appropriate dropbox.

Activity Complete