Instance variables, and overriding superclass functions

Questions
1

Hi

I’m new to Codea, and Lua… but I am really enjoying it.

I have a question about inheritance.

I want to create a class that encapsulates a physics body, and draws a sprite where the body is. To do this, I created a base class, which includes a string for the desired sprite.

The problem is that if I create two objects, each from a different subclass the sprite string always changes to that of the last object that was instantiated.

Block base class

Block = class()

body = nil
srcSprite = nil

function Block:init(x, y, w, h, aSprite)
    self.body = physics.body(POLYGON, vec2(0,0), vec2(0,h), vec2(w,h), vec2(w,0))
    self.body.x = x
    self.body.y = y
    self.type = STATIC
    srcSprite = aSprite
end

function Block:draw()
    pushMatrix()
    translate(self.body.x, self.body.y)
    sprite(srcSprite, 0, 0)
    popMatrix()
end

Subclass 1

WoodBlock = class(Block)

function WoodBlock:init(x, y, w, h)
    -- you can accept and set parameters here
    Block.init(self, x, y, w, h, "Planet Cute:Wood Block")
end

function WoodBlock:draw()
    Block.draw(self)
end

Subclass 2

GrassBlock = class(Block)

function GrassBlock:init(x, y, w, h)
    -- you can accept and set parameters here
    Block.init(self, x, y, w, h, "Planet Cute:Grass Block")
end

function GrassBlock:draw()
    Block.draw(self)
end

Main

supportedOrientations(LANDSCAPE_ANY)

woodBlock = nil
grassBlock=nil

function setup()
    woodBlock = WoodBlock(0, 0, 101, 160)
    grassBlock = GrassBlock(400, 0, 101, 160)
end

function draw()
    woodBlock:draw()
    grassBlock:draw()
end

Both blocks will show the second image (grass). If I switch the order in setup(), they both show the wood sprite.

Please help.

2

Your Block base class needs to store its members on its table, rather than globally.

Note that srcSprite is moved to self.srcSprite

Block = class()

function Block:init(x, y, w, h, aSprite)
    self.body = physics.body(POLYGON, vec2(0,0), vec2(0,h), vec2(w,h), vec2(w,0))
    self.body.x = x
    self.body.y = y
    self.type = STATIC
    self.srcSprite = aSprite
end

function Block:draw()
    pushMatrix()
    translate(self.body.x, self.body.y)
    sprite(self.srcSprite, 0, 0)
    popMatrix()
end

```


The behaviour you were seeing had the last-instantiated class overwriting the srcSprite global variable.
3

Of course (feels dumb now). Too obvious! Thank you. :slight_smile:

4

@nomore

See the changes and additions I made. (changed) (added)


Block = class()

body = nil
srcSprite = nil

function Block:init(x, y, w, h, aSprite)
    self.body = physics.body(POLYGON, vec2(0,0), vec2(0,h), vec2(w,h), vec2(w,0))
    self.body.x = x
    self.body.y = y
    self.body.type = STATIC    -- changed
    self.srcSprite = aSprite    -- changed
end

function Block:draw()
    pushMatrix()
    translate(self.body.x, self.body.y)
    sprite(self.srcSprite, 0, 0)    --changed
    popMatrix()
end
--Subclass 1

WoodBlock = class(Block)

function WoodBlock:init(x, y, w, h)
    -- you can accept and set parameters here
    Block.init(self, x, y, w, h, "Planet Cute:Wood Block")
end

function WoodBlock:draw()
    Block.draw(self)
end
--Subclass 2

GrassBlock = class(Block)

function GrassBlock:init(x, y, w, h)
    -- you can accept and set parameters here
    Block.init(self, x, y, w, h, "Planet Cute:Grass Block")
end

function GrassBlock:draw()
    Block.draw(self)
end
--Main

supportedOrientations(LANDSCAPE_ANY)

woodBlock = nil
grassBlock=nil

function setup()
    woodBlock = WoodBlock(100, 0, 101, 160)    -- changed
    grassBlock = GrassBlock(400, 0, 101, 160)
end

function draw()
    background(40,40,50)    --added
    woodBlock:draw()
    grassBlock:draw()
end
5

@dave1701. Thanks for the clarifications.

6

This may be obvious, but unless you need to take additional steps, the WoodBlock:draw() and GrassBlock:draw() functions aren’t needed.