How to access methods for instances of a class?

MrMells · January 24, 2012, 1:03am

Hi,

I’m just starting so I’m still trying to get a good understanding of the basics.
here is a simplified version of what I am trying to do : basically adding a number of balls to the screen and move them.
My questions are contained in the code and will be easier to understand in the context.

Any pointer is greatly appreciated

Code

main.lua
[lua]require (“Balls”)
require (“Ball”)

local Game = {}

function Game:startSetup()
local Balls = Balls:new()
Balls:setup()
end

Game:startSetup()

Balls:animate()

– How can I change the position of an instance of my Ball class?
– I would like to move my objects (circles) independantly
– -> …[/lua]

balls.lua
[lua]Balls = {}
Balls_mt = {__index = Balls}

function Balls:new()
local new_inst = {}
setmetatable(new_inst, Balls_mt)
return new_inst
end

– SETUP

function Balls:setup()
– Set Balls Defaults
local params = {
{ radius=20, xdir=1, ydir=1, xspeed=2.8, yspeed=6.1, r=255, g=0, b=0, posx=400, posy=150 },
{ radius=50, xdir=-1, ydir=-1, xspeed=1.8, yspeed=1.5, r=115, g=23, b=33, posx=200, posy=200 }
}

for i, j in pairs(params) do
ball = Ball:new(j)
end

end

– ANIMATE

function Balls:animate()

local _Balls=self
– Question : How to access .move method for each instance of the Ball class?
for i, ball in pairs(_Balls) do
ball:move() – Doesn’t work | Error : attempt to index local ‘ball’ (a function value)
end

end

return Balls[/lua]

ball.lua
[lua]Ball = {}
Ball_mt = {__index = Ball}

function Ball:new(params)

local new_inst = {}
setmetatable(new_inst, Ball_mt)

– Default Location
local default_xpos = display.contentWidth*0.5
local default_ypos = display.contentHeight*0.5

– Draw Ball
local new_inst = display.newCircle(default_xpos, default_ypos, params.radius )
new_inst.x = params.posx
new_inst.y = params.posy

– Copy params to new_inst
for i, j in pairs(params) do
new_inst[i] = j
end
return new_inst
end

– I need to access that function from main.lua and balls.lua
function Ball:move()
print (“Move Ball”)
end

return Ball[/lua]

My questions

I believe that if I understand how to access methods for instances of a class I will be able to make progress
Is there a table that stores all the instances of a class?

Thank you [import]uid: 95346 topic_id: 20854 reply_id: 320854[/import]

dmekersa · January 24, 2012, 1:22am

When you call a function with “:” like ball:move() there is an hidden parameter call “self” which point to the instance. Use self.[anyproperty] to access instance properties.
Hope it helps. [import]uid: 77660 topic_id: 20854 reply_id: 82174[/import]

MrMells · January 24, 2012, 1:58am

Thank you for the answer.
I am not sure how your suggestion is suited to my case?

[lua]local _Balls=self
– Question : How to access .move method for each instance of the Ball class?
for i, ball in pairs(_Balls) do
ball:move() – Doesn’t work | Error : attempt to index local ‘ball’ (a function value)
end[/lua]

ball.move() doesn’t work either.

I want to access the method for my instances (are methods same as properties in programming?), and I believe that I already pass the instance to the move function because ball is taken from _Balls and local _Balls=self .

I can’t see where I would need to modify my script and the relevancy of your answer doesn’t appear clearly to my eyes.
Can you elaborate? [import]uid: 95346 topic_id: 20854 reply_id: 82184[/import]

Beloudest · January 24, 2012, 2:28am

I am also finding this a little confusing, I can easily do what I want to do with a single instance of a display object e.g. ball but when I create multiple instances its becomes unclear how to access a single instance. On day day 2 of learning will check back here to hopefully get a clearer picture. [import]uid: 118379 topic_id: 20854 reply_id: 82187[/import]

dmekersa · January 24, 2012, 3:00am

Hum, why are you using:

local \_Balls=self

?

As pairs need the table containing your balls.

I’m not confortable with your sample. Here is mine:

balls.lua

[code]
Ball = {}
Ball_mt = {__index = Ball}

– Note the use of ‘.’ instead of ‘:’ because it’s not a method of an instance
function Ball.new()

local new_inst = {}
– setmetatable(new_inst, Ball_mt) – removed!

– Default Location
local default_xpos = display.contentWidth*0.5
local default_ypos = display.contentHeight*0.5

– Create Display Object for Ball
local new_inst = display.newCircle(default_xpos, default_ypos, params.radius )
new_inst.x = params.posx
new_inst.y = params.posy

return setmetatable(new_inst,Ball_mt)
end

– I need to access that function from main.lua and balls.lua
function Ball:move(dist)
print (“Move Ball”)
self.x = self.x + dist – example of accessing a property
end

return Ball[/code]

Now how to create 2 balls and move them :

[code]
_balls = require(“balls”)

local ball1 = _balls.new()
local ball2 = _balls.new()

ball1:move(5)
ball2:move(10)
[/code] [import]uid: 77660 topic_id: 20854 reply_id: 82191[/import]

dmekersa · January 24, 2012, 3:03am

– Edit -> I replaced

function Ball:new()

by

function Ball.new()

because new is not a class method and do not need to receive the hidden self parameter.

Read more here:
http://www.lua.org/pil/16.html [import]uid: 77660 topic_id: 20854 reply_id: 82192[/import]

Beloudest · January 24, 2012, 3:10am

HAHA, much better. Thx for the insight. I found this tutorial which also has a great approach to OOP if any one needs further help.

http://www.ludicroussoftware.com/blog/2011/07/06/simple-oop-with-inheritance-in-corona/

On the way to being able to get stuck in. [import]uid: 118379 topic_id: 20854 reply_id: 82193[/import]

Beloudest · January 24, 2012, 3:10am

HAHA, much better. Thx for the insight. I found this tutorial which also has a great approach to OOP if any one needs further help.

http://www.ludicroussoftware.com/blog/2011/07/06/simple-oop-with-inheritance-in-corona/

On the way to being able to get stuck in. [import]uid: 118379 topic_id: 20854 reply_id: 82194[/import]

Beloudest · January 24, 2012, 3:11am

HAHA, much better. Thx for the insight. I found this tutorial which also has a great approach to OOP if any one needs further help.

http://www.ludicroussoftware.com/blog/2011/07/06/simple-oop-with-inheritance-in-corona/

On the way to being able to get stuck in. [import]uid: 118379 topic_id: 20854 reply_id: 82195[/import]

Beloudest · January 24, 2012, 3:12am

Opps, it kept asking me to verify. BUG [import]uid: 118379 topic_id: 20854 reply_id: 82196[/import]

MrMells · January 24, 2012, 3:31am

dmekersa >
Thank you very much for taking the time to answer. I appreciate.
I will take the time to study (started a few days ago and I was stuck).

I have a question :
In your example each ball is manually created
local ball1 = _balls.new()

How would you create a function so that you can populate Balls (10 at once for example) and still access each instance easily later?
Would you store the names in a table or, what would be the best practice? [import]uid: 95346 topic_id: 20854 reply_id: 82200[/import]

Beloudest · January 24, 2012, 3:41am

Me too, same question. Does not seem intuitive to manually declare each ball? [import]uid: 118379 topic_id: 20854 reply_id: 82202[/import]

dmekersa · January 24, 2012, 4:22am

In a table, using table.insert then browsing the table with pairs, example :

for k in pairs(listBall) do  
 listBall[k]:move()  
end

read http://lua-users.org/wiki/TablesTutorial [import]uid: 77660 topic_id: 20854 reply_id: 82207[/import]

MrMells · January 26, 2012, 6:23am

dmekersa
Thank you. I have read the docs at lua-users (and more) but I guess I will learn more with time.

I have a last question : I can’t access methods on my instances of Ball after I have stored them in a table, but I can access some of their properties.
-> attempt to call method ‘move’ (a nil value)

Where am I mistaking?

If someone could point me in the right direction, that would be great.
Thank you!

Code

main.lua
[lua]_balls = require (“Balls”)
_ball = require (“Ball”)
require (“libraries”)

local Game = {}
function Game:startSetup()
local Balls = _balls.new()
local listBalls = _balls:setup()

print (listBalls[1].x) – Displays “400”, great until there

listBalls[1]:move(50) – Doesn’t work.How can I access function move() in ball.lua
end

Game:startSetup()[/lua]

balls.lua

[lua]Balls = {}
Balls_mt = {__index = Balls}

local listBalls = {}

function Balls.new()
local new_inst = {}
setmetatable(new_inst, Balls_mt)
return (new_inst)
end

– SETUP

function Balls:setup()
– Set Balls Defaults
local params = {
{ radius=20, xdir=1, ydir=1, xspeed=2.8, yspeed=6.1, r=255, g=0, b=0, posx=400, posy=150 },
{ radius=50, xdir=-1, ydir=-1, xspeed=1.8, yspeed=1.5, r=115, g=23, b=33, posx=200, posy=200},
{ radius=20, xdir=1, ydir=1, xspeed=2.8, yspeed=6.1, r=255, g=0, b=0, posx=400, posy=350 }
}

for i, j in pairs(params) do
local ball = _ball.new(j)
table.insert(listBalls, ball)
ball = nil
end

return (listBalls)

end

return Balls[/lua]

ball.lua

[lua]Ball = {}
Ball_mt = {__index = Ball}

function Ball.new(params)

local new_inst = {}
setmetatable(new_inst, Ball_mt)

– Default Location
local default_xpos = display.contentWidth*0.5
local default_ypos = display.contentHeight*0.5

– Create Display Object for Ball
new_inst = display.newCircle(default_xpos, default_ypos, params.radius )
new_inst.x = params.posx
new_inst.y = params.posy

return (new_inst)
end

function Ball:move(dist)
print (“Move Ball”)
–self.x = self.x + dist – example of accessing a property
end

return Ball[/lua]

Note

On top of my previous question here is another one :

When I use
[lua]return setmetatable(new_inst,Ball_mt)[/lua]
I can access my “move” method but print (listBalls[1].x) gives me a nil

On the other hand :
[lua]return (new_inst)[/lua]
I can’t access my “move” method but print (listBalls[1].x) gives me 400 (correct value)

[code]attempt to call method ‘move’ (a nil value)
>> I would like to store access .x and other properties on my ball objects AND access the method from the class move().
How would I achieve that?
Also in the docs Lua Simple classes setmetatable is not done on return.

Can somebody explain me what I should learn from that difference?
Thank you! [import]uid: 95346 topic_id: 20854 reply_id: 82448[/import]

MrMells · January 26, 2012, 6:23am

If anyone could please take a look at my last message above, it probably shouldn’t take more than 2 minutes for someone a bit experienced.
This guy on the picture is worried that I will never be able to start working on my platformer.
Help him, help me.

Thank you [import]uid: 95346 topic_id: 20854 reply_id: 82734[/import]

jeremyapplebaum12 · January 27, 2012, 3:03pm

Do you have a copy of require libraries.lua?

Thanks. [import]uid: 23649 topic_id: 20854 reply_id: 83006[/import]

MrMells · January 28, 2012, 6:06am

Hi Jeremy,

in fact libraries.lua is nothing special, just my own lua file for testing purposes. [import]uid: 95346 topic_id: 20854 reply_id: 83118[/import]

jeremyapplebaum12 · February 3, 2012, 3:11pm

Try this:

[code]

Ball = {}
Ball_mt = {__index = Ball}

function Ball.new(params)

local new_inst = {}

– Default Location
local default_xpos = display.contentWidth*0.5
local default_ypos = display.contentHeight*0.5

– Create Display Object for Ball
new_inst = display.newCircle(default_xpos, default_ypos, params.radius )
new_inst.x = params.posx
new_inst.y = params.posy
setmetatable(new_inst, Ball_mt)
return (new_inst)
end

function Ball:move(dist)
print (“Move Ball”)
–self.x = self.x + dist – example of accessing a property
end

return Ball
[/code] [import]uid: 23649 topic_id: 20854 reply_id: 84534[/import]