problem in inheritence and using "self" keyword

aimoi · August 2015

Hi everyone,
why making classes for getting instances from them is this much hard in lua and gideros?
I knew a method to simulate classes in lua. it was making a table (for example "boat") with the fields and functions we want and then making a methode like this:

function boat:new()
	return self
end

and whenever we wanted to use the boat object we simply wrote:

theBoat=boat:new()

but in this case when I write

local boat={
	side="left",
	pic=Bitmap.new(Texture.new("boat.png")),
}
 
function boat:GoX()
	if self.pic:getX()<1150 then
		self.pic:setX(self.pic:getX()+10)
		self:move()
	else 
		self.side="right" 
	end
end

and when I use "boat:GoX()" function I get this error

gamesCore.lua:50: attempt to index local 'self' (a nil value)
stack traceback:
	gamesCore.lua:50: in function 'func'
	[string "compatibility.lua"]:32: in function <[string "compatibility.lua"]:31>

so how can I use "self" keyword to be able to use a object's fields inside it's functions?
If anyone could explain what is the easiest and efficient way to simulate java-like classes in lua, it would a great help for me.
Thanks in advance.

keszegh · August 2015

don't worry, it's simple. look here about inheritance.. http://docs.giderosmobile.com/classes_in_gideros.html

ar2rsawseen · August 2015

Boat = Core.class()
 
function Boat:init()
    self.side = "left"
    self.pic = Bitmap.new(Texture.new("boat.png"))
end
 
function Boat:GoX()
	if self.pic:getX()<1150 then
		self.pic:setX(self.pic:getX()+10)
		self:move()
	else 
		self.side="right" 
	end
end

and usage is like

local boat = Boat.new()
boat:GoX()

aimoi · August 2015

well, thanks a lot for your answers.
But even with the "core.class" things I still have problems using self keyword.

I wrote:

boat=Core.class()
function boat:init()
	self.side="left"
	self.pic=Bitmap.new(Texture.new("boat.png"))
	self.done=false
end

and defined a function:

function boat:GoX()
	if self.pic:getX()<1150 then
		self.pic:setX(self.pic:getX()+10)
		self:move()
		--print(theBoat.pic:getX())
	else 
		self.side="right" 
		--print("AA"..theBoat.side)
	end
end

but again when I call:

boat:GoX()

I get:

gamesCore.lua:47: attempt to index local 'self' (a nil value)
stack traceback:
	gamesCore.lua:47: in function 'func'
	[string "compatibility.lua"]:32: in function <[string "compatibility.lua"]:31>

line 47 is the exact place that I have written:

if self.pic:getX()<1150 then

So it seems like whenever I'm trying to access a function from a field (which is a "Sprite" in this case) of my class (which is the "boat" here) by using "self" keyword, self gives a "nil" instead of pointing to my class object.

I was wondering about making a metatable for all my boat objects and then set their metatables to that. Is that a better approach that using Core.class?

totebo · August 2015

Make sure you use a . instead of : when creating the object:

local boat = Boat.new()

totebo · August 2015

Oh, and also make sure you use a different instance name than the class (so not use "boat" for both).

vitalitymobile · August 2015

Quick typing @totebo comment, here you are :P

Boat=Core.class()
function Boat:init()
	self.side="left"
	self.pic=Bitmap.new(Texture.new("boat.png"))
	self.done=false
end
 
function Boat:GoX()
	if self.pic:getX()<1150 then
		self.pic:setX(self.pic:getX()+10)
		self:move()
		--print(theBoat.pic:getX())
	else 
		self.side="right" 
		--print("AA"..theBoat.side)
	end
end
function Boat:move()
end 
 
-- using it
local my_boat = Boat.new()
my_boat:GoX()
 
-- look out, enemy come
local pirate_boat = Boat.new()
pirate_boat:GoX()

aimoi · August 2015

Thank you for your complete answers. (:
well, first after changing the things you said, I still had the

attempt to index local 'self' (a nil value)

problem. Then I remembered somewhere in my code I have used

Timer.delayedCall(2, self.GoX)

so I thought let's call the boat:GoX() function out of this structure.. and whooho! It worked.

But seems like the problem has not been solved yet. I need to make delay there but with this function I get a nil value in 'self' when I call the GoX function of a specific boat. I think it's because the timer function is not sending the object inside the function the way when using ':' it does. So is there a solution for this too?

vitalitymobile · August 2015

OK, here you are. Solution is wrap call method inside a function() end. In case of inside Class function call, you can using self (move function example), use instance when call outside Class (pirate_boat example )

Boat=Core.class()
function Boat:init(params)
	params = params or {}
	self.id = params.id or "a boat";
	self.side="left"
	self.pic=Bitmap.new(Texture.new("boat.png"))
	self.done=false
	self.vx = 1
end
 
function Boat:GoX()
	if self.pic:getX()<1150 then
		self.pic:setX(self.pic:getX()+10)
		self:move()
		--print(theBoat.pic:getX())
	else 
		self.side="right" 
		--print("AA"..theBoat.side)
	end
end
function Boat:move()
	Timer.delayedCall(2000, function() 
		self:changeSide()
	end)
end 
 
function Boat:changeSide()
	self.vx = -self.vx
	print("changeSide called from", self.id)
end
 
-- using it
local my_boat = Boat.new({id = "My Boat"})
my_boat:GoX()
 
-- look out, enemy come
local pirate_boat = Boat.new({id = "Enemy Board"})
 
--- Call out of Class method 
Timer.delayedCall(2000, function() 
	pirate_boat:changeSide()
end)

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