I'm new to Lua so I'm sure I'm missing something but I have this class and it seems to be behaving unexpectedly.
Item = {elm = nil, __index = {}}
function Item:new(obj)
setmetatable({}, Item)
self.elm = obj.elm
return self
end
function Item:collectItem()
print(self.elm);
end
local itm = Item:new{elm = "val1"}
local itm2 = Item:new{elm = "val2"}
itm:collectItem()
itm2:collectItem()
This outputs:
>val2
>val2
When I would expect:
val1
val2
What am I missing here?
The issue here in that your Item:new function keeps modifying the same table: Item (self in the context of Item:new). What you want to do is create a new table for each new Item object you create. Here is one way you can do this:
Item = {elm = nil}
function Item:new(obj)
-- Create a new table whose metatable's __index is the Item table
local instance = setmetatable({}, {
__index = self
})
-- Modify the new table, not Item (self)
instance.elm = obj.elm
-- Return the new object
return instance
end
Related
I'm new to Lua, and I'm trying to understand its OO part, for example :
lkw = {}
lkw.la= 0
function lkw:func(ge)
self.la = self.la + ge
end
function lkw:new()
local res = {}
setmetatable(res, self)
self.__index = self
return res
end
mylkw = lkw:new()
in this example the "class" lkw can create object using new, but what do self and index mean ?
should consider self as this in java/C++ and what is the index ?
This style of OOP is frequent in Lua. I do not like it because it is not explicit enough for me, but let me try to explain.
There are two confusing things: the use of the : sugar in function definitions and the use of the "class" as the metatable for its instances.
First, function a:b(...) is the same as a.b = function(self, ...), so let us remove all sugar:
lkw = {}
lkw.la = 0
lkw.func = function(self, ge)
self.la = self.la + ge
end
lkw.new = function(self)
local res = {}
setmetatable(res, self)
self.__index = self
return res
end
mylkw = lkw.new(lkw)
Now, this is "prototypal inheritance". lkw is the "prototype" for instances like mylkw. This is similar but slightly different from a "class".
When the new constructor is called, lkw is passed as the self argument.
The second and third lines of the constructor are weird. This is probably easier to understand:
lkw.new = function(self)
local res = {}
setmetatable(res, {__index = lkw})
return res
end
i.e.: if we do not find something in the instance we go look for it inside the prototype.
This explains how func works. The first time it is called, the instance will not contain a la key so lkw.la will be used.
The reason the code is not written this way is that the weird construction allows "prototypal inheritance": you could call "new" on mylkw and get an "instance of the instance" (i.e. in prototypal inheritance an instance and a child class are the same thing).
I think this is a very confusing feature. For reference this is about how I would write code that does about the same thing, with no inheritance:
local methods = {
func = function(self, ge)
self.la = self.la + ge
end
}
local lkw = {
new = function()
return setmetatable({la = 0}, {__index = methods})
end
}
local mylkw = lkw.new()
got some problem with metatable. This is my simple metatable:
local mt = {}
function mt:add(n)
return setmetatable({n = n}, {__index = mt})
end
function mt:get() return self.n end
Now I want to add some division like:
mt.math
mt.effect
Which each one has some own methods like:
mt.math:floor() return math.floor(self:get()) end
mt.effect:show(args) onMapShowEffect(self:get(), {x = x + (args[1] ~= nil or 0), ...) end
mt.effect:get() return getCurrentPos() end
Any ideas?
OK, trying make all details to share my problem.
Player = {}
function Player:add(this)
return setmetatable({this = this}, {__index = Player})
end
Player:get() return self.this end
Above code works perfectly on this example
function enterToGame(player1, player2)
local p1 = Player:add(player1)
local p2 = Player:add(player2)
print(p1:get()) -- ID1
print(p2:get()) -- ID2
Now I want to create some helpfully methods(functions) for table Player. I want to make it more flexible, so I want divide it for classes. Example:
Player.info = {
id = function() return Player:get() end,
}
Player.pos = {
get = function() return getPosition(Player:get()) end,
set = function(args) setPosition(Player:get(), args) end,
}
Player.speed = {
get = function() return getSpeed(Player:get()) end,
set = function(value) setSpeed(value) end,
improve = function(value) setSpeed(Player.speed.get() + value) end,
}
But its not work exactly what I want:
function enterToGame(player1, player2)
local p1 = Player:add(player1)
local p2 = Player:add(player2)
print(p1:get()) -- ID1
print(p2:get()) -- ID2
print(p1.info.id()) -- ID2 instead of ID1
print(p2.info.id()) -- ID2
When I put Player:get() in my methods its return last object declaration.
Based on what you state, if you do
mt.math = mt:add(123)
You don't need themt:get() because mt is the metatable for mt.math. Then
mt.math.floor = function(self) return math.floor(self.n) end
will work as expected. For example,
print(mt.math:floor())
prints 123.
EDIT 1: So now that I have a better understanding of what you are trying to do: normally you would do
p1:id()
p1:getPos()
p1:setPos()
p1:getSpeed()
p1:improveSpeed()
Note the colon, this is important, so that each method gets a "self" as first parameter, thereby given them the table instance to operate on (p1, in the above example). Instead you want to group methods so
p1.info:id()
p1.pos:get()
p1.pos:set()
p1.speed:improve()
p1.speed:get()
These methods will get a self that points to p1.info, p1.pos, etc. But those sub-tables have no knowledge of the container table (p1). The info and pos tables are in the Player class: they are shared by all instances of Player (p1, p2 etc). You have to make the info and pos tables non-shared:
function Player:add(player)
local pN= setmetatable( {n = player, info={}, pos={}}, {__index = Player})
pN.info.id = function() return pN.n end
pN.pos.set = function(x) return setPosition(pN, x) end
return pN
end
Then you get
> p1=mt:add(player1)
> p2=mt:add(player2)
> print(player1)
table: 0024D390
> print(p1.info.id())
table: 0024D390
> print(player2)
table: 0024D250
> print(p2.info.id())
table: 0024D250
All that said, I don't really like the idea of having to use closures like this, perhaps there are gotchas since not everything will be in Player.
Below is the gist of the system i'm having issues with. I seem to understand self, ., and :. I just seem to be missing something. What's happening is that when it calls "Object:setSomeObjectIsAttachedTo()" if I simply print "self" i'll get a table address printed. If i go a step further and try to print "self.someObject" i get nil, which shouldn't happen because in Object it has a key someObject which was created at the start in "Object.new(args)" of course if tried to go a step further it wouldn't even be able to go there since its nil. Please Help!!
Object File
Object = {};
ObjectMeta = {__index = Object};
function Object.new(args)
Obj = {};
Object.someObject = OtherObject.new(args)
return setmetatable(Obj,ObjectMeta );
end
function Object:setSomeObjectIsAttachedTo()
--OtherObject instance Should set its attached property to
--This instance of Object
self.someObject.ObjectImAttachedTo = self;
end
--Calls after new to set the ObjectImAttachedTo Property, So it isnt nil
Object:setSomeObjectIsAttachedTo();
return Object;
OtherObject File
OtherObject = {};
OtherObjectMeta = {__index = OtherObject};
function OtherObject.new(args)
Obj = {};
Obj.ObjectImAttachedTo =nil;
return setmetatable(Obj,ObjectMeta );
end
return Object;
UPDATE
Scene
Scene = {};
ObjectContainer = {};
function Scene.new()
end
function Scene.addObjects()
local Object= require "Object"
local StartX = 50;
local StartY = 20;
local counter = 0;
for i=0, 17 do
ObjectContainer[i] = Object.new({x=StartX,y=StartY});
end
end
Scene.addObjects();
return Scene
end
The table Object does not have a field named someObject, though instances returned by Object.new() do have that field. These are two different tables, roughly corresponding to a class and one of its instances.
ADDENDUM
With the revised code, Object doesn't have the field someObject until you call Object.new(). So, you must call Object.new() before you call Object:setSomeObjectIsAttachedTo();. Note that OtherObject.new() must be defined before you can call Object.new().
ADDENDUM2 in answer to question "so what would you suggest i do to fix this?"
function Object.new(args)
Obj = {};
Obj.someObject = OtherObject.new(args)
Obj.someObject.ObjectImAttachedTo = Obj;
return setmetatable(Obj,ObjectMeta );
end
and get rid of Object:setSomeObjectIsAttachedTo();
Im just alittle curious, as well as a bit confused. In my lua code im setting a new object like so from the start.
enemy = {};
enemy.__index = enemy;
function enemy.new(args)
Obj = {};
setmetatable(Obj,enemy);
Obj.name = "bullet";
Obj.x = args.x;
Obj.y = args.y;
Obj.spriteTexFile= "Invader.png";
Obj.sprite = display.newImage( Obj.spriteTexFile);
Obj.sprite:setReferencePoint ( display.TopLeftReferencePoint );
Obj.sprite.x = Obj.x;
Obj.sprite.y = Obj.y;
Obj.sprite.alpha = 0;
Obj.health = 100;
Obj.activeBul = false;
Obj.bullet = Bullet.new({x=Obj.sprite.x,y=Obj.sprite.y});
return Obj;
end
...
return enemy;
end
So when instantiating a new Enemy obj I call the new function above. NOW in the same file, a function in the Enemy Object I have the following function for example which allows me to acces the "self.bullet", a Bullet Object created when the Enemy is created. It also allows me to call the function trajectBullet in this Bullet instants.
function enemy:shoot()
local Bullet = require "Bullet";
local DEFAULTTIME = 5;--Movement time per space
self.bullet:trajectBullet({x=self.sprite.x,y=display.contentHeight, time =
DEFAULTTIME*display.contentHeight-self.sprite.y)});
end
My Question comes with a call like the following. If I try setting a property of a Bullet in this case, the owner property, i get a nil error and wont let me change it. If someone could help me understand alittle how accessing keys and properties really works that would help me out alot.
function enemy:setBulletOwner()
self.bullet.owner = self;
end
UPDATE:
bullet = {};
bullet.__index = bullet;
function bullet.new(arg)
local Obj = {};
setmetatable ( Obj, bullet );
Obj.sprite = display.newRect( 0, 0, 3, 7 );
Obj.sprite.x = arg.x;
Obj.sprite.y = arg.y;
Obj.sprite:setFillColor ( 255, 255, 255 );
Obj.sprite:setReferencePoint ( display.TopLeftReferencePoint );
Obj.owner = nil;
return Obj;
end
function bullet:trajectBullet(arg)
self.sprite.tween = transition.to(self.sprite,{ tansistion = easing.outExpo, y = arg.y, x=arg.x,time= arg.time,onComplete = function() bullet:cancelTween(self.sprite);
self.owner.sprite:dispatchEvent( {name = "canShootAgain"} ); end});
end
Keep in mind Obj.owner should be getting set from the function below.
function enemy:setBulletOwner()
print("BULLET MADE");
self.bullet.owner = self;
end
You should have your classes set up like this
Bullet
Bullet = {}
Bullet_mt = { __index = Bullet }
function Bullet:new(co_ordinates)
local obj = {x=co_ordinates[1],y=co_ordinates[2]}
obj.owner = "You" --etc...
return setmetatable(obj,Bullet_mt)
end
Enemy
Enemy = {slogan="Gettm!'"}
Enemy_mt = {__index = Enemy;}
function Enemy:new(args)
local obj = {}
--etc..
obj.bullet = Bullet:new({0,0})
return setmetatable(obj,Enemy_mt)
--alert return setmetatable(obj,getmetatable(self))
end
function Enemy:getBulletOwner()
return self.bullet.owner;
end
You shouldn't be requiring "Bullet" each time the enemy shoots in enemy:shoot. When you want to create a bullet for an enemy if you only want the enemy to have one bullet you should
create a new 'instance' of the bullet class and associate it with the key bullet like you've been doing obj.bullet= Bullet.new(...) but also introduce this functionality into a method of Enemy (so you can add a new bullet after the old one goes out of range etc...).
If a index doesn't exist within a table, it will go looking for the index in the table associated with __index in the metatable assigned to the table in question. As an example say a = Enemy:new(), and we wanted to find out the slogan of the enemy, via a.slogan we would look for the index slogan in a but not find it. So we would then go check what __index was associated with in the metatable of a, in this case Enemy. So we look for slogan in Enemy, it exists so we end up with `"Gettm!'".
Adding the following code beneath the class definitions
en = Enemy:new()
print(en:getBulletOwner())
print(en.slogan)
Produces
You
Gettm!'
Also be weary of the difference between a:b(arg1,arg2) and a.b(arg1,arg2). a:b(arg1,arg2) is essentially equivalent to a.b(a,arg1,arg2) where a is bound to self within the function. An example of this would be:
print(en.getBulletOwner())
produces
lua: l.lua:22: attempt to index local 'self' (a nil value)
while
print(en:getBulletOwner())
produces
You
I'm trying to write a metatable so that all indexes into the table are shifted up one position (i.e. t[i] should return t[i+1]). I need to do this because the table is defined using index 1 as the first element, but I have to interface with a program that uses index 0 as the first element. Since reading Programming in Lua, I think that I can accomplish what I want with a proxy table, but I can't seem to get it working. So far, I have this:
t = {"foo", "bar"}
local _t = t
t = {}
local mt = {
__index = function(t, i)
return _t[i+1]
end
}
setmetatable(t, mt)
However, this does not create the expected result. In fact, it doesn't return any values at all (every lookup is nil). Is there a better way to do this, or am I just missing something?
t = {"foo", "bar"}
local _t = t
t = {}
local mt = {
__index = function(t, i)
return _t[i+1]
end
}
setmetatable(t, mt)
print(t[0])
outputs "foo" for me when run here: http://www.lua.org/cgi-bin/demo