What to use: self.something or local something?

eezing · October 2013

New to Lua and Gideros and have a question about classes:

function button:init()
self.texture = Texture.new("texture.png")
self.bitmap = Bitmap.new(self.texture)
self:addChild(self.bitmap)
end

-- or --

function button:init()
local texture = Texture.new("texture.png")
local bitmap = Bitmap.new(texture)
self:addChild(self.bitmap)
end

Let me see if I understand this correctly:

'self.texture' and 'self.bitmap' would be tables that belong to the button class?

'local texture' and 'local bitmap' are variables that belong to the 'function button:init()'?

They both appear to work, so what is the better method and why? Or maybe I just got this all wrong?

Thanks in advance for the help!

Mells · October 2013

@eeznig
I don't think the second works, because self.bitmap does not exist.
Should be the following instead :

self:addChild(bitmap)

After that change, both versions would work.
The difference is that the first version allows you to keep an easy access to the texture and the bitmap outside of the function, which is not possible in the second case.
Understanding that subtlety means a lot.

You can learn more by studying the following link : Scope Tutorial at lua-users.org.

So you need to know which variables you are going to need again, later.

function button:init(texture) 
    local tempCount = 5
    for i = 1, tempCount, 1 do
        print ("count : ", i)
    end
    self.texture = texture
    self.bitmap = Bitmap.new(self.texture) 
    self:addChild(self.bitmap) 
    tempCount = 1
end
 
local myTexture = Texture.new("texture.png")
local myButton = button.new(myTexture)
print ("myButton texture >", myButton.texture) -- Is possible thanks to the line : self.texture = texture
print ("myButton bitmap >", myButton.bitmap) -- Is possible thanks to the line : self.bitmap = bitmap

Notes :

to enjoy more flexibility, you see I gave the texture as a parameter to your button init().
In this case I didn't need to do anything with tempCount, but as you can see this is not necessarily something that I am going to need later so it makes sense to keep it local.

'self.texture' and 'self.bitmap' would be tables that belong to the button class?

That belong to the instance of the button class you are creating.

'local texture' and 'local bitmap' are variables that belong to the 'function button:init()'?

I would not say that "belong" is the right word. The scope of those variables is local to the init() function which means they are not accessible from outside of the init() function.

Do those answers cover your questions?

[Please someone correct me if/where I'm wrong]

ar2rsawseen · October 2013

@Mells is completely correct

In most cases, if you need to use the value stored in variable in other methods, etc you store it as self.property
but if you just create and forget about it, then you can make it local

eezing · October 2013

@Mells

oops, yep that second one was supposed to be 'self:addChild(bitmap)'... typo.

Thanks a lot for the thorough explanation... I'm starting to grasp the scope concept.

Ninjadoodle · October 2015

Hi guys

Just found this thread and I understand the difference between .self and local.

I do have one question tho ...

I have some sprites that I need to assess between functions, yet other just sit there.

Is there any downside / performance issues in setting everything as self. ?

Should I still be setting the ones I don't need to local?

Thank you in advance!

pie · October 2015

Yes, you should use local everytime you can: it's faster, and it gets collected as soon as it is no longer referenced.

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