Simple Objects

[FortyFourDigital]

I have been trying to work on a very simple object system for a game. I would like to create a new class for a Player/Enemy/Controls however I am having problems understanding metatables and groups. For example
 
Secondly, if I use
 
As a basic example:
 

local Controls = {} local ControlsMeta = {\_\_index = Controls} function Controls:newJoystick(options) local joystick = display.newGroup() -- container joystick.border = display.newCircle(.....) -- thumb control joystick.thumb = display.newCircle(.....) joystick.onMove = function(event) print(self) end joystick.thumb:addEventListener("touch",joystick.onMove) -- Insert parts joystick:insert(joystick.border) joystick:insert(joystick.thumb) -- setmetatable(joystick,self) -- Returns error if I uncomment this return joystick end return Controls

 
 
When I run the code and hover over the control, Corona throws an error:
 

Corona Runtime Error attempt to call a nil value Do you want to relaunch the project?

 
Why can’t I insert the items into the joystick group?

[paulscottrobson]

Well, first thing I would do is to encapsulate your OOP in a base class, which allows the creation of new classes/prototypes and nothing else, and then derive everything from that ; I tend to use :

G.Base = \_G.Base or { new = function(s,...) local o = { } setmetatable(o,s) s.\_\_index = s o:initialise(...) return o end, initialise = function() end }

Metatables are kind of like ‘behaviour modifier’ for tables. So when you do .__index on the metatable, it is like saying, for the table that uses this, look here for unknown table members - the net effect of this is it looks in the metatable for the unknown element, allowing you to create a chain.

The above code breaks down as 

local o = {} create a new table - this will either be a new class or an instance - in reality they are all prototypes 

setmetatable(o,s) use ‘s’ as this objects metatable, this is the parent prototype.

s.__index = s when you can’t find something look for it in the metatable.

o:initialise(…) call the constructor

then there’s just a dummy constructor.

The answer to your question, I think, is that you have your event listener as a function listener, you should make it a table listener, so it calls joystick.touch with two parameters self and e (or call joystick:touch with one parameter, same difference).

Generally when writing OOP code (not always) you should be using : for methods and . for members.

Also, it is not a good idea to create mixin classes out of Corona objects unless you really have to. This may involve writing some adapter code - a move() method rather than assigning x and y, but it does save a lot of issues, because you cannot guarantee what Corona will do with their objects. The most obvious problem is when you try to set the metatable of joystick (itself a displayGroup), I’m pretty sure Corona have already done this as it is the descendant of some other object. When you do mixin classes you really have to decorate.

I wrote something similar https://github.com/autismuk/Executive/blob/master/utils/controller.lua ; this is for my own object system which has its own constructors and destructors but it should help you get the general idea Corona does not normally have automatic destructors. Aside from the constructor() destructor() stuff (and the self:name() call) it is pretty much normal lua.

[FortyFourDigital]

Hi Paul,

Thanks for the thorough response. I have been reading a few more articles and have since swapped the listener to read:

joystick.thumb:addEventListener("touch",Controls)

and then wrote my method

function Controls:touch(event) ...

This works fine as the event has a reference to the “thumb” and hence I can use event.target.parent to access my original “joystick” group as defined in Controls:newJoystick().

I’ll have a go with the code you provided (many thanks), but for my own curiosity, how would you create a class without the wrapper? For the sake of keeping lines of code down, how could I “fix” my original code so that I can have a few display objects, properties and also methods.

[roaminggamer]

@FortyFourDigital,

There are lots of wonderful Lua OOP modules and wrappers out there, but I find this one very simple and functional at the  same time:

https://github.com/bjorn/lua-oop/blob/master/object.lua

[FortyFourDigital]

Thanks for this roaminggamer - I guess it’s similar to Paul’s wrapper.

I’ve done a quick test and still can’t access a method:

main.lua:

\_G.Object = require("object") \_G.Controls = require("controls") local joystick = Controls:new({name="myStick"}) joystick:testMethod() -- This gives "attempt to call method 'testMethod' (a nil value)

My controls.lua

local Controls = Object({ someProperty = 0 }) function Controls:new(options)    local joystick = display.newGroup()    -- deleted extraneous code joystick.thumb = display.newCircle(0,0,30) joystick.thumb:addEventListener("touch",Controls) end function Controls:touch(event)    -- touch events work and update the instance properties correctly end function Controls:testMethod()    print("Hello") -- Trying to call this end return Controls

Perhaps I missed something obvious?

[bobcook47]

I wrote a Lua class implementation  tutorial that is posted at http://professorcook.org/lua/ClassDesign.html

to continue your discussion, i am trying to implement a Cartesian graph class that would be a sub-class of a display group.

the following code shows that it is possible.

g=display.newGroup()
g.x=display.contentCenterX
g.y=display.contentCenterY
display.newRect(g,0,0,50,50)
g.apple=22
print(g.apple)
function g:test()
  print(55, self.x, self.y)
end
g:test() --prove props/methods can be added to system object
for a,b in ipairs(g) do print(a,b) end

OUTPUT

22

55    360    640

a problem arises in implementing a copy constructor.

i know that the docs say that ipairs doesn’t work on system objects.

i think that is a bug that prevents sub-classing groups, which is very desirable.

Can the behavior be modified to only iterate over user-defined fields?

[paulscottrobson]

Hi, my micro version is at the top of a file - it defines a class ‘Base’ which has two methods, new(), which creates subclasses or instances, and a constructor initialise(). The Executive thing is a very different approach, but feel free to nick any of the code

There are plenty of good tutorials and libraries, just find one which suits your coding style

[The_Lemon]

I think the problem lies in the metatable fourty four. Try this

– joystick.lua

-- base table local Controls = {} -- metatable which is associated with table which cruicially calls the testmethod function local Controls\_mt = {\_\_index = Controls} -- private function, no need to be accesed by other modules local function onTouch(event)     print( "touched" ) end -- constructor, i prefer using the dot operator but its up to you function Controls:new(options)    local joystick = display.newGroup()    -- deleted extraneous code    joystick.thumb = display.newCircle(0,0,30)    joystick.thumb:addEventListener("touch",onTouch)    return setmetatable(joystick,Controls\_mt) end -- method instance function Controls:testMethod()    print("Hello") end -- you know what this does return Controls

and

– main.lua

  local require = require local control = require("joystick") local joystick = control:new({}) joystick:testMethod() -- prints hello  

[FortyFourDigital]

Hi @86lemonade68

I had tried to use your code before (I’ve re-written my controller class several times!). I can call the test method now, but as soon as I hover my mouse over the joystick control in the simulator - it crashes and says:

“Corona Runtime Error - attempt to call a nil value Do you want to relaunch the project”

What is causing this? I am doing nothing else aside from moving the mouse over the circle.

EDIT:

if I change my controller class to use joystick = {} rather than joystick = display.newGroup() then I don’t get an error. I have to use table.insert(joystick,joystick.border) and table.insert(joystick,joystick.thumb) to add display objects into the joystick table.

The only problem now is that if I set an event listener on joystick.thumb, I cannot use event.target.parent in the onTouch function.

[FortyFourDigital]

@paulscottrobson

I created a simple test control class using your code, and it still gives me the same issue. I cannot call any of my own methods on the joystick object “Corona - attempt to call method ‘someMethod’ (a nil value)”

The controls show up fine but I’m obviously missing something important.

[paulscottrobson]

Difficult to say because I don’t know how you are using it - the module works, but without seeing your version of it I can’t tell you

[FortyFourDigital]

Hi Paul, This is my demo code

main.lua

display.setStatusBar(display.HiddenStatusBar) \_G.screenWidth = display.contentWidth - (display.screenOriginX\*2) \_G.screenHeight = display.contentHeight - (display.screenOriginY\*2) \_G.screenTop = 0 + display.screenOriginY \_G.screenRight = display.contentWidth - display.screenOriginX \_G.screenBottom = display.contentHeight - display.screenOriginY \_G.screenLeft = 0 + display.screenOriginX \_G.screenCenterX = display.contentWidth/2 \_G.screenCenterY = display.contentHeight/2 local Controls = require("controls") local joystickMove, joystickShoot local player = display.newGroup() local head = display.newRect(0,-20,10,10) local body = display.newRect(0,0,50,50) head:setFillColor(1,0,0) body:setFillColor(0,1,0) player:insert(body) player:insert(head) player.x = screenCenterX player.y = screenCenterY joystickMove = Controls:newJoystick({name="movement",x=screenLeft+60, y=screenBottom-60}) joystickShoot = Controls:newJoystick({name="shoot",x=screenRight-60,y=screenBottom-60}) local function doStuff(event) if(joystickMove.isActive) then print("Angle",joystickMove:getAngle()) -- THIS THROWS THE ERROR end end Runtime:addEventListener("enterFrame",doStuff)

And this is my controls.lua

\_G.Base =  \_G.Base or { new = function(s,...) local o = { } setmetatable(o,s) s.\_\_index = s o:initialise(...) return o end, initialise = function() end } local Controls = Base:new() function Controls:initialise(options) -- I have tried to move my newJoystick code into here - same problem end function Controls:newJoystick(options) local joystick = display.newGroup() -- Properties joystick.name = options.name joystick.maxDist = 50 joystick.snapBackSpeed = 0.7 joystick.x = options.x joystick.y = options.y -- Create the container joystick.border = display.newCircle(0,0,48) joystick.border:setFillColor  (0.6,0.6,1.0,0.18) joystick.border:setStrokeColor(0.6,0.6,1.0,1) -- Create the thumb control joystick.thumb = display.newCircle(0,0,30) joystick.thumb:setFillColor  (0.6,0.6,1.0,0.38) joystick.thumb:setStrokeColor(0.6,0.6,1.0,1) joystick.thumb.x0 = 0 joystick.thumb.y0 = 0 -- Event listeners joystick.thumb:addEventListener("touch",Controls) -- Insert parts joystick:insert(joystick.border) joystick:insert(joystick.thumb) return joystick end function Controls:touch(event) local phase = event.phase local target = event.target local parent = target.parent local ex = event.x - target.x0 local ey = event.y - target.y0 if(phase=="began") then display.getCurrentStage():setFocus(target,event.id) target.isFocus = true target.x0 = ex - target.x target.y0 = ey - target.y parent.isActive = true transition.cancel(target) elseif target.isFocus then if(phase == "moved") then parent.distance = math.sqrt (ex\*ex + ey\*ey) if parent.distance \> parent.maxDist then  parent.distance = parent.maxDist end parent.angle = ((math.atan2( ex-0,ey-0 )\*180 / math.pi) - 180 ) \* -1 parent.percent = parent.distance / parent.maxDist target.x = math.cos( math.rad(parent.angle-90) ) \* (parent.maxDist \* parent.percent)  target.y = math.sin( math.rad(parent.angle-90) ) \* (parent.maxDist \* parent.percent) elseif(phase=="ended" or phase=="cancelled") then target.x0 = 0 target.y0 = 0 target.isFocus = false display.getCurrentStage():setFocus(nil,event.id) parent.isActive = false transition.to(target,{ delay = 0, time = 200, x = 0, y = 0 }) end end end function Controls:getAngle() print("Calling getAngle") end return Controls

What have I missed?

[paulscottrobson]

lua OOP doesn’t work like that - it works a bit like Python’s, using the self keyword.

I’ve rewritten it at https://github.com/autismuk/Executive/blob/master/utils/d44.lua - you probably need to read through it in conjunction with this. http://www.lua.org/pil/16.html 

Fundamentally, don’t tinker with Corona objects - treat them as things you manipulate in an object. 

[paulscottrobson]

Well, first thing I would do is to encapsulate your OOP in a base class, which allows the creation of new classes/prototypes and nothing else, and then derive everything from that ; I tend to use :

G.Base = \_G.Base or { new = function(s,...) local o = { } setmetatable(o,s) s.\_\_index = s o:initialise(...) return o end, initialise = function() end }

Metatables are kind of like ‘behaviour modifier’ for tables. So when you do .__index on the metatable, it is like saying, for the table that uses this, look here for unknown table members - the net effect of this is it looks in the metatable for the unknown element, allowing you to create a chain.

The above code breaks down as 

local o = {} create a new table - this will either be a new class or an instance - in reality they are all prototypes 

setmetatable(o,s) use ‘s’ as this objects metatable, this is the parent prototype.

s.__index = s when you can’t find something look for it in the metatable.

o:initialise(…) call the constructor

then there’s just a dummy constructor.

The answer to your question, I think, is that you have your event listener as a function listener, you should make it a table listener, so it calls joystick.touch with two parameters self and e (or call joystick:touch with one parameter, same difference).

Generally when writing OOP code (not always) you should be using : for methods and . for members.

Also, it is not a good idea to create mixin classes out of Corona objects unless you really have to. This may involve writing some adapter code - a move() method rather than assigning x and y, but it does save a lot of issues, because you cannot guarantee what Corona will do with their objects. The most obvious problem is when you try to set the metatable of joystick (itself a displayGroup), I’m pretty sure Corona have already done this as it is the descendant of some other object. When you do mixin classes you really have to decorate.

I wrote something similar https://github.com/autismuk/Executive/blob/master/utils/controller.lua ; this is for my own object system which has its own constructors and destructors but it should help you get the general idea Corona does not normally have automatic destructors. Aside from the constructor() destructor() stuff (and the self:name() call) it is pretty much normal lua.

[FortyFourDigital]

Hi Paul,

Thanks for the thorough response. I have been reading a few more articles and have since swapped the listener to read:

joystick.thumb:addEventListener("touch",Controls)

and then wrote my method

function Controls:touch(event) ...

This works fine as the event has a reference to the “thumb” and hence I can use event.target.parent to access my original “joystick” group as defined in Controls:newJoystick().

I’ll have a go with the code you provided (many thanks), but for my own curiosity, how would you create a class without the wrapper? For the sake of keeping lines of code down, how could I “fix” my original code so that I can have a few display objects, properties and also methods.

[roaminggamer]

@FortyFourDigital,

There are lots of wonderful Lua OOP modules and wrappers out there, but I find this one very simple and functional at the  same time:

https://github.com/bjorn/lua-oop/blob/master/object.lua

[FortyFourDigital]

Thanks for this roaminggamer - I guess it’s similar to Paul’s wrapper.

I’ve done a quick test and still can’t access a method:

main.lua:

\_G.Object = require("object") \_G.Controls = require("controls") local joystick = Controls:new({name="myStick"}) joystick:testMethod() -- This gives "attempt to call method 'testMethod' (a nil value)

My controls.lua

local Controls = Object({ someProperty = 0 }) function Controls:new(options)    local joystick = display.newGroup()    -- deleted extraneous code joystick.thumb = display.newCircle(0,0,30) joystick.thumb:addEventListener("touch",Controls) end function Controls:touch(event)    -- touch events work and update the instance properties correctly end function Controls:testMethod()    print("Hello") -- Trying to call this end return Controls

Perhaps I missed something obvious?

[bobcook47]

I wrote a Lua class implementation  tutorial that is posted at http://professorcook.org/lua/ClassDesign.html

to continue your discussion, i am trying to implement a Cartesian graph class that would be a sub-class of a display group.

the following code shows that it is possible.

g=display.newGroup()
g.x=display.contentCenterX
g.y=display.contentCenterY
display.newRect(g,0,0,50,50)
g.apple=22
print(g.apple)
function g:test()
  print(55, self.x, self.y)
end
g:test() --prove props/methods can be added to system object
for a,b in ipairs(g) do print(a,b) end

OUTPUT

22

55    360    640

a problem arises in implementing a copy constructor.

i know that the docs say that ipairs doesn’t work on system objects.

i think that is a bug that prevents sub-classing groups, which is very desirable.

Can the behavior be modified to only iterate over user-defined fields?

[paulscottrobson]

Hi, my micro version is at the top of a file - it defines a class ‘Base’ which has two methods, new(), which creates subclasses or instances, and a constructor initialise(). The Executive thing is a very different approach, but feel free to nick any of the code

There are plenty of good tutorials and libraries, just find one which suits your coding style

[The_Lemon]

I think the problem lies in the metatable fourty four. Try this

– joystick.lua

-- base table local Controls = {} -- metatable which is associated with table which cruicially calls the testmethod function local Controls\_mt = {\_\_index = Controls} -- private function, no need to be accesed by other modules local function onTouch(event)     print( "touched" ) end -- constructor, i prefer using the dot operator but its up to you function Controls:new(options)    local joystick = display.newGroup()    -- deleted extraneous code    joystick.thumb = display.newCircle(0,0,30)    joystick.thumb:addEventListener("touch",onTouch)    return setmetatable(joystick,Controls\_mt) end -- method instance function Controls:testMethod()    print("Hello") end -- you know what this does return Controls

and

– main.lua

  local require = require local control = require("joystick") local joystick = control:new({}) joystick:testMethod() -- prints hello