passing parameter to function in event listener

Other
#1

Hi.

i’ve been curious for a while, but is it actually okay, or should i rephrase it, return 100% possibility that giving parameter into listener function will work?

here is the snippet code to help understand my question

local function getGlow(num) return function(event) if "began" == event.phase then buttonTable[num] = display.newImage(group, symbols[num].glow, symPos[num].x, symPos[num].y) elseif "moved" == event.phase then elseif "ended" == event.phase or "cancelled" == phase then buttonTable[num] = display.newImage(group, symbols[num].origin, symPos[num].x, symPos[num].y) count = count + 1 print(count) end end end local function makeButton() --local right = symbols[symbolNum] --load all symbols for i = 1, #symbols do buttonTable[i] = display.newImage(group, symbols[i].origin, symPos[i].x, symPos[i].y) buttonTable[i]:addEventListener('touch', getGlow(i)) //is this will return every time?? end end
#2

That is fine. This is called lua closures and is one of the top features of lua in my opinion.

#3

so you don’t need to wrap it up like this?

--load all symbols for i = 1, #symbols do buttonTable[i] = display.newImage(group, symbols[i].origin, symPos[i].x, symPos[i].y) buttonTable[i]:addEventListener('touch', function() getGlow(i) end) //wrapped it up with a function instead end end
#4

Nope @jonathan, look that get glow returns already wrappex function.

#5

oh, okay then. cool. thanks! :smiley:

#6

No beacuse getGlow(num) return a function so that is fine. The way you wrote it now is not actually going to work for many reasons:

  1. getGlow(i) returns a function so you would have to call it like getGlow(i)(event)

  2. you didn’t specify a parameter for the function thus you would loose the event table.

  3. this would create a new function every time an event is fired which I think is not very good practice

#7

if i have to attach a function on a listener that does not create a 

return function(event) end

does it mean that I have to called the function using wrappers?

#8

Only if you want custom parameters in it. But you could set those on the objec itself and access them in the event function using event.target.myParam.

Usualy you don’t need a wrapper you can define like this:

local function myTouch(event)

–proccess event

end

object:addEventListener(“touch”, myTouch)

or evene

object:addEventListener(“touch”, function(event)

    --proccess event

  end;)

if the function is short and you don’t need it anywhere else

in the case above where you need a param the first coed you posted is fine or you could set a field on the object like

 local function getGlow(event) local num = event.target.myIndex if "began" == event.phase then buttonTable[num] = display.newImage(group, symbols[num].glow, symPos[num].x, symPos[num].y) elseif "moved" == event.phase then elseif "ended" == event.phase or "cancelled" == phase then buttonTable[num] = display.newImage(group, symbols[num].origin, symPos[num].x, symPos[num].y) count = count + 1 print(count) end end --load all symbols for i = 1, #symbols do buttonTable[i] = display.newImage(group, symbols[i].origin, symPos[i].x, symPos[i].y) buttonTable[i].myIndex = i buttonTable[i]:addEventListener('touch', getGlow) end end
#9

neat. thanks :smiley: i will try to implement it

#10

That is fine. This is called lua closures and is one of the top features of lua in my opinion.

#11

so you don’t need to wrap it up like this?

--load all symbols for i = 1, #symbols do buttonTable[i] = display.newImage(group, symbols[i].origin, symPos[i].x, symPos[i].y) buttonTable[i]:addEventListener('touch', function() getGlow(i) end) //wrapped it up with a function instead end end
#12

Nope @jonathan, look that get glow returns already wrappex function.

#13

oh, okay then. cool. thanks! :smiley:

#14

No beacuse getGlow(num) return a function so that is fine. The way you wrote it now is not actually going to work for many reasons:

  1. getGlow(i) returns a function so you would have to call it like getGlow(i)(event)

  2. you didn’t specify a parameter for the function thus you would loose the event table.

  3. this would create a new function every time an event is fired which I think is not very good practice

#15

if i have to attach a function on a listener that does not create a 

return function(event) end

does it mean that I have to called the function using wrappers?

#16

Only if you want custom parameters in it. But you could set those on the objec itself and access them in the event function using event.target.myParam.

Usualy you don’t need a wrapper you can define like this:

local function myTouch(event)

–proccess event

end

object:addEventListener(“touch”, myTouch)

or evene

object:addEventListener(“touch”, function(event)

    --proccess event

  end;)

if the function is short and you don’t need it anywhere else

in the case above where you need a param the first coed you posted is fine or you could set a field on the object like

 local function getGlow(event) local num = event.target.myIndex if "began" == event.phase then buttonTable[num] = display.newImage(group, symbols[num].glow, symPos[num].x, symPos[num].y) elseif "moved" == event.phase then elseif "ended" == event.phase or "cancelled" == phase then buttonTable[num] = display.newImage(group, symbols[num].origin, symPos[num].x, symPos[num].y) count = count + 1 print(count) end end --load all symbols for i = 1, #symbols do buttonTable[i] = display.newImage(group, symbols[i].origin, symPos[i].x, symPos[i].y) buttonTable[i].myIndex = i buttonTable[i]:addEventListener('touch', getGlow) end end
#17

neat. thanks :smiley: i will try to implement it