Removing events listeners in functions

Getting Started Newbie Questions
1

Hi, I am creating a game and I have a conceptual question that I’d like to know.

I am using the composer library to change between scenes

This is part of my code. 

local coin local function spawnCoins() local sceneGroup = scene.view coin = display.newImageRect("level1/bitcoin.png",58,59) coin.x = math.random(300,17\*480) coin.y = math.random(y/4,y\*2-20) coin.name = "coin" sceneGroup:insert(coin) coin.enterFrame = scrollSnowBackground Runtime:addEventListener("enterFrame", coin) physics.addBody(coin, "dynamic", {bounce = 0, filter = filtersTable.coinFilter ,friction = 0, isSensor = true, radius = 30}) coin.gravityScale = 0 coin:addEventListener("collision", onCollisionCoin) end

Then, at the beginning of the game I call the function as follows when the scene is shown

for i = 0,8,1 do spawnCoins() end

I didn’t include other functions since I don’t think they are important to my question.

I am creating the object as a local variable to the function spawnCoins() and I am adding event listeners to it.

Do I have to remove the event listener at any point? Like when changing the scene? Or I don’t have to worry about it?

Thanks

2

You must remove that enterFrame listener from every coin when the coin is destroyed.

You can do that this way:

function coin.finalize( self ) Runtime:removeEventListener( "enterFrame", self ) end coin:addEventListener("finalize")
3

The full code would look like this:

--local coin -- this is pointless since you are making multiple local function spawnCoins() local sceneGroup = scene.view local coin = display.newImageRect("level1/bitcoin.png",58,59) coin.x = math.random(300,17\*480) coin.y = math.random(y/4,y\*2-20) coin.name = "coin" sceneGroup:insert(coin) coin.enterFrame = scrollSnowBackground Runtime:addEventListener("enterFrame", coin) physics.addBody(coin, "dynamic", { bounce = 0, filter = filtersTable.coinFilter, friction = 0, isSensor = true, radius = 30 }) coin.gravityScale = 0 coin:addEventListener("collision", onCollisionCoin)    function coin.finalize( self )    Runtime:removeEventListener( "enterFrame", self )    end    coin:addEventListener("finalize")    return coin end
4

Thanks!

5

Why do I need the return coin?

6

Why not? I always return an object if I make it, in case I should need to use the function that way later.

7

You must remove that enterFrame listener from every coin when the coin is destroyed.

You can do that this way:

function coin.finalize( self ) Runtime:removeEventListener( "enterFrame", self ) end coin:addEventListener("finalize")
8

The full code would look like this:

--local coin -- this is pointless since you are making multiple local function spawnCoins() local sceneGroup = scene.view local coin = display.newImageRect("level1/bitcoin.png",58,59) coin.x = math.random(300,17\*480) coin.y = math.random(y/4,y\*2-20) coin.name = "coin" sceneGroup:insert(coin) coin.enterFrame = scrollSnowBackground Runtime:addEventListener("enterFrame", coin) physics.addBody(coin, "dynamic", { bounce = 0, filter = filtersTable.coinFilter, friction = 0, isSensor = true, radius = 30 }) coin.gravityScale = 0 coin:addEventListener("collision", onCollisionCoin)    function coin.finalize( self )    Runtime:removeEventListener( "enterFrame", self )    end    coin:addEventListener("finalize")    return coin end
9

Thanks!

10

Why do I need the return coin?

11

Why not? I always return an object if I make it, in case I should need to use the function that way later.