Passing params from event listeners

Solar2D General Questions/Discussion
#1

Hi all, I’ve looked around the forums and havent been able to suss this out (and i’ve so far run into a few situations where this would simplify code)

Lets say I have the following code

local function myFunction( abc ) print( abc ) end local param = "myString" test:addEventListener( "touch", myFunction( param ) )

The idea is to have have the event listener pass a variable or parameter to the function which then uses that variable.

I found a similar snippet of code in an older post but havent been able to make it work.

Basically I’m a bit stuck on how passing params works with event listeners, any clues?

thanks!

#2

Hi Trakyan

 

I don’t think I fully understand what you are trying to achieve with this, but what you would normally do if you want parameters to be sent with an event listener is attaching them to the object that you add the event listener to.

 

Eg. like this:

local function eventHandler(event) print ( event.target.param ) end local obj = display.newCircle(0,0,100) obj.param = "myString" obj:addEventListener(“touch”, eventHandler)
#3

what im trying to achieve is just like you can pass params to a function usually by going myFunction( param1, param2 ) when its been called. I was wondering why you cant do this when calling a function with an event listener. 

Correct me if I’m wrong on how i thing passing params works.

local function myFunction( firstParam, secondParam ) print( "firstParam" ) print( "secondParam" ) end myFunction( one, two ) myFunction( three, four ) myFunction( five, six)

from what I understand the above code should print first “one” "two, second “three” “four” and third “five” “six”

Here the params in the brackets where the function is being called are being passed to the function.

what i was wondering was if this is possible when calling a function from an event listener

#4

Hi, Trakyan.

In your first post, you’re calling myFunction , which then prints its input and returns nothing. Assuming you also do this in your normal code, you’re never actually adding an event listener.

What you might be after is this sort of thing:

local function myFunction( abc ) return function(event) -- Other stuff print( abc ) end end local param = "myString" test:addEventListener( "touch", myFunction( param ) )

This will capture the parameter and return a new function with it baked in.

#5

the second bit of code i posted was just to show my understanding of passing params, i didnt add an event listener there. 

Your code seems to do what im after but could you explain what the return function(event) thats nested in myFunction does?

thanks!

What I mean here is why is return function(event) nessecary, why cant you just do

local function myFunction( abc ) print( abc ) end 
#6

The touch listener is dispatched somewhere in the guts of Corona, and needs to make do with the limited information it has available. The machinery that sends and delivers events, such as touch, is in a similar boat. The developers decided that a sane way to achieve this, while still allowing for reasonable flexibility, was to pass everything through a table as the first parameter.

So, to answer your second question, you can’t do that because that parameter is already reserved for something else.

Also, the most obvious way to feed your parameter into the function is to call it, but then its action happens immediately. This is where the new function (not yet called, obviously) comes in.

The line in question creates a new, one-parameter function. I gave the parameter the name “event” to emphasize that this one is meant to honor the event listener contract. This is what gets returned from myFunction (because of the return , naturally), and thus added as a listener.

We don’t pass abc as an argument to this one. Rather, Lua sees that it’s using abc, and also available in the function creating it (in this case myFunction ), so it makes a reference to abc’s current value and the new function gets to keep it. (This might be a bit clearer if you follow the link in my previous post.)

Functions are just another object in Lua, and get created like tables or strings. For comparison, the code in your last post is just a nicer syntax Lua gives us for

local myFunction myFunction = function( abc ) print( abc ) end

Luckily we don’t usually have to think about this, but it’s good to know for situations like yours.

Don’t worry if this seems confusing. It really is one of those things that takes a little time, and then it suddenly clicks.  :slight_smile:

#7

Also, you might find it simpler to use the table listener syntax, for example as in Fix #2 here. Then you can just stuff the parameters into your object, much like @ojnab did above.

#8

Hi Trakyan

 

I don’t think I fully understand what you are trying to achieve with this, but what you would normally do if you want parameters to be sent with an event listener is attaching them to the object that you add the event listener to.

 

Eg. like this:

local function eventHandler(event) print ( event.target.param ) end local obj = display.newCircle(0,0,100) obj.param = "myString" obj:addEventListener(“touch”, eventHandler)
#9

what im trying to achieve is just like you can pass params to a function usually by going myFunction( param1, param2 ) when its been called. I was wondering why you cant do this when calling a function with an event listener. 

Correct me if I’m wrong on how i thing passing params works.

local function myFunction( firstParam, secondParam ) print( "firstParam" ) print( "secondParam" ) end myFunction( one, two ) myFunction( three, four ) myFunction( five, six)

from what I understand the above code should print first “one” "two, second “three” “four” and third “five” “six”

Here the params in the brackets where the function is being called are being passed to the function.

what i was wondering was if this is possible when calling a function from an event listener

#10

Hi, Trakyan.

In your first post, you’re calling myFunction , which then prints its input and returns nothing. Assuming you also do this in your normal code, you’re never actually adding an event listener.

What you might be after is this sort of thing:

local function myFunction( abc ) return function(event) -- Other stuff print( abc ) end end local param = "myString" test:addEventListener( "touch", myFunction( param ) )

This will capture the parameter and return a new function with it baked in.

#11

the second bit of code i posted was just to show my understanding of passing params, i didnt add an event listener there. 

Your code seems to do what im after but could you explain what the return function(event) thats nested in myFunction does?

thanks!

What I mean here is why is return function(event) nessecary, why cant you just do

local function myFunction( abc ) print( abc ) end 
#12

The touch listener is dispatched somewhere in the guts of Corona, and needs to make do with the limited information it has available. The machinery that sends and delivers events, such as touch, is in a similar boat. The developers decided that a sane way to achieve this, while still allowing for reasonable flexibility, was to pass everything through a table as the first parameter.

So, to answer your second question, you can’t do that because that parameter is already reserved for something else.

Also, the most obvious way to feed your parameter into the function is to call it, but then its action happens immediately. This is where the new function (not yet called, obviously) comes in.

The line in question creates a new, one-parameter function. I gave the parameter the name “event” to emphasize that this one is meant to honor the event listener contract. This is what gets returned from myFunction (because of the return , naturally), and thus added as a listener.

We don’t pass abc as an argument to this one. Rather, Lua sees that it’s using abc, and also available in the function creating it (in this case myFunction ), so it makes a reference to abc’s current value and the new function gets to keep it. (This might be a bit clearer if you follow the link in my previous post.)

Functions are just another object in Lua, and get created like tables or strings. For comparison, the code in your last post is just a nicer syntax Lua gives us for

local myFunction myFunction = function( abc ) print( abc ) end

Luckily we don’t usually have to think about this, but it’s good to know for situations like yours.

Don’t worry if this seems confusing. It really is one of those things that takes a little time, and then it suddenly clicks.  :slight_smile:

#13

Also, you might find it simpler to use the table listener syntax, for example as in Fix #2 here. Then you can just stuff the parameters into your object, much like @ojnab did above.