Drag object returning nil value

Audio and Graphics
#1

Hi,

I have a function setup that is similar to the basic object drag function - https://coronalabs.com/blog/2011/09/24/tutorial-how-to-drag-objects/

It works fine if it is set with the addeventlistener assigned to an object. However I want to move the object by dragging anywhere on the screen. So essentially the object will mirror your touch and drag anywhere on the screen. But I keep getting an error saying that event.target is a nil value. I have set it up as runtime:addeventlistener () given that it’s not an object that I am clicking on… Can that still work?

Also, I notice in the example given that they refer to self.markX. I gather markX is just a variable? But they don’t define it anywhere prior. I had to create markX as a local variable to get it to work. I gather in this example using self or event.target doesn’t make any difference?

Thank you to those that take the time to respond.

#2

show your exact code so folks can help.

However, if you want to see a conversion of the code you referenced that would work for the whole screen, here is one possibility (of many; Corona + LUA == Very flexible and many alternate solutions):

-- touch listener function function touch( self, event ) if event.phase == "began" then self.markX = self.x -- store x location of object self.markY = self.y -- store y location of object elseif event.phase == "moved" then local x = (event.x - event.xStart) + self.markX local y = (event.y - event.yStart) + self.markY self.x, self.y = x, y -- move object based on calculations above end return true end local myObject = display.newRect( 0, 0, 100, 100 ) myObject:setFillColor( 255 ) myObject.touch = touch Runtime:addEventListener( "touch", myObject )
#3

Fantastic! It works! However I am still unsure of how exactly… I don’t quite understand the myObject.touch = touch part… how does that produce the end result?

I would be very interested in some further explanation of what you have provided compared to the track that I was on… i’m sure you could shed some light on where I may have been going wrong in general…

Here is what I had previously… but I was yet to get it to not return an error of nil value… i was then going to try storing the value of (event.x - event.xStart) and use that to move the object:

[lua]

function touchM(event)

local objectM = event.target

local markX, markY

if(event.phase == “began”) then

display.getCurrentStage( ):setFocus(objectM)

objectM.markX = objectM.x

objectM.markY = objectM.y

elseif(event.phase == “moved”) then

objectM.x = (event.x - event.xStart) + objectM.markX

objectM.y = (event.y - event.yStart) + objectM.markY

elseif(event.phase == “ended” or event.phase == “cancelled”) then

display.getCurrentStage( ):setFocus(nil)

end

return true

end

Runtime:addEventListener(“touch”, touchM)

[/lua]

I now have the following which works:

[lua]

function touchM(self, event)

if(event.phase == “began”) then

display.getCurrentStage( ):setFocus(self)

self.markX = self.x

self.markY = self.y

elseif(event.phase == “moved”) then

self.x = (event.x - event.xStart) + self.markX

self.y = (event.y - event.yStart) + self.markY

elseif(event.phase == “ended” or event.phase == “cancelled”) then

display.getCurrentStage( ):setFocus(nil)

end

return true

end

objectM.touch = touchM

Runtime:addEventListener(“touch”, objectM)

[/lua]

#4

You were using a function listener, whereas I prefer table listeners wherever possible.  I’d walk through the various guides on the site and read up.  That’s how I learned of these features:

Sorry, but research, trial and error, and lots and lots of practice are the only way to really get this. 

#5

I’ve already read through the corona university pages. I looked through the lua syntax again and finally understand how that code works.

#6

show your exact code so folks can help.

However, if you want to see a conversion of the code you referenced that would work for the whole screen, here is one possibility (of many; Corona + LUA == Very flexible and many alternate solutions):

-- touch listener function function touch( self, event ) if event.phase == "began" then self.markX = self.x -- store x location of object self.markY = self.y -- store y location of object elseif event.phase == "moved" then local x = (event.x - event.xStart) + self.markX local y = (event.y - event.yStart) + self.markY self.x, self.y = x, y -- move object based on calculations above end return true end local myObject = display.newRect( 0, 0, 100, 100 ) myObject:setFillColor( 255 ) myObject.touch = touch Runtime:addEventListener( "touch", myObject )
#7

Fantastic! It works! However I am still unsure of how exactly… I don’t quite understand the myObject.touch = touch part… how does that produce the end result?

I would be very interested in some further explanation of what you have provided compared to the track that I was on… i’m sure you could shed some light on where I may have been going wrong in general…

Here is what I had previously… but I was yet to get it to not return an error of nil value… i was then going to try storing the value of (event.x - event.xStart) and use that to move the object:

[lua]

function touchM(event)

local objectM = event.target

local markX, markY

if(event.phase == “began”) then

display.getCurrentStage( ):setFocus(objectM)

objectM.markX = objectM.x

objectM.markY = objectM.y

elseif(event.phase == “moved”) then

objectM.x = (event.x - event.xStart) + objectM.markX

objectM.y = (event.y - event.yStart) + objectM.markY

elseif(event.phase == “ended” or event.phase == “cancelled”) then

display.getCurrentStage( ):setFocus(nil)

end

return true

end

Runtime:addEventListener(“touch”, touchM)

[/lua]

I now have the following which works:

[lua]

function touchM(self, event)

if(event.phase == “began”) then

display.getCurrentStage( ):setFocus(self)

self.markX = self.x

self.markY = self.y

elseif(event.phase == “moved”) then

self.x = (event.x - event.xStart) + self.markX

self.y = (event.y - event.yStart) + self.markY

elseif(event.phase == “ended” or event.phase == “cancelled”) then

display.getCurrentStage( ):setFocus(nil)

end

return true

end

objectM.touch = touchM

Runtime:addEventListener(“touch”, objectM)

[/lua]

#8

You were using a function listener, whereas I prefer table listeners wherever possible.  I’d walk through the various guides on the site and read up.  That’s how I learned of these features:

Sorry, but research, trial and error, and lots and lots of practice are the only way to really get this. 

#9

I’ve already read through the corona university pages. I looked through the lua syntax again and finally understand how that code works.