Drag and drop

I am trying to learn by starting with a drag and drop game. I want to do this using classes. I have objects dragging using info from others in these forums but as you'll see I get errors - attempt to perform arithmetic on field 'x0' (a nil value) - but I am having enough success to be hopeful this can work.

Eventually I want to detect collisions between sprites and labels as per a matching game.

------------
-- main.lua
------------

-- Create multiple blocks.

scene = gideros.class(Sprite)

--on scene initialization
function scene:init()
--create world instance
self.actors = {}
local actors = {}
for i = 0, 5 do
-- The block image.
local blockSprite = Bitmap.new(Texture.new("_tanker.png"))

-- Create new block.
local block = Block.new(blockSprite)

-- Place blocks in the bottom screen side by side.
block:setPosition((i * 50) + 32, 240)
stage:addChild(block)
self.actors[block] = block
end

end
--Input (sprite2): another sprite to check for collision detection
--Return: return true if the rectangles overlap otherwise return false
function Sprite:collision(sprite2)

-- self bottom < other sprite top
if self:getY() + self:getHeight() < sprite2:getY() then
return false
end
-- self top > other sprite bottom
if self:getY() > sprite2:getY() + sprite2:getHeight() then
return false
end
-- self left > other sprite right
if self:getX() > sprite2:getX() + sprite2:getWidth() then
return false
end
-- self right < other sprite left
if self:getX() + self:getWidth() < sprite2:getX() then
return false
end

return true
end
--running the world
function scene:onEnterFrame()

for sprite in pairs(self.actors) do
--get specific sprite
--sprite:setPosition(body:getPosition())
--sprite:setRotation(body:getAngle() * 180 / math.pi)

--sprite:setPosition(body:getPosition())
--sprite:setRotation(body:getAngle() * 180 / math.pi)
end
end


--removing event on exiting scene
--just in case you're using SceneManager
function scene:onExitBegin()
self:removeEventListener(Event.ENTER_FRAME, self.onEnterFrame, self)
end

--add created scene to stage or sceneManager
local mainScene = scene.new()
stage:addChild(mainScene)

----------

Block = Core.class(Sprite)
local sprite



function Block:init(sprite)

self:addChild(sprite)


-- Register touch events.
sprite:addEventListener(Event.TOUCHES_BEGIN, self.onTouchesBegin, self)
sprite:addEventListener(Event.TOUCHES_MOVE, self.onTouchesMove, self)
sprite:addEventListener(Event.TOUCHES_END, self.onTouchesEnd, self)
sprite:addEventListener(Event.TOUCHES_CANCEL, self.onTouchesCancel, self)
local dragging=false
--sprit.
end

-- If block is on focus, stop propagation of touch events
function Block:onTouchesBegin(event)
if self.id == nil then
self.id = event.touch.id

print("BEGIN")
if self:hitTestPoint(event.touch.x, event.touch.y) then
dragging = true
-- Get start postion.
self.x0 = event.touch.x
self.y0 = event.touch.y
else
--self.x0 = 1
--self.y0 = 1
end
end
end

-- If block is on focus, stop propagation of touch events.
function Block:onTouchesMove(event)
if self.id == event.touch.id then


if self:hitTestPoint(event.touch.x, event.touch.y) then
if event.touch.x == nil then
event:stopPropagation()

end
print( self.x0)
local dx = event.touch.x - self.x0--get the diff of initial pos and current touch pos
print("MOVE2")
local dy = event.touch.y - self.y0
print("MOVE3")
self:setX(self:getX() + dx) -- add this diff to img's x and y pos
self:setY(self:getY() + dy)
print("MOVE4")

self.x0 = event.touch.x -- change the initial pos with new one
self.y0 = event.touch.y


event:stopPropagation()
end

end
end

-- If block is on focus, stop propagation of touch events.
function Block:onTouchesEnd(event)
if self.id - event.touch.id then

print("END")

self.id = nil
end
end

-- If touches are cancelled, reset the state of the button.
function Block:onTouchesCancel(event)
if self.focus then
self.focus = false;
self:updateVisualState(false)
event:stopPropagation()
end
end