Code assistance

Getting Started Newbie Questions
#1 
module(...,package.seeall) force\_multiplier=4 state={} ready=false pop={} thresholds={} function newBullet() local sceneGroup=display.newGroup() local bullet=display.newImage(sceneGroup,"bulleth.png") bullet.x=\_W-240 bullet.y=\_H-50 bullet.xScale=.09 bullet.yScale=.09 bullet.type="bullet" physics.addBody(bullet,"kinematic",{density=0.2, friction=.2, bounce=.5, radius=20}) bullet.linearDampling=.3 bullet.angularDampling=.8 bullet.isBullet=true --forces continous collison detection bullet.isSensor=true function bullet:touch(event) if (ready) then if(event.phase=="began") then display.getCurrentStage():setFocus( event.target ) self.isFocus=true self.bodyType="kinematic" --Stop current motion if any self:setLinearVelocity(0,0) self.angularVelocity=0 myLine=nil elseif(self.isFocus) then if(event.phase=="moved") then if (myLine) then myLine.parent:remove(myLine) --erase previous line, if any end --Check this myLine=display.newLine(sceneGroup,self.x,self.y, event.x, event.y) myLine:setStrokeColor(255,255,255,50) myLine.alpha=.3 myLine.strokeWidth=8 elseif(event.phase=="ended" or event.phase=="cancelled") then display.getCurrentStage():setFocus(nil) self.isFocus=false --audio.play(shot) if(myLine) then myLine.parent:remove(myLine) end --Launch bullet self.bodyType="dynamic" self:applyForce((self.x-event.x)\*force\_multiplier,(self.y-event.y)\*force\_multiplier, self.x, self.y) self.timer=timer.performWithDelay(1000,function(event) state:dispatchEvent({name="change",state="fire"}) if(event.count==1) then timer.cancel(self.timer) self.timer=nil end end, 1) end end end end function bullet:collision( event ) if(event.phase=="began") then if(event.other.type=="balloon") then local s=event.other.points local m=event.other.multiplier state:dispatchEvent({name="change",state="score", points=s, multiplier=m}) elseif(event.other.type=="wall") then end elseif(event.phase=="ended") then if(event.other.type=="balloon") then audio.play(pop1) event.other.pop() elseif(event.other.type=="wall") then self:removeSelf() self=nil end end end bullet:addEventListener("collision", bullet) bullet:addEventListener("touch", bullet) return bullet end

Error i get:

bullet.lua:52: attempt to index field ‘parent’ (a nil value)

In my game i drag the bullet back and a line appears behind the bullet to show how much power is going into the shot (longer the line higher the power) and the trajectory of the bullet. You can think of the way the line forms behind a bird in Angry Birds. Sometimes it works perfectly but if i pull back sometimes very quickly or just normally, in no hurry, i get that error. I want to make sure that error never comes so what do i do?

The code the error applies to is where it says myLine=nil and from then on, the variable that makes up the line is myLine.

#2

Try https://forums.coronalabs.com/topic/37641-attempt-to-index-field-parent-a-nil-value it may help.

#3

first, as @ldurniat points out, display.remove() is “safer” than group.remove() or object.removeSelf()

but source of trouble appears to be:  your “ended” phase removes the line, but doesn’t nil the value.  once removed, your line no longer has a parent.  so next time through, your test in the “moved” phase is just “if not nil” (which will be true), so it attempts to access its parent (which is no longer assigned, so nil) to remove it, thus the error.  setting “myLine=nil” after every place where it has been removed should clear that up.  (related: don’t assume that you cannot get a “moved” event after an “ended” event)

(aside: each bullet also seems to be creating its own local “sceneGroup” instance that never get destroyed)

#4

Try https://forums.coronalabs.com/topic/37641-attempt-to-index-field-parent-a-nil-value it may help.

#5

first, as @ldurniat points out, display.remove() is “safer” than group.remove() or object.removeSelf()

but source of trouble appears to be:  your “ended” phase removes the line, but doesn’t nil the value.  once removed, your line no longer has a parent.  so next time through, your test in the “moved” phase is just “if not nil” (which will be true), so it attempts to access its parent (which is no longer assigned, so nil) to remove it, thus the error.  setting “myLine=nil” after every place where it has been removed should clear that up.  (related: don’t assume that you cannot get a “moved” event after an “ended” event)

(aside: each bullet also seems to be creating its own local “sceneGroup” instance that never get destroyed)