Runtime error

Games
#1

Hi, I downloaded Christian Peeters Space Shooter game.

http://www.christianpeeters.com/complete-app-tutorial/create-a-simple-space-shooter-with-corona-sdk/

Runtime error.PNG
Runtime error.PNG1403×867 222 KB

It started out okay then the error message started appearing, but not at the same intervals sometimes I can get up to 12 hits sometimes less.

Did you want me to enter the code?

Thanks

#2

Guessing from the message, it looks like part of the code is trying to operate/modify on an object that is no longer valid.

If an object can be deleted, and if you have a timer or a listener that modifies the object, you need to add code to either:

  • cancel the timer and or listener BEFORE deleting the object.
  • better/safer - Check the object is valid before you try to modify it.

A basic ‘is valid checker’ looks like this:

local function isValid ( obj )
	return ( obj and obj.removeSelf and type(obj.removeSelf) == "function" )
end

You can use it like this:

local function rotationAmmo()
   if( isValid(ammo) ) then
       ammo.rotation = ammo.rotation + 45
   end
end
#3

Hi, Roaminggamer

Can you please explain this isValid function?
I don’t understand why is obj.removeSelf returned from the check?

Thanks.

#4

You’ve read that wrong. It is returning the result of a conditional check, it isn’t returning the elements in the condition. e.g. It is not returning obj.removeSelf, instead it is checking if obj.removeSelf exists.

Let me break it down
return ( obj and obj.removeSelf and type(obj.removeSelf) == "function" )
left to right.

  • return false if obj is nil
  • return false if obj.removeSelf (handle to function) does not exists (is nil)
  • return false if obj.removeSelf handle is present, but does not reference a function.

The obj is nil case should be pretty clear, but what about the other two?

When you delete a display object, the object handle obj is cleaned up. In that cleaning, all references to functions are removed. Thus, the second check above.

The last check is a bit over-the-top, but protects you from the weird case where obj exists and it has a field named removeSelf, but removeSelf is not a function. i.e. This is not a display object.

#5

Ok, get it now.Thank you.