How To Set Position as a Variable (Corona SDK)

Question

So I am making a game like pong in Corona and I am stuck at how to get the paddles to trace the ball. I have defined the ball's x coordinate as ball.x = ballx and ballx = 160 but when I use

local myListener = function( event )
   upperpad.x = ballx
   lowerpad.x = ballx
end

Runtime:addEventListener( "enterFrame", myListener )

to keep the paddles x value updated to the ball's x value, it stays constant. How can I express ballx as a changing value and get the paddles to mirror the ball's x value?

Answer 1

The ballx value stays constant because you're probably not updating it, and changing directly ball.x instead. Indeed, this is the way to do it: you should use ball.x as this is is the real value that's being changed, and not ballx.

Yet, if you still want to use a reference value to ball.x, you can set ballx = ball.x and then update the ballx value directly.

When you're done updating the ballx value, you can update the ball.x value as well, and the paddles just like you did.

local updateBall = function()

    ballx = ballx + 20 --update the ballx value

end

Runtime:addEventListener("enterFrame", updateBall)

local myListener = function(event)

   ball.x = ballx --then update the ball itself with the ballx value

   upperpad.x = ballx --and make the paddles follow
   lowerpad.x = ballx

end

Runtime:addEventListener("enterFrame", myListener)

EDIT: If you want to manually set the paddles' position depending on the ball's (which is handled by the physics engine), simply make them follow the original ball.x value. You don't need ballx.

local movePaddles = function(event)

   upperpad.x = ball.x
   lowerpad.x = ball.x

end

Runtime:addEventListener("enterFrame", movePaddles)