Detection FOR the entered fields on a scene

Games
#1

Hi, Solarians
I have a square which being placed inside a circle field produce a notification after touch ended phase.

Look at this:

local circle = display.newCircle(150,150,40)
    circle.alpha = 0.5

local square = display.newRect(50,50,20,20)

local function moveSquare( event )

    	local t = event.target
    	local phase = event.phase
    	
    	if "began" == phase then               
    	
    	local parent = t.parent
    	parent:insert( t )
    	display.getCurrentStage():setFocus( t )
    	t.isFocus = true
    	
             	elseif t.isFocus then
    			
    			if "moved" == phase then
    			
    			t.x = event.x
    			t.y = event.y
    				
    			elseif "ended" == phase then
    			
    			local bounds = circle.contentBounds
    	
    			if square.x > bounds.xMin and
    			square.x < bounds.xMax and
    			square.y > bounds.yMin and
    			square.y < bounds.yMax then
    			
    			print("Inside circle!")

    			end
    	  end
      end
  return true
end

square:addEventListener ( "touch", moveSquare )

Ok, it works perfectly.
But, I need a circle to detect that an object has just entered the circle’s area.
Help me, please, to make inverse for the code.

Which event listener and event.name would you recommend in this case, because a static circle is not performed by touch or tap… only a square, only hardcore…

One more question is: how to detect some different objects, I mean a situation like a circle “detects” if a red square entered the circle’s area or a green triangle, etc.

As for me, it is better to describe one Class Prototype in code for every of 64 square of a chessboard as “a detector” than make every chess piece like a source of situational messages:

chessPiece:touch ( event )
( event.phase == “ended” )
…
print ( “Now I am at E4!” )
…
print ( "Now I am at G7! ")

etc.

Could it be resolved by any “object ID”?

Thanks for any ideas, sorry for newbee kuestions))