Box2d DstanceJoint

require "box2d"
 
local physicsScale = 30
b2.setScale(physicsScale)
 
local world = b2.World.new(0, 10, true)
 
-- for creating objects using shape
-- as example - bounding walls
function wall(x, y, width, height, name)
	local wall = Shape.new()
	--define wall shape
	wall:beginPath()
	--we make use (0;0) as center of shape,
	--thus we have half of width and half of height in each direction
	wall:moveTo(-width/2,-height/2)
	wall:lineTo(width/2, -height/2)
	wall:lineTo(width/2, height/2)
	wall:lineTo(-width/2, height/2)
	wall:closePath()
	wall:endPath()
	wall:setPosition(x,y)
 
	--create box2d physical object
	local body = world:createBody{type = b2.STATIC_BODY}
	body:setPosition(wall:getX(), wall:getY())
	body.name= name
	--body:setAngle(wall:getRotation() * math.pi/180)
	local poly = b2.PolygonShape.new()
	poly:setAsBox(wall:getWidth()/2, wall:getHeight()/2)
	local fixture = body:createFixture{shape = poly, density = 1.0, 
	friction = 0.1, restitution = 0.8}
	wall.body = body
	wall.body.type = "wall"
 
	--add to scene
	stage:addChild(wall)
 
	--return created object
	return wall
end
 
local screenH = application:getContentHeight()
local screenW= application:getContentWidth()
 
--create bounding walls outside the scene
wall(-dx,screenH/2,10,screenH) --left
wall(screenW/2,-dy,screenW+2*dx,10) --top
wall(screenW+dx,screenH/2,10,screenH)--right
wall(screenW/2,screenH,screenW+2*dx,10) --bottom
 
--create empty box2d body for joint
--since mouse cursor is not a body
--we need dummy body to create joint
local ground = world:createBody({})
 
--joint with dummy body
local mouseJoint = nil
local distanceJoint = nil
	-- create a mouse joint on mouse down
function onMouseDown(event)
	if ball:hitTestPoint(event.x, event.y) then
			--ball.body:setPosition(event.x, event.y)
		local jointDef = b2.createMouseJointDef(ground, ball.body, event.x, event.y, 100000)
		mouseJoint = world:createJoint(jointDef)	
	end
end
 
	-- update the target of mouse joint on mouse move
function onMouseMove(event)
	if mouseJoint ~= nil then
		mouseJoint:setTarget(event.x, event.y)
	end
end
 
	-- destroy the mouse joint on mouse up
function onMouseUp(event)
	if mouseJoint ~= nil then
		world:destroyJoint(mouseJoint)
		mouseJoint = nil
	end
end
-- for creating objects using image
-- as example - ball
function createBall(x, y)
	--create ball bitmap object from ball graphic
	local ball = Bitmap.new(Texture.new("ball.png"))
	--reference center of the ball for positioning
	ball:setAnchorPoint(0.5,0.5)
 
	ball:setPosition(x,y)
 
	--get radius
	local radius = ball:getWidth()/2
 
	--create box2d physical object
	local body = world:createBody{type = b2.DYNAMIC_BODY}
	body.name ='ball'
	print(ball:getX()..'    '.. ball:getY())
	body:setPosition(ball:getX(), ball:getY())
	body:setAngle(ball:getRotation() * math.pi/180)
	local circle = b2.CircleShape.new(0, 0, radius)
	local fixture = body:createFixture{shape = circle, density = 1.0, 
	friction = 0.1, restitution = 0.8}
	ball.body = body
	ball.body.type = "ball"
 
	--add to scene
	stage:addChild(ball)
 
	--return created object
	return ball
end
 
ball =createBall(application:getContentWidth()/2, 100)
ball1 =createBall(200, 200)
 
local dj = b2.createDistanceJointDef(ball.body, ball1.body, 0, 0, 0, 0, 100)
distanceJoint = world:createJoint(dj)
--distanceJoint.setDampingRatio(0) not work 
 
 
function onEnterFrame() 
	-- edit the step values if required. These are good defaults!
	world:step(1/60, 8, 3)
	--iterate through all child sprites
	for i = 1, stage:getNumChildren() do
		--get specific sprite
		local sprite = stage:getChildAt(i)
		-- check if sprite HAS a body (ie, physical object reference we added)
		if sprite.body then
			--update position to match box2d world object's position
			--get physical body reference
			local body = sprite.body
			--get body coordinates
			local bodyX, bodyY = body:getPosition()
			--apply coordinates to sprite
			sprite:setPosition(bodyX, bodyY)
			--apply rotation to sprite
			sprite:setRotation(body:getAngle() * 180 / math.pi)
		end
	end
end
 
local debugDraw = b2.DebugDraw.new()
world:setDebugDraw(debugDraw)
stage:addChild(debugDraw)
 
 
stage:addEventListener(Event.ENTER_FRAME, onEnterFrame)
-- register for mouse events
stage:addEventListener(Event.MOUSE_DOWN, onMouseDown)
stage:addEventListener(Event.MOUSE_MOVE, onMouseMove)
stage:addEventListener(Event.MOUSE_UP, onMouseUp)