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Track Velocity

Posted by Gilbert, Posted on August 4, 2010, Last updated October 7, 2010

This code sample creates a circle that bounces off the edges of the screen. You can touch the circle and throw it around. It tracks velocity. When the user touches the circle and starts moving it, the change in distance over time is captured and given to the circle as it's new speed when the user lets go.

Code

local screenW, screenH = display.stageWidth, display.stageHeight
local friction = 0.8
local gravity = .9
local speedX, speedY = 0, 0
local prevX, prevY = 0, 0
 
local ball = display.newCircle( 0, 0, 25)
ball:setFillColor(255, 255, 0)
ball.x = screenW*.5
ball.y = 20
 
function onMoveCircle(event) 
        speedY = speedY + gravity
 
        ball.x = ball.x + speedX
        ball.y = ball.y + speedY        
 
        if( ball.x >= screenW - ball.width*.5 ) then
                ball.x = screenW - ball.width*.5
                speedX = speedX*friction
                speedX = speedX*-1 --change direction     
        elseif( ball.x <= ball.width*.5) then
            ball.x = ball.width*.5
                speedX = speedX*friction
                speedX = speedX*-1 --change direction     
        end
        if( ball.y >= screenH - ball.height*.5 ) then
                ball.y = screenH - ball.height*.5 
                speedY = speedY*friction
                speedX = speedX*friction
                speedY = speedY*-1  --change direction  
        elseif( ball.y <= ball.height*.5 ) then
                ball.y = ball.height*.5
                speedY = speedY*friction
                speedY = speedY*-1 --change direction     
        end
end
 
-- A general function for dragging objects
local function startDrag( event )
        local t = event.target
 
        local phase = event.phase
        if "began" == phase then
                display.getCurrentStage():setFocus( t )
                t.isFocus = true
 
                -- Store initial position
                t.x0 = event.x - t.x
                t.y0 = event.y - t.y
                                
                -- Stop current motion, if any
                Runtime:removeEventListener("enterFrame", onMoveCircle)
                -- Start tracking velocity
                Runtime:addEventListener("enterFrame", trackVelocity)
 
        elseif t.isFocus then
                if "moved" == phase then
                        t.x = event.x - t.x0
                        t.y = event.y - t.y0
 
                elseif "ended" == phase or "cancelled" == phase then
                        display.getCurrentStage():setFocus( nil )
                        t.isFocus = false
                        
                        Runtime:removeEventListener("enterFrame", trackVelocity)
                        Runtime:addEventListener("enterFrame", onMoveCircle)
 
                end
        end
 
        -- Stop further propagation of touch event!
        return true
end
 
function trackVelocity()        
        speedX = ball.x - prevX
        speedY = ball.y - prevY
 
        prevX = ball.x
        prevY = ball.y
end                     
 
ball:addEventListener("touch", startDrag)
Runtime:addEventListener("enterFrame", onMoveCircle)