Corona SDK object follows "swipe" direction? - lua

Question

I'm trying to do something very simple as I'm new to Corona SDK, but I'm having a lot of issues with it.
What I'm trying to accomplish is when a user "swipes", "flings", "flicks" a certain area of the screen (an image set to half the screen) a ball in the middle of the screen moves in that direction at a set speed. No physics or anything special. So basically, if you swipe the image at an angle, the ball will move to that angle and eventually off the screen unless you "swipe" the ball again in a different direction.
What i have so far is:
physics.addBody( ball, "dynamic" )
function flick(event)
if event.phase == "moved" then
ball:applyForce( (ball.x) * 0.5, (ball.y) * 0.5, ball.x, ball.y )
end
end
Currently when you move the ball it just shoots in one direction (the 0.5) how would i get the direction "flung"?
I'm new to corona and still trying to figure out the touch events. Still a little confusing to me.
Any help and advice is appreciated! Thanks everyone!

Answer 1

You need to track where the touch starts and ends: this is the flick. The difference between start and end along x and y will give direction of motion of ball. If ball speed depends on how fast user flicked, you need to track time too. Also, I'm not sure why you are making the force proportional to the object's position: this means that if the object is at 0,0 there is no force, that's probably not what you want. You would just want a force proportional to the flick vector. The amount of time would determine the time. It would look something like this:
local flickStartTime = 0
local function touch(event)
if event.phase == "began" then
flickStartTime = system.timer()
elseif event.phase == "ended" then
local flickDuration = system.timer() - flickStartTime
if flickDuration <= 0 then return end -- ignore flick
local speedX = (event.xStart - event.x)/flickDuration
local speedY = (event.yStart - event.y)/flickDuration
local coef = 0.1 -- adjust this via "trial and error" so moves at "reasonable" speed
-- only react to flick if it was large enough
if math.abs(event.xStart - event.x) > 10 or math.abs(event.yStart - event.y) > 10 then
ball:applyForce( coef * speedX, coef * speedY, ball.x, ball.y )
end
end
end
However you said you don't want any physics, yet you apply force, do you mean you just want to set the velocity? Then instead of applyForce you would call setLinearVelocity(speedX, speedY)
local speedX = (event.xStart - event.x)/flickDuration
local speedY = (event.yStart - event.y)/flickDuration
local coef = 0.1 -- adjust this via "trial and error" so moves at "reasonable" speed
-- only react to flick if it was large enough
if math.abs(event.xStart - event.x) > 10 or math.abs(event.yStart - event.y) > 10 then
ball:setLinearVelocity( coef * speedX, coef * speedY )
end

Answer 2

By the looks of it, flick is a touch listener. You must filter the event that is passed on, and determine which direction the user swiped. Read more about touch listeners here
What you are interested in, is the xStart and yStart properties, and measure how much the user moved, something like this:
local function flick(event)
if event.phase == "moved" then
local diffX = event.xStart - event.x
local diffY = event.yStart - event.y
if math.abs(diffX) > 10 or math.abs(diffY) > 10 then
ball:applyForce( (ball.x) * diffX, (ball.y) * diffY, ball.x, ball.y )
end
end
end