Corona SDK: How to create untouchable area at very the top and bottom of the screen?

Question

1. If I touch the very top (say, y=5), an object should stop moving when it reaches y=50
2. If I touch the bottom (let's say y=300), an object should stop moving when it reaches y=250

Here is what i have done so far:

local function movePlayer(event)
    if(event.phase == "ended") then
        transition.to(player, {y=event.y, time=3000})
    end
 end

How do i do that?

Answer 1

this is easy to solve with simple comparison logic, add this function and use it with the arguments you like for (player x and y for example), to limit their values before you apply to the player.

This function accepts three arguments - number value, low limit and high limit. And it returns a number, which is guranteed to be within these boundaries.

this is the function

function math.clamp(value, low, high)  
    if low and value <= low then
        return low
    elseif high and value >= high then
        return high
    end
    return value
end 

usage example

local actualSpeed = math.clamp(calculatedSpeed, 0, 200)

local temperature = math.clamp(temperature, -270)

player.x = math.clamp(player.x, 0, map.width)

cannon.rotation = math.clamp(cannon.rotation, -90, 90)

local age = math.clamp(age, 1, 120)

-- Six has higher probability than any other number
local luckyDice = math.clamp(math.random(1, 10), nil, 6)  

source for further reading

EDIT :

local backGround = display.newRect(display.actualContentWidth/2,display.actualContentHeight/2,display.actualContentWidth,display.actualContentHeight)
backGround:setFillColor( 0.5, 0.5, 0.5, 1.0 )


local player = display.newCircle(100,100,50)

function math.clamp(value, low, high)  
    if low and value <= low then
        return low
    elseif high and value >= high then
        return high
    end
    return value
end 


function movePlayer(event)
    if(event.phase == "moved") then
        local xPosition = event.x
        local yPosition = event.y

        xPosition =  math.clamp(xPosition, 0, 150)
        yPosition =  math.clamp(yPosition, 0, 150)

        transition.to(player, {x=xPosition,y=yPosition,time = 30})

    end
 end


player.touch = movePlayer

player:addEventListener( "touch", movePlayer )
backGround:addEventListener( "touch", movePlayer )