Question

We’re using objects that do not necessarily need to leverage the physics engine, but we still need to detect collision. Here’s a similar example to what we’re trying to build:

In solitaire, the cards are draggable objects. When they are released over top of another stack of cards (the collision), they will “stick” to that deck. The target hotspots (the card stacks) are not always known ahead of time – they are dynamic.

What would be the best approach to this problem in Corona SDK?

Answer 1

If you do not want to use physics engine, you could code up a touch event listener to check for overlapping cards. I assume no difference in a stack of cards or a single card.

local cards={} --a list of display objects
cards[1]=display.newRect(100,100,100,100)
cards[2]=display.newRect(100,210,100,100)
cards[3]=display.newRect(100,320,100,100)

local tolerance=20 --20px radius
local cardAtPointer=0  --the index of the card stuck to user hand

local function onOverlap(self,event)
    if event.phase == "began" then
        cardAtPointer=self.index    --hold one card only    
    elseif event.phase == "moved" then
        if cardAtPointer > 0 and self.index == cardAtPointer then
            self.x,self.y = event.x,event.y     --drag card around
        end
    elseif event.phase == "ended" or event.phase == "cancelled" then
        local s=self

        for i=1,#cards do
            local t=cards[i]
            if s.index ~= t.index then  --dont compare to self
                if math.pow(tolerance,2) >= math.pow(t.x-s.x,2)+math.pow(t.y-s.y,2) then
                    print(s.index.." overlap with "..t.index)
                    break   --compare only 2 overlapping cards, not 3,4,5...
                end
            end
        end
        cardAtPointer=0     --not holding any cards
    end
end

for i=1,#cards do 
    cards[i].index=i
    cards[i].touch=onOverlap
    cards[i]:addEventListener("touch",cards[i])
end