Region tracker

local touches = {}

local function monitor(event)
    if event.phase == 'began' or event.phase == 'moved' then
        touches[event.id] = event.target
    else
        touches[event.id] = nil
    end
end

function touchesInRegion(r)
    local n = 0
    for touch, target in pairs(touches) do
        if target == r then
            n = n + 1
        end
    end
    return n
end

function trackRegion(r)
    r:addEventListener('touch', monitor)
end

function ignoreRegion(r)
    r:removeEventListener('touch', monitor)
end