在Corona中移除移动物体

Question

local heli = display.newCircle(x_pos, y_pos, radius)

local y_direction = 1
local y_speed = 5

local function animate(event)
    y_pos = y_pos + (y_direction * y_speed);
    if(y_pos > screen_bottom - radius) then
        y_direction = y_direction * -1;
        heli.fill = green_paint;
    end
    if(y_pos < screen_top + radius) then
        y_direction = y_direction * -1;
        heli.fill = red_paint;
    end
    heli:translate( x_pos - heli.x, y_pos - heli.y )
end

local function touchpop(self, event)
    if(event.phase == "began") then
        self:removeSelf( )
    end
    return true 
end

Runtime:addEventListener("enterFrame", animate);
heli.onTouch = touchpop
heli:addEventListener("touchpop", heli)

我试图在触摸时移除heli对象。但是当我触摸它时它并没有消失。如何确保被触摸的物体消失？