function init(self)
    msg.post(".", "acquire_input_focus")
    
    -- list of boxes to drag
    self.boxes = {
        gui.get_node("box1"),
        gui.get_node("box2"),
        gui.get_node("box3"),
    }

    -- variable where the currently dragged box is stored
    self.dragged_box = nil
end

function on_input(self, action_id, action)
    if action_id == hash("touch") then
        -- update the position of the currently dragged box
        if self.dragged_box then
            local mouse_position = vmath.vector3(action.x, action.y, 0)
            gui.set_position(self.dragged_box, mouse_position)
        end

        -- check if the mouse button was pressed
        if action.pressed then
            -- iterate the list of boxes and check if the mouse was
            -- clicked on a box
            for i=1,#self.boxes do
                local box = self.boxes[i]
                -- this will return true if the x and y is within the
                -- bounds of the box
                if gui.pick_node(box, action.x, action.y) then
                    -- keep track of the box as being dragged
                    self.dragged_box = box
                    break
                end
            end
        -- check if the mouse button was released
        -- clear the variable which keeps track of which box is dragged
        elseif action.released then
            self.dragged_box = nil
        end
    end
end