虚拟摇杆

教程一：
http://blog.flashgen.com/gaming/general-gaming-articles/virtual-controllers-for-touch-based-devices-pt-1/
http://blog.flashgen.com/gaming/general-gaming-articles/virtual-controllers-for-touch-based-devices-pt-2/?

教程二：
http://active.tutsplus.com/tutorials/mobile/creating-a-virtual-joystick-for-touch-devices/