将图像旋转180度

I have one image like drop down. Initially it will look like drop down image So when user press drop down list some option will show. So what I need is, when drop-down is true.I means when user press drop down image and when the list of option are showing down I need to show the drop down image to 180 degree.Same like when drop down is false I need to show the image as normal position.I have one image like drop down. Initially it w