如何检测设备何时从纵向模式切换到横向模式？

I have an app which shows fullscreen bitmaps in an activity. In order to provide fast loading time, I load them in the memory. But when the screen changes orientation, I would like to clear the cache in order to fill it again with bitmaps that fit inside the new dimensions. The only problem is that in order to do this, I need to detect when an orientation change occurs. Do anyone know how to detect this?I have an app which shows fullscreen bitmaps in