I want to use animation to change UIView's color to make it fade, however, somehow i can't access and set opacity, but i can set alpha. I wonder if alpha and opacity are the same? Search on the web and I didnt find a good answer. Thanks a lot for your help!
Actually "opacity" means "value of alpha-channel" of your UIView. When a view is fully opaque this means its alpha = 1, when a view is fully transparent (non-opaque) its alpha = 0.
As about properties of CALayer and UIView in Cocoa, yes, they provide the same functionality.
Just try by yourself!
CALayer has opacity from 0 to 1, while UIView has alpha from 0 to 1. They have the same visual effect.
Alpha affects everything drawn on the view.The background color's alpha affects the background color's transparency and anything else drawn on the view.i.e it brings about a frosty effect to whole view. Opacity means don't draw anything underneath, even if you are transparent, it just effects the current view.
来源：https://stackoverflow.com/questions/15381436/is-the-opacity-and-alpha-the-same-thing-for-uiview