A Demo for Shimmer Effect with two methods
Use Blend Mode + CAGradientLayer to Get Shimmer Effect
if you want to know more about the blend mode, you can get the details in apple’s document here, remember to click every entry to get the explain, you will find some thing fun！
Use MaskLayer + CAGradientLayer to Get Shimmer Effect
if you want to know more about the mask layer, you can read this chapter for free, and you will get peace with mask skill in iOS!