Shimmer

Shimmer is a lightweight Objective-C library by Facebook that adds an elegant, animated shimmering effect to views—used as a subtle loading indicator, such as in the Paper app. There's also an example project. In the example, you can swipe horizontally and vertically to try various shimmering parameters, or tap to start or stop shimmering. (To build the example locally, you'll need to open FBShimmering.xcworkpace rather than the .xcodeproj.) Shimmer uses the -[CALayer mask] property to enable shimmering, similar to what's described in John Harper's 2009 WWDC talk (unfortunately no longer online). Shimmer uses CoreAnimation's timing features to smoothly transition "on-beat" when starting and stopping the shimmer.