Bitrise is a cloud-hosted CI/CD as-a-service platform built for mobile application projects. It is a mature product, and is widely used for mobile engineering projects. It does have some small disadvantages over Circle CI and other build platforms (see below), but the built-in mobile features make it worth consideration.
Cross platform mobile development
SnapKit is an Auto Layout DSL (Domain Specific Language) for iOS and OSX. It allows the use of written code to lay out views in Swift, as opposed to Apple's GUI-based Interface Builder tool. In iOS development, views are your UI elements and constraints are the relationships between views.