• Remove internal access control level bacause it is default access control level in Swift 4

Default Access Levels
All entities in your code (with a few specific exceptions, as described later in this chapter) have a default access level of internal if you don’t specify an explicit access level yourself. As a result, in many cases you don’t need to specify an explicit access level in your code.

https://docs.swift.org/swift-book/LanguageGuide/AccessControl.html

• Refactor ConstraintAttributes using enum's Dot syntax

// as is
return ConstraintItem(target: self.target, attributes: ConstraintAttributes.centerX)
// to be
return ConstraintItem(target: self.target, attributes: .centerX)