Extension

• Extensions add new functionality to an existing class, structure, enumeration, or protocol type.
• You can extend types for which you do not have access to the original source code
• Extensions are similar to categories in Objective-C. (Unlike Objective-C categories, Swift extensions do not have names.)

Extensions in Swift can:

• Add computed instance properties and computed type properties
• Can not add instance properties
• Define instance methods and type methods
• Provide new initializers
• Define subscripts
• Make an existing type conform to a protocol