Video 13

Global and nested functions, as introduced in Functions, are actually special cases of closures. Closures take one of three forms:

• Global functions are closures that have a name and do not capture any values.
• Nested functions are closures that have a name and can capture values from their enclosing function.
• Closure expressions are unnamed closures written in a lightweight syntax that can capture values from their surrounding context.

Swift’s closure expressions have a clean, clear style, with optimizations that encourage brief, clutter-free syntax in common scenarios. These optimizations include:

• Inferring parameter and return value types from context
• Implicit returns from single-expression closures
• Shorthand argument names
• Trailing closure syntax

This video was developed by Skip Wilson. You can follow him on Twitter (@SkipAllMighty).

Generic code enables you to write flexible, reusable functions and types that can work with any type, subject to requirements that you define. You can write code that avoids duplication and expresses its intent in a clear, abstracted manner.

Generics are one of the most powerful features of Swift, and much of the Swift standard library is built with generic code. In fact, you’ve been using generics throughout this Language Guide, even if you didn’t realize it. For example, Swift’s Array and Dictionary types are both generic collections. You can create an array that holds Int values, or an array that holds String values, or indeed an array for any other type that can be created in Swift. Similarly, you can create a dictionary to store values of any specified type, and there are no limitations on what that type can be.

This video was developed by Skip Wilson. You can follow him on Twitter (@SkipAllMighty).

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol doesn’t actually provide an implementation for any of these requirements—it only describes what an implementation will look like. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol.

Protocols can require that conforming types have specific instance properties, instance methods, type methods, operators, and subscripts.

This video was developed by Skip Wilson. You can follow him on Twitter (@SkipAllMighty).

Classes, structures, and enumerations can define subscripts, which are shortcuts for accessing the member elements of a collection, list, or sequence. You use subscripts to set and retrieve values by index without needing separate methods for setting and retrieval. For example, you access elements in an Array instance as someArray[index] and elements in a Dictionary instance as someDictionary[key].

You can define multiple subscripts for a single type, and the appropriate subscript overload to use is selected based on the type of index value you pass to the subscript. Subscripts are not limited to a single dimension, and you can define subscripts with multiple input parameters to suit your custom type’s needs.

This video was developed by Skip Wilson. You can follow him on Twitter (@SkipAllMighty).