Extensions and code organization | Swift

An extension adds functionality to an existing type. You can extend your own types, standard library types, and framework types.

Adding methods with extensions

struct Article {
    let title: String
    let body: String
}

extension Article {
    var estimatedReadingMinutes: Int {
        max(1, body.split(separator: " ").count / 200)
    }
}

This keeps derived behavior near the type without crowding the core stored properties.

Organizing protocol conformance

Extensions are often used to group protocol conformance:

struct Article {
    let id: String
    let title: String
}

extension Article: Identifiable {}
extension Article: Codable {}

This makes it easier to scan what the type supports.

Extending standard types

You can add helpers to standard types:

extension String {
    var trimmed: String {
        trimmingCharacters(in: .whitespacesAndNewlines)
    }
}

Use this carefully. Extensions on common types are visible everywhere in a module, so they should be broadly useful and unsurprising.

File organization

For small types, one file is enough. For larger features, group by feature instead of by technical layer:

Features/
  Articles/
    Article.swift
    ArticleListView.swift
    ArticleViewModel.swift

Feature-based organization keeps related model, UI, and logic close together.

What to carry forward

extensions add behavior to existing types
computed properties and helper methods fit well in extensions
protocol conformance can be grouped in extensions
global extensions should be broadly useful
organize app code around features when projects grow

Next, you will model operations that can fail with Swift errors.