Writing Documentation for a Swift Project: From A to Z

Introduction: Why Documentation is Important for Swift Projects

High-quality documentation is not just a nice addition to your Swift project, but a critically important element of professional development. Whether you are creating a library for CocoaPods, writing an app for iOS or macOS, documentation helps:

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• Accelerate onboarding of new developers to the team;
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• Reduce the number of bugs through a clear understanding of the API;
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• Increase code reusability;
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• Create a professional image of your product.
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In this article, we will cover the full cycle of writing documentation for a Swift project: from basic comments to advanced documentation generation using DocC.

1. Basics of Documenting Code in Swift

1.1 Comments in Swift: Three Levels

Swift supports three types of comments used for documentation:

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• Single-line (//) — for quick notes;
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• Multi-line (/* ... */) — for blocks of text;
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• Documentation (/// or /** ... */) — for generating documentation.
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For writing documentation, use exclusively the third type. Here is an example:

/// Calculates the area of a circle given a radius.\n///\n/// - Parameter radius: The radius of the circle (must be greater than 0).\n/// - Returns: The area of the circle, accurate to two decimal places.\n/// - Throws: `InvalidRadiusError` if radius <= 0.\nfunc calculateCircleArea(radius: Double) throws -> Double {\n    guard radius > 0 else {\n        throw InvalidRadiusError.negativeOrZero\n    }\n    return Double.pi * radius * radius\n}

1.2 Basic Documentation Tags

Swift supports standard Markdown tags within documentation comments:

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• - Parameter [name]: — description of a parameter;
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• - Returns: — description of the return value;
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• - Throws: — description of errors;
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• - Note: — additional note;
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• - Warning: — warning.
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Example using Markdown for formatting:

/// Creates a personalized greeting.\n///\n/// Use this method to generate welcome messages.\n/// \n/// **Usage example:**\n/// ```swift\n/// let greeting = makeGreeting(name: "Anna")\n/// print(greeting) // "Hello, Anna!"\n/// ```\n///\n/// - Parameter name: The user's name.\n/// - Returns: A greeting string.\nfunc makeGreeting(name: String) -> String {\n    return "Hello, \\(name)!"\n}

2. Documenting Classes, Structures, and Protocols

2.1 Documenting Types

Every public type in your project should have a description. This is especially important for libraries and frameworks:

/// Represents a user in the system.\n///\n/// The class contains basic user information and methods\n/// for authentication. Supports the `Codable` and `Equatable` protocols.\nclass User: Codable, Equatable {\n    /// The unique identifier of the user.\n    let id: UUID\n    \n    /// The user's name (from 2 to 50 characters).\n    let name: String\n    \n    /// The user's email.\n    let email: String\n    \n    /// The user's age (if provided).\n    var age: Int?\n}

2.2 Documenting Protocols

Protocols require special attention, as they define a contract for all implementations:

/// Protocol for objects that can be serialized to JSON.\n///\n/// Implement this protocol to add support for\n/// JSON serialization for your types.\nprotocol JSONSerializable {\n    /// Converts the object to a dictionary for JSON.\n    ///\n