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Swift Package Manager - Modular Programming

Let me frame this article with a couple of not-so-bold predictions for software development. First, agentic coding will be responsible for the majority of code written. This is already achievable today with an optimised development harness and spec-driven development.

As a consequence, I expect feature development to accelerate dramatically resulting in codebases that continue to grow in both size and complexity. This is a particular concern for enterprise applications, which are already operating at significant scale.

To prepare for this new era of code growth, architecture must prioritise modularity, isolation, and clear dependency boundaries.

This iOS architecture demonstrates a scalable approach that uses Swift Package Manager (SPM) as the foundation for modular application design.

Core Concept

At the heart of this architecture is a simple idea:

The Xcode project should be almost empty — everything lives in Swift Packages.

Key principles:

Project Structure

App.swift only contains enough code to hand off the the first feature package which handles the entry/launch functionality before routing to the next package

Xcode Project
├── App/
│   └── App.swift
├── Feature Packages/ (local SPM)
│   └── ...
├── Shared Packages/ (local or remote SPM)
│   └── ...
├── Package Dependencies/ (3rd party SPM)
│   └── ...
├── AGENTS.md
└── Harness/

Feature Package Structure

Each feature is fully self-contained:

Package/
├── Package.swift
├── Sources
│   ├── Concurrent/
│   │   ├── Models/ (Sendable)
│   │   └── Service.swift (actor)
│   │
│   └── Main/ (UI layer)
│       ├── Views/
│       ├── PackageCoordinator.swift
│       └── PackageRoute.swift
│
└── Tests/

Harness

A harness is essential to accurate agentic contributions. AGENTS.md and the Harness/ directory work together to help agents accurately and efficiently contribute features. As your project grows the harness must be maintained and optimised to remain effective. Read more here https://openai.com/index/harness-engineering/

Concurrency Model

Swift Structured Concurrency can be difficult to adopt correctly, but when used well it provides significant benefits in predictability, stability and UI responsiveness. Each feature package is split into two logical layers:

This provides a clear separation between:

Example responsibilities:

Benefits

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