SwiftCallKit

SwiftCallKit is a production-grade iOS reference project showcasing a state-driven CallKit architecture built with SwiftUI, Combine, and Coordinator-based MVVM, and developed using Test-Driven Development.

The project focuses on explicit state management, side-effect isolation, and testability, reflecting real-world iOS platform engineering practices rather than feature-driven implementation.

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Goals

• Demonstrate senior-level iOS architecture suitable for large, long-lived codebases
• Showcase correct isolation of Apple system frameworks (CallKit)
• Integrate a WebRTC media layer using LiveKit, isolated behind protocol-based abstractions
• Provide a clean MVVM + Coordinator reference implementation
• Enable unit testing of business logic and navigation flows
• Reflect real-world engineering practices used in production fintech apps

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Architecture Overview

The application follows a strict layered architecture:

Architecture Diagram

graph TD
    UI --> ViewModels
    ViewModels --> Domain
    Domain --> Coordinators
    Coordinators --> Services
    Services --> System["System (Apple Framework & SDK Wrappers)"]
    System --> Infrastructure

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Call Lifecycle & Coordinator Flow

sequenceDiagram
    participant User
    participant View
    participant ViewModel
    participant StateMachine
    participant Coordinator
    participant CallKit
    participant Media

    User->>View: Tap Call
    View->>ViewModel: startCall()
    ViewModel->>StateMachine: send(.calling)
    StateMachine->>Coordinator: stateDidChange(.calling)
    Coordinator->>CallKit: startOutgoingCall()

    StateMachine->>Coordinator: stateDidChange(.connecting)
    Coordinator->>Media: prepareMedia(token)

    StateMachine->>Coordinator: stateDidChange(.active)
    Coordinator->>Media: startMedia()

    CallKit-->>Coordinator: systemEndCall
    Coordinator->>StateMachine: send(.ended)
    StateMachine->>Coordinator: stateDidChange(.ended)
    Coordinator->>Media: stopMedia()
    Coordinator->>CallKit: endCall()

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Core Principles

• Views render UI only
• ViewModels expose user intent and observe domain state
• Domain owns business rules and the call state machine
• Coordinators react to state changes and orchestrate side effects
• Services provide protocol-based capabilities (media, tokens, system access)
• System wraps Apple frameworks and third-party SDKs (CallKit, AVAudioSession, LiveKit)
• Infrastructure constructs and wires dependencies

This separation ensures that no UI code depends directly on system frameworks, all side effects are centralized, and all business logic remains testable and deterministic.

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Testing Strategy

This project is developed using TDD.

• Domain is tested in isolation (pure state machine tests)
• Coordinators are unit tested by asserting side effects
• System frameworks (CallKit, LiveKit) are fully mocked
• Async behavior is tested deterministically using structured concurrency

No Apple system APIs are exercised in tests.