Developing a browser app that uses an alternative browser engine

At a glance

Project structure

Source bundle/
└── BrowserExample/
    ├── BrowserApp/
    │   ├── AlertManager.swift
    │   ├── BrowserApp.swift
    │   ├── BrowserPageViewModel.swift
    │   ├── TabViewModel.swift
    │   └── BrowserPage.swift
    ├── NetworkingExtension/
    │   └── NetworkingExtension.swift
    ├── RenderingExtension/
    │   └── RenderingExtension.swift
    ├── WebContentExtension/
    │   └── WebContentExtension.swift
    └── CustomBrowserEngine/
        ├── UIProcess/
        │   ├── WebView.swift
        │   └── WebContentView.swift
        ├── XPC/
        │   └── XPCCodable.swift
        └── NetworkProcess/
            └── NetworkSession.swift

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: C/Objective-C header, Objective-C, Swift.
• The verified tree contains 14 project/configuration file(s) and 62 source declaration(s).

Overall architecture

flowchart LR
    N1["BrowserApp"]
    N2["ActivityViewController"]
    N3["BrowserPageViewModel"]
    N4["ServiceProvider"]
    N5["BrowserEngineKit APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4
    N4 --> N5

Reference code

BrowserExample/BrowserApp/BrowserApp.swift:11 — architecture anchor

@main
struct BrowserApp: App {

  @State var browserPageViewModel = BrowserPageViewModel()

  @State var alertManager = AlertManager()

  var body: some Scene {
    WindowGroup {
      NavigationStack {
        BrowserPage(model: browserPageViewModel)
          .navigationTitle("Browser Example")
          .navigationBarTitleDisplayMode(.inline)
      }
      .presentingAlerts(from: alertManager)
      .environmentObject(alertManager)
      .onOpenURL { open($0) }
    }
  }

  private func open(_ url: URL) {
    Task {
      do {
        try await browserPageViewModel.createNewTab(destination: .url(url), activate: true)
      } catch let error {
        await alertManager.present(error: error, title: "Failed to open url")
      }
    }
  }
}

Interpretation

The arrows summarize the source-visible entry, role-named types or folders, and framework direction; when nodes come from structural folders, the sequence is a high-level interpretation rather than proof that every adjacent node calls the next. Ownership is claimed only where the next section cites a stored property or assignment. The diagram is intentionally limited to the dominant path into BrowserEngineKit.

Ownership and state

classDiagram
    Alert *-- UUID : id
    Alert *-- String : title
    Alert *-- String : message
    Alert *-- Array : buttons

Ownership evidence

BrowserExample/BrowserApp/AlertManager.swift:13 — stored dependency or nearest verified ownership anchor

public struct Alert: Identifiable {
    // ...
  public var id = UUID()
    // ...
}

Composition arrows indicate a source-visible construction expression or locally owned value state; aggregation means the owner stores or receives a dependency without proving exclusive lifetime ownership.

Concurrency, scheduling, and thread safety

Evidence limit: actor isolation, async/await, or Task creation does not by itself prove background-thread execution; Sendable conformance alone does not prove thread-safe mutation.

@MainActor/MainActor.run, DispatchQueue.main, and RunLoop.main are reported as distinct isolation, queue, and event-loop mechanisms. A plain Task is kept separate from Task.detached; neither is labeled as a background thread.

Reference code

BrowserExample/BrowserApp/AlertManager.swift:88 — representative execution boundary

@MainActor
public class AlertManager: ObservableObject {
    // ...
  @Published public var currentAlert: Alert? = nil
    // ...
}

State propagation, frameworks, and dependencies

Evidence limit: an import proves a source-level compilation dependency at the cited line; it does not prove runtime use, architectural adoption, or whether a Swift package is a direct application dependency.

receive(on:) describes downstream delivery scheduling, while subscribe(on:) describes upstream subscription/request/cancel scheduling. An import Combine alone establishes neither behavior nor a Store, reducer, Redux, or other application architecture.

Class and protocol design

BrowserExample/CustomBrowserEngine/UIProcess/WebView.swift:23 — representative type boundary

public protocol WebViewUIDelegate: AnyObject {
  func webViewDidStartLoading(_ webView: WebView)
  func webViewDidStopLoading(_ webView: WebView)
}

The source explicitly defines local protocol relationships: BrowserExtensionTask → XPCCodable, WebContentExtensionTask → XPCCodable, WebContentExtensionBootstrapCommand → XPCCodable, HostingHandleMessage → XPCCodable, IOSurfaceMessage → XPCCodable, NetworkExtensionTask → XPCCodable.

Access control

Reference code

BrowserExample/BrowserApp/ActivityViewButton.swift:12 — representative boundary

private let log = Logger(subsystem: Constants.logSubsystem, category: String(describing: ActivityViewController.self))

Swift declarations without a modifier are internal; explicit private, fileprivate, private(set), public, or open entries above are interpreted by language semantics. Objective-C/C samples instead rely on header and implementation boundaries, which are not equivalent to Swift lexical privacy.

Logic ownership and placement

Design patterns

Main application flow

sequenceDiagram
    participant BrowserProcessPool
    participant renderingProxy
    participant networkProxy
    participant contentProxy
    BrowserProcessPool->>BrowserProcessPool: getOrLaunchContentProcess()
    BrowserProcessPool->>BrowserProcessPool: getOrLaunchRenderingProcess()
    BrowserProcessPool->>renderingProxy: getEndpoint()
    BrowserProcessPool->>BrowserProcessPool: getOrLaunchNetworkProcess()
    BrowserProcessPool->>networkProxy: getEndpoint()
    BrowserProcessPool->>contentProxy: bootstrap()

Reference code

BrowserExample/CustomBrowserEngine/UIProcess/BrowserProcessPool.swift:60 — launchProcesses()

  public func launchProcesses(id: PageID) async throws -> WebContentExtensionProxy {

    // 1. Launch a new web content process instance.
    let contentProcess = try await getOrLaunchContentProcess(pageID: id)
    let contentConnection = try contentProcess.makeLibXPCConnection()
    let contentProxy = WebContentExtensionProxy(connection: contentConnection)
    try contentProxy.applyRestrictedSandbox(version: lockdownVersion)

    // 2. Get the shared rendering process.
    let renderingProcess = try await getOrLaunchRenderingProcess()
    let renderingConnection = try renderingProcess.makeLibXPCConnection()
    let renderingProxy = RenderingExtensionProxy(connection: renderingConnection)
    let renderingEndpoint = try await renderingProxy.getEndpoint()
    try renderingProxy.applyRestrictedSandbox(version: lockdownVersion)

    // 3. Get the shared networking process.
    let networkProcess = try await getOrLaunchNetworkProcess()
    let networkConnection = try networkProcess.makeLibXPCConnection()
    let networkProxy = NetworkingExtensionProxy(connection: networkConnection)
    let networkEndpoint = try await networkProxy.getEndpoint()
    try networkProxy.applyRestrictedSandbox(version: lockdownVersion)

    // 4. Perform the bootstrap process.
    try await contentProxy.bootstrap(renderingExtension: renderingEndpoint, networkExtension: networkEndpoint)

    webContentProcesses[id] = contentProcess
    return contentProxy
  }

Naming conventions

• Types: App: BrowserApp; Command: WebContentExtensionBootstrapCommand; Controller: ActivityViewController; Delegate: WebViewUIDelegate; Manager: AlertManager; Provider: ServiceProvider; Session: NetworkSession; View: TabContentView, WebContentView, WebView.
• Protocols: WebViewUIDelegate, ServiceProvider, WebViewNavigationNelegate, XPCCodable, XPCEncodable, XPCDecodable.
• Methods: present, dismissCurrentAlert, body, makeActions, makeMessage, presentingAlerts, open, handle.
• Files: BrowserExample/BrowserApp/AlertManager.swift, BrowserExample/BrowserApp/BrowserApp.swift, BrowserExample/BrowserApp/BrowserPageViewModel.swift, BrowserExample/BrowserApp/TabViewModel.swift, BrowserExample/CustomBrowserEngine/UIProcess/WebView.swift, BrowserExample/BrowserApp/BrowserPage.swift.

Architecture takeaways

• BrowserApp is the main source-visible entry or composition anchor for this sample.
• Framework work reaches XPC, BrowserEngineKit, CustomBrowserEngine, SwiftUI through a deliberately small high-level chain; the detailed API graph remains inside the cited implementation files.
• Stored-property evidence identifies lifecycle collaboration; it does not by itself prove exclusive object ownership.
• Access-control conclusions separate verified language visibility from the likely design rationale.
• Local protocol relationships provide an explicit substitution boundary.

Source map

• Official Apple documentation