BrowserEngineKit

Framework for building web browsers with alternative (non-WebKit) rendering engines on iOS and iPadOS. Provides process isolation, XPC communication, capability management, and system integration for browser apps that implement their own HTML/CSS/JavaScript engine. Targets Swift 6.3 / iOS 26+.

BrowserEngineKit is a specialized framework. Alternative browser engines are currently supported for distribution in the EU. Japan requires Memory Integrity Enforcement (MIE) for alternative browser engine distribution. Development and testing can occur anywhere. The companion frameworks BrowserEngineCore (low-level primitives) and BrowserKit (eligibility checks, data transfer) support the overall browser engine workflow.

Contents

• Overview and Eligibility
• Entitlements
• Architecture
• Process Management
• Extension Types
• Capabilities
• Layer Hosting and View Coordination
• Text Interaction
• Sandbox and Security
• Downloads
• Common Mistakes
• Review Checklist
• References

Overview and Eligibility

Eligibility Checking

Use BEAvailability from the BrowserKit framework to check whether the device is eligible for alternative browser engines:

import BrowserKit

BEAvailability.isEligible(for: .webBrowser) { eligible, error in
    if eligible {
        // Device supports alternative browser engines
    } else {
        // Fall back or show explanation
    }
}

Eligibility depends on the device region and OS version. Do not hard-code region checks; rely on the system API.

Related Frameworks

Entitlements

Browser App (Host)

The host app requires two entitlements:

Both must be requested from Apple. The request process varies by region.

Extension Entitlements

Each extension target requires its type-specific entitlement set to true:

Optional Entitlements

Embedded Browser Engine (Non-Browser Apps)

Apps that are not browsers but embed an alternative engine for in-app browsing use different entitlements:

Embedded engines use arm64 only (not arm64e), cannot include browser extensions, and cannot use JIT compilation.

Japan-Specific Requirements

Browser apps distributed in Japan must enable hardware memory tagging via com.apple.security.hardened-process.checked-allocations. Apple strongly recommends enabling this in the EU as well.

Architecture

A browser built with BrowserEngineKit consists of four components running in separate processes:

Host App (UI, coordination)
  |
  |-- XPC --> Web Content Extension (HTML parsing, JS, DOM)
  |-- XPC --> Networking Extension (URLSession, sockets)
  |-- XPC --> Rendering Extension (Metal, GPU, media)

The host app launches and manages all extensions. Extensions cannot launch other extensions. Extensions communicate with each other through anonymous XPC endpoints brokered by the host app.

Bootstrap Sequence

1. Host launches web content, networking, and rendering extensions
2. Host creates XPC connections to each extension
3. Host requests anonymous XPC endpoints from networking and rendering
4. Host sends both endpoints to the web content extension via a bootstrap message
5. Web content extension connects directly to networking and rendering

This architecture follows the principle of least privilege: the web content extension works with untrusted data but has no direct OS resource access.

Process Management

Launching Extensions

Each extension type has a corresponding process class in the host app:

import BrowserEngineKit

// Web content (one per tab or iframe)
let contentProcess = try await WebContentProcess(
    bundleIdentifier: nil,
    onInterruption: {
        // Handle crash or OS interruption
    }
)

// Networking (typically one instance)
let networkProcess = try await NetworkingProcess(
    bundleIdentifier: nil,
    onInterruption: {
        // Handle interruption
    }
)

// Rendering / GPU (typically one instance)
let renderingProcess = try await RenderingProcess(
    bundleIdentifier: nil,
    onInterruption: {
        // Handle interruption
    }
)

Pass nil for bundleIdentifier to use the default extension target. The interruption handler fires if the extension crashes or is terminated by the OS.

Creating XPC Connections

let connection = try contentProcess.makeLibXPCConnection()
// Use connection for inter-process messaging

Each process type provides makeLibXPCConnection() to create an xpc_connection_t for communication.

Stopping Extensions

contentProcess.invalidate()

After calling invalidate(), no further method calls on the process object are valid.

Extension Types

Web Content Extension

Hosts the browser engine's HTML parser, CSS engine, JavaScript interpreter, and DOM. Subclass WebContentExtension to handle incoming XPC connections:

import BrowserEngineKit

final class MyWebContentExtension: WebContentExtension {
    override func handle(xpcConnection: xpc_connection_t) {
        // Set up message handlers on the connection
    }
}

Configure via WebContentExtensionConfiguration in the extension's EXAppExtensionAttributes.

Networking Extension

Handles all network requests using URLSession or socket APIs. One instance serves all tabs:

import BrowserEngineKit

final class MyNetworkingExtension: NetworkingExtension {
    override func handle(xpcConnection: xpc_connection_t) {
        // Handle network request messages
    }
}

Configure via NetworkingExtensionConfiguration.

Rendering Extension

Accesses the GPU via Metal for video decoding, compositing, and complex rendering. One instance typically serves the entire browser:

import BrowserEngineKit

final class MyRenderingExtension: RenderingExtension {
    override func handle(xpcConnection: xpc_connection_t) {
        // Handle rendering commands
    }
}

The rendering extension can enable optional features:

// Enable CoreML in the rendering extension
extension.enableFeature(.coreML)

Configure via RenderingExtensionConfiguration.

Capabilities

Grant capabilities to extensions so the OS schedules them appropriately:

// Grant foreground priority to an extension
let grant = try contentProcess.grantCapability(.foreground)

// ... extension does foreground work ...

// Relinquish when done
grant.invalidate()

Available Capabilities

Media Environment

For media capabilities, create a MediaEnvironment tied to a page URL. The environment supports AVCaptureSession for camera/mic access and is XPC-serializable for cross-process transport:

let mediaEnv = MediaEnvironment(webPage: pageURL)
let grant = try contentProcess.grantCapability(
    .mediaPlaybackAndCapture(environment: mediaEnv)
)
try mediaEnv.activate()
let captureSession = try mediaEnv.makeCaptureSession()

Visibility Propagation

Attach a visibility propagation interaction to browser views so extensions know when content is on screen. Both WebContentProcess and RenderingProcess provide createVisibilityPropagationInteraction().

Layer Hosting and View Coordination

The rendering extension draws into a LayerHierarchy, whose content the host app displays via LayerHierarchyHostingView. Handles are passed over XPC. Use LayerHierarchyHostingTransactionCoordinator to synchronize layer updates atomically across processes.

See references/browserenginekit-patterns.md for detailed layer hosting examples and transaction coordination.

Text Interaction

Adopt BETextInput on custom text views to integrate with UIKit's text system. This enables standard text selection, autocorrect, dictation, and keyboard interactions.

Key integration points:

• asyncInputDelegate for communicating text changes to the system
• handleKeyEntry(_:completionHandler:) for keyboard events
• BETextInteraction for selection gestures, edit menus, and context menus
• BEScrollView and BEScrollViewDelegate for custom scroll handling

See references/browserenginekit-patterns.md for detailed text interaction implementation.

Sandbox and Security

Restricted Sandbox

After initialization, lock down content extensions using the restricted sandbox:

// In the web content extension, after setup:
extension.applyRestrictedSandbox(revision: .revision2)

This removes access to resources the extension used during startup but no longer needs. Use the latest revision (.revision2) for the strongest restrictions.

JIT Compilation

Web content extensions that JIT-compile JavaScript toggle memory between writable and executable states. Use the BE_JIT_WRITE_PROTECT_TAG from BrowserEngineCore:

import BrowserEngineCore

// BE_JIT_WRITE_PROTECT_TAG is used with pthread_jit_write_protect_np
// to control JIT memory page permissions

Requires the com.apple.security.cs.allow-jit and com.apple.developer.kernel.extended-virtual-addressing entitlements on the web content extension only.

arm64e Requirement

All executables (host app and extensions) must be built with the arm64e instruction set for distribution. Build as a universal binary to also support arm64 iPads.

In Xcode build settings or xcconfig:

ARCHS[sdk=iphoneos*]=arm64e

Do not use arm64e for Simulator targets.

Downloads

Report download progress to the system using BEDownloadMonitor. Create an access token, initialize the monitor with source/destination URLs and a Progress object, then call beginMonitoring() to show the system download UI. Use resumeMonitoring(placeholderURL:) to resume interrupted downloads.

See references/browserenginekit-patterns.md for full download management examples.

Common Mistakes

DON'T: Skip the bootstrap sequence

// WRONG - content extension has no path to other extensions
let contentProcess = try await WebContentProcess(
    bundleIdentifier: nil, onInterruption: {}
)
// Immediately start sending work without connecting to networking/rendering

// CORRECT - broker connections through the host app
let networkEndpoint = try await networkProxy.getEndpoint()
let renderEndpoint = try await renderProxy.getEndpoint()
try await contentProxy.bootstrap(
    renderingExtension: renderEndpoint,
    networkExtension: networkEndpoint
)

DON'T: Launch extensions from other extensions

// WRONG - extensions cannot launch other extensions
// (inside a WebContentExtension)
let network = try await NetworkingProcess(...)

// CORRECT - only the host app launches extensions
// Host app creates all processes, then brokers connections

DON'T: Use extension process objects after invalidation

// WRONG
contentProcess.invalidate()
let conn = try contentProcess.makeLibXPCConnection()  // Error

// CORRECT - create a new process if needed
let newProcess = try await WebContentProcess(
    bundleIdentifier: nil, onInterruption: {}
)

DON'T: Apply JIT entitlements to non-content extensions

JIT compilation entitlements (com.apple.security.cs.allow-jit) are valid only on web content extensions. Adding them to the host app, rendering extension, or networking extension causes App Store rejection.

DON'T: Hard-code region eligibility

// WRONG
if Locale.current.region?.identifier == "DE" {
    useAlternativeEngine()
}

// CORRECT - use the system eligibility API
BEAvailability.isEligible(for: .webBrowser) { eligible, _ in
    if eligible { useAlternativeEngine() }
}

DON'T: Forget to set UIRequiredDeviceCapabilities

Without web-browser-engine in UIRequiredDeviceCapabilities, users on unsupported devices can download the app and hit runtime failures.

Review Checklist

• com.apple.developer.web-browser-engine.host entitlement on host app
• Each extension has its type-specific entitlement
• UIRequiredDeviceCapabilities includes web-browser-engine
• arm64e instruction set configured for all iOS device targets
• arm64e is not set for Simulator targets
• Swift packages built with iOSPackagesShouldBuildARM64e workspace setting
• Extension point identifiers set correctly in each extension's Info.plist
• Interruption handlers implemented for all process types
• Bootstrap sequence connects content extension to networking and rendering
• Capabilities granted before work begins and invalidated when done
• Visibility propagation interaction added to browser content views
• Restricted sandbox applied to content extensions after initialization
• BEAvailability used for eligibility checks instead of manual region logic
• Memory attribution configured if rendering extension memory is high
• Download progress reported via BEDownloadMonitor for active downloads
• Memory tagging enabled for Japan distribution (recommended for EU)

References

• Extended patterns (text interaction, layer hosting, scroll views, XPC communication, content filtering): references/browserenginekit-patterns.md
• BrowserEngineKit framework
• Designing your browser architecture
• Creating browser extensions in Xcode
• Managing the browser extension life cycle
• Using XPC to communicate with browser extensions
• Web Browser Engine Entitlement
• BrowserKit framework
• BrowserEngineCore framework
• Sample: Developing a browser app with an alternative engine