Browserenginekit

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. Examples target Swift 6.3 and current Apple SDKs.

BrowserEngineKit is a specialized framework. Alternative browser engines are available only through Apple-approved entitlement profiles and supported-region device eligibility. EU support applies to eligible users on iOS 17.4+ and iPadOS 18+; Japan support starts with iOS 26.2 and adds explicit PAC/MIE security requirements for browser apps. Development and testing can occur anywhere. The companion frameworks BrowserEngineCore (low-level primitives) and BrowserKit (eligibility checks, data transfer) support the overall 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. BEAvailability is available on iOS/iPadOS 18.4+:

import BrowserKit

do {
    let eligible = try await BEAvailability.isEligible(for: .webBrowser)
    guard eligible else { return /* fall back or explain */ }
    // Device supports alternative browser engines
} catch {
    // Handle eligibility lookup failure
}

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

Availability anchors: process APIs are iOS/iPadOS 17.4+, BEDownloadMonitor is iOS 18.2+, .revision2 restricted sandbox is iOS 26+, and RenderingExtensionFeature.coreML is iOS 26.2+.

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:

engine-association is available starting iOS/iPadOS/Mac Catalyst 26.2 and is set to first-party when you own the engine or third-party when another developer owns it. Embedded engines use arm64 only (not arm64e), cannot include browser extensions, and cannot use JIT compilation.

Japan-Specific Requirements

Browser apps distributed in Japan are supported on iOS 26.2+ and must adopt the current security mitigations Apple lists for Japan, including Pointer Authentication Codes and Memory Integrity Enforcement for relevant allocators and extension processes. Enable hardware memory tagging with com.apple.security.hardened-process.checked-allocations; Apple strongly recommends enabling it 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. Conform to WebContentExtension to handle incoming XPC connections:

import BrowserEngineKit

@main
struct MyWebContentExtension: WebContentExtension {
    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

@main
struct MyNetworkingExtension: NetworkingExtension {
    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