Filtering Network Traffic

At a glance

Project structure

Source bundle/
├── SimpleFirewallExtension/
│   ├── main.swift
│   ├── IPCConnection.swift
│   ├── FilterDataProvider.swift
│   └── Info.plist
├── SimpleFirewall/
│   ├── AppDelegate.swift
│   ├── ViewController.swift
│   ├── Base.lproj/
│   │   └── Main.storyboard
│   ├── Info.plist
│   └── SimpleFirewall.entitlements
├── Configuration/
│   └── SampleCode.xcconfig
└── SimpleFirewall.xcodeproj/
    ├── .xcodesamplecode.plist
    └── project.pbxproj

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: Swift.
• The verified tree contains 8 project/configuration file(s) and 8 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["ViewController"]
    N3["FilterDataProvider"]
    N4["NetworkExtension / Network APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

SimpleFirewall/AppDelegate.swift:10 — architecture anchor

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {

}

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 Network Extension.

Ownership and state

classDiagram
    IPCConnection o-- NSXPCListener : listener
    IPCConnection o-- NSXPCConnection : currentConnection
    IPCConnection o-- AppCommunication : delegate
    IPCConnection *-- IPCConnection : shared

Ownership evidence

SimpleFirewallExtension/IPCConnection.swift:34 — stored dependency or nearest verified ownership anchor

class IPCConnection: NSObject {
    // ...
    var listener: NSXPCListener?
    // ...
}

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

SimpleFirewall/ViewController.swift:205 — representative execution boundary

                DispatchQueue.main.async {
                    // ...
                        os_log("Failed to disable the filter configuration: %@", error.localizedDescription)
                    // ...
                }

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

SimpleFirewallExtension/IPCConnection.swift:13 — representative type boundary

@objc protocol ProviderCommunication {

    func register(_ completionHandler: @escaping (Bool) -> Void)
}

The source explicitly defines local protocol relationships: ViewController → AppCommunication, IPCConnection → ProviderCommunication.

Access control

Reference code

SimpleFirewallExtension/IPCConnection.swift:46 — representative boundary

    private func extensionMachServiceName(from bundle: Bundle) -> String {
        // ...
                fatalError("Mach service name is missing from the Info.plist")
        // ...
    }

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

Naming conventions

• Types: Controller: ViewController; Delegate: AppDelegate; Provider: FilterDataProvider.
• Protocols: ProviderCommunication, AppCommunication.
• Methods: register, promptUser, extensionMachServiceName, startListener, listener, viewWillAppear, viewWillDisappear, updateStatus.
• Files: SimpleFirewall/AppDelegate.swift, SimpleFirewallExtension/IPCConnection.swift, SimpleFirewall/ViewController.swift, SimpleFirewallExtension/FilterDataProvider.swift.

Architecture takeaways

• AppDelegate is the main source-visible entry or composition anchor for this sample.
• Framework work reaches NetworkExtension, Cocoa, Network, SystemExtensions 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