Build Mail App Extensions

At a glance

Project structure

Source bundle/
├── MailExtensions/
│   ├── MailExtensions/
│   │   ├── AppDelegate.swift
│   │   └── ViewController.swift
│   └── SampleMailExtensions/
│       ├── Compose Session/
│       │   ├── ComposeSessionHandler.swift
│       │   └── ComposeSessionViewController.swift
│       ├── Message Security/
│       │   ├── MessageSecurityHandler.swift
│       │   ├── ExampleSigningViewController.swift
│       │   └── MockEncoder.swift
│       ├── SpecialProjectHandler.swift
│       ├── Message Actions/
│       │   └── MessageActionHandler.swift
│       ├── Content Blocker/
│       │   └── ContentBlocker.swift
│       └── MailExtension.swift
└── Configuration/
    └── SampleCode.xcconfig

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 9 project/configuration file(s) and 15 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["ComposeSessionViewController"]
    N3["ComposeSessionHandler"]
    N4["MailKit APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

MailExtensions/MailExtensions/AppDelegate.swift:10 — architecture anchor

@main
class AppDelegate: NSObject, NSApplicationDelegate {

    func applicationDidFinishLaunching(_ aNotification: Notification) {}

    func applicationWillTerminate(_ aNotification: Notification) {}

}

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 MailKit.

Ownership and state

classDiagram
    MessageSecurityHandler *-- MessageSecurityHandler : shared
    SpecialProjectHandler *-- SpecialProjectHandler : sharedHandler
    SpecialProjectHandler *-- Array : verifiedEmails
    SpecialProjectHandler o-- SpecialProject : selectedSpecialProject

Ownership evidence

MailExtensions/SampleMailExtensions/Message Security/MessageSecurityHandler.swift:12 — stored dependency or nearest verified ownership anchor

class MessageSecurityHandler: NSObject, MEMessageSecurityHandler {
    // ...
    static let shared = MessageSecurityHandler()
    // ...
}

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

MailExtensions/SampleMailExtensions/Compose Session/ComposeSessionHandler.swift:22 — representative execution boundary

    func annotateAddressesForSession(_ session: MEComposeSession) async -> [MEEmailAddress: MEAddressAnnotation] {
        var annotations: [MEEmailAddress: MEAddressAnnotation] = [:]
        // ...
    }

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

MailExtensions/MailExtensions/AppDelegate.swift:11 — representative type boundary

@main
class AppDelegate: NSObject, NSApplicationDelegate {
    // ...
    func applicationDidFinishLaunching(_ aNotification: Notification) {}
    // ...
}

No local protocol conformance is claimed as protocol-oriented design; external framework conformances are listed only as dependencies.

Access control

Reference code

MailExtensions/SampleMailExtensions/Message Security/MockEncoder.swift:16 — representative boundary

    private func shouldEncode(_ message: MEMessage) -> Bool {
        true
    }

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: ComposeSessionViewController, ExampleSigningViewController, ViewController; Delegate: AppDelegate; Handler: ComposeSessionHandler, MessageActionHandler, MessageSecurityHandler, SpecialProjectHandler.
• Protocols: no local protocol declaration in the scanned source.
• Methods: applicationDidFinishLaunching, applicationWillTerminate, mailComposeSessionDidBegin, mailComposeSessionDidEnd, annotateAddressesForSession, viewController, additionalHeaders, allowMessageSendForSession.
• Files: MailExtensions/MailExtensions/AppDelegate.swift, MailExtensions/SampleMailExtensions/Compose Session/ComposeSessionHandler.swift, MailExtensions/SampleMailExtensions/Message Security/MessageSecurityHandler.swift, MailExtensions/SampleMailExtensions/SpecialProjectHandler.swift, MailExtensions/MailExtensions/ViewController.swift, MailExtensions/SampleMailExtensions/Compose Session/ComposeSessionViewController.swift.

Architecture takeaways

• AppDelegate is the main source-visible entry or composition anchor for this sample.
• Framework work reaches MailKit, Cocoa 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.
• The source does not justify labeling the design protocol-oriented.

Source map

• Official Apple documentation