appmigrationkit

SKILL.md

AppMigrationKit

One-time cross-platform data transfer for app resources. Enables apps to
export data from one platform (e.g., Android) and import it on iOS during
device setup or onboarding. iOS 26+ / iPadOS 26+ / Swift 6.3.

Beta-sensitive. AppMigrationKit is new in iOS 26 and may change before GM.
Re-check current Apple documentation before relying on specific API details.

AppMigrationKit uses an app extension model. The system orchestrates the
transfer between devices. The app provides an extension conforming to export
and import protocols, and the system calls that extension at the appropriate
time. The app itself never manages the network connection between devices.

Contents

Architecture Overview

Setup and Entitlements

App Migration Extension

Exporting Resources

Importing Resources

Migration Status

Progress Tracking

Testing

Common Mistakes

Review Checklist

References

Architecture Overview

AppMigrationKit operates through three layers:

App extension -- An AppMigrationExtension conforming type that the
system invokes during migration. It handles data export and import.

System orchestration -- The OS manages the device-to-device session,
transport, and scheduling. The extension does not control when it runs.

Containing app -- After migration completes, the app checks
MigrationStatus.importStatus on first launch to determine whether
migration occurred and whether it succeeded.

Key types:

Type
Role

AppMigrationExtension
Protocol for the app extension entry point

ResourcesExportingWithOptions
Protocol for exporting files via archiver

ResourcesExporting
Simplified export protocol (no custom options)

ResourcesImporting
Protocol for importing files on the destination

ResourcesArchiver
Streams files into the export archive

MigrationDataContainer
Access to the containing app's data directories

MigrationStatus
Check import result from the containing app

MigrationPlatform
Identifies the other device's platform (e.g., .android)

MigrationAppIdentifier
Identifies the source app by store and bundle ID

AppMigrationTester
Test-only actor for validating export/import logic

Setup and Entitlements

Entitlement

The app extension requires the com.apple.developer.app-migration.data-container-access
entitlement. Its value is a single-element string array containing the bundle
identifier of the containing app:

<key>com.apple.developer.app-migration.data-container-access</key>
<array>
    <string>com.example.myapp</string>
</array>

No other values are valid. This entitlement grants the extension read access
to the containing app's data container during export and write access during
import.

Extension Target

Add a new App Extension target to the Xcode project. The extension conforms
to one or more of the migration protocols (ResourcesExportingWithOptions,
ResourcesExporting, ResourcesImporting).

App Migration Extension

The extension entry point conforms to AppMigrationExtension. During
migration, the system prevents launching the containing app and its other
extensions to ensure exclusive data access.

Accessing the Data Container

The extension accesses the containing app's files through appContainer:

import AppMigrationKit

struct MyMigrationExtension: ResourcesExportingWithOptions, ResourcesImporting {
    // Access the containing app's directories
    let container = appContainer

    // container.bundleIdentifier     -- app's bundle ID
    // container.containerRootDirectory -- root of the app container
    // container.documentsDirectory    -- Documents/
    // container.applicationSupportDirectory -- Application Support/
}

MigrationDataContainer provides containerRootDirectory, documentsDirectory,
and applicationSupportDirectory as URL values pointing into the containing
app's sandbox.

Exporting Resources

Conform to ResourcesExportingWithOptions (or ResourcesExporting for no
custom options) to package files for transfer. The system calls
exportResources(to:request:) with a ResourcesArchiver and a
MigrationRequestWithOptions.

Declaring Export Properties

struct MyMigrationExtension: ResourcesExportingWithOptions {
    typealias OptionsType = MigrationDefaultSupportedOptions

    var resourcesSizeEstimate: Int {
        // Return estimated total bytes of exported data
        calculateExportSize()
    }

    var resourcesVersion: String {
        "1.0"
    }

    var resourcesCompressible: Bool {
        true  // Let the system compress during transport
    }
}

resourcesSizeEstimate -- Estimated total bytes. The system uses this for
progress UI and free-space checks.

resourcesVersion -- Format version string. The import side receives this
to handle versioned data formats.

resourcesCompressible -- When true, the archiver may compress files
during transport.

Implementing Export

func exportResources(
    to archiver: sending ResourcesArchiver,
    request: MigrationRequestWithOptions<MigrationDefaultSupportedOptions>
) async throws {
    let docsDir = appContainer.documentsDirectory

    // Check destination platform if needed
    if request.destinationPlatform == .android {
        // Platform-specific export logic
    }

    // Append files one at a time -- make continuous progress
    let userDataURL = docsDir.appending(path: "user_data.json")
    try await archiver.appendItem(at: userDataURL)

    // Append with a custom archive path
    let settingsURL = docsDir.appending(path: "settings.plist")
    try await archiver.appendItem(at: settingsURL, pathInArchive: "preferences/settings.plist")

    // Append a directory
    let photosDir = docsDir.appending(path: "photos")
    try await archiver.appendItem(at: photosDir, pathInArchive: "media/photos")
}

The archiver streams files incrementally. Call appendItem(at:pathInArchive:)
repeatedly as each resource is ready. The system may terminate the extension
if it appears hung, so avoid long gaps between append calls.

Cancellation

ResourcesArchiver handles task cancellation automatically by throwing
cancellation errors. Do not catch these errors -- doing so causes the system
to kill the extension.

Migration Platform

MigrationRequestWithOptions exposes destinationPlatform as a
MigrationPlatform value. Use this to tailor exported data:

if request.destinationPlatform == .android {
    // Export in a format the Android app expects
}

MigrationPlatform provides .android as a static constant. Custom
platforms can be created with MigrationPlatform("customPlatform").

Importing Resources

Conform to ResourcesImporting to receive transferred files on the
destination device. The system calls importResources(at:request:) after
app installation but before the app is launchable.

struct MyMigrationExtension: ResourcesImporting {
    func importResources(
        at importedDataURL: URL,
        request: ResourcesImportRequest
    ) async throws {
        let sourceVersion = request.sourceVersion
        let sourceApp = request.sourceAppIdentifier

        // sourceApp.platform        -- e.g., .android
        // sourceApp.bundleIdentifier -- source app's bundle ID
        // sourceApp.storeIdentifier  -- e.g., .googlePlay

        // Copy imported files into the app container
        let docsDir = appContainer.documentsDirectory

        let userData = importedDataURL.appending(path: "user_data.json")
        if FileManager.default.fileExists(atPath: userData.path()) {
            try FileManager.default.copyItem(
                at: userData,
                to: docsDir.appending(path: "user_data.json")
            )
        }
    }
}

Error Handling During Import

On import error, the system clears the containing app's data container to
prevent partial state. However, app group containers are not cleared. The
import implementation should clear any app group containers before writing
imported content:

func importResources(
    at importedDataURL: URL,
    request: ResourcesImportRequest
) async throws {
    // Clear shared app group data first
    let groupURL = FileManager.default.containerURL(
        forSecurityApplicationGroupIdentifier: "group.com.example.myapp"
    )
    if let groupURL {
        try? FileManager.default.removeItem(at: groupURL.appending(path: "shared_data"))
    }

    // Then import
    try await performImport(from: importedDataURL)
}

Source App Identifier

ResourcesImportRequest provides sourceAppIdentifier as a
MigrationAppIdentifier with three properties:

platform -- The source device's platform (e.g., .android)

bundleIdentifier -- The source app's bundle identifier

storeIdentifier -- The app store (e.g., .googlePlay)

Migration Status

After migration completes, the containing app checks the result on first
launch:

import AppMigrationKit

func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
) -> Bool {
    if let status = MigrationStatus.importStatus {
        switch status {
        case .success:
            showMigrationSuccessUI()
            MigrationStatus.clearImportStatus()
        case .failure(let error):
            showMigrationFailureUI(error: error)
            MigrationStatus.clearImportStatus()
        }
    }
    return true
}

MigrationStatus.importStatus is nil if no migration occurred.

Call clearImportStatus() after handling the result to prevent showing
the notification on subsequent launches.

The enum has two cases: .success and .failure(any Error).

Progress Tracking

The import side exposes a Progress object via resourcesImportProgress.
The system uses this to display transfer progress to the user. Update
completedUnitCount incrementally during import:

var resourcesImportProgress: Progress {
    Progress(totalUnitCount: 100)
}

func importResources(
    at importedDataURL: URL,
    request: ResourcesImportRequest
) async throws {
    let progress = resourcesImportProgress
    let files = try FileManager.default.contentsOfDirectory(
        at: importedDataURL, includingPropertiesForKeys: nil
    )
    let increment = Int64(100 / max(files.count, 1))
    for file in files {
        try processFile(file)
        progress.completedUnitCount += increment
    }
    progress.completedUnitCount = 100
}

Testing

AppMigrationTester is a test-only actor for validating migration logic
in unit tests hosted by the containing app. Do not use it in production.

import Testing
import AppMigrationKit

@Test func testExportImportRoundTrip() async throws {
    let tester = try await AppMigrationTester(platform: .android)

    // Export
    let result = try await tester.exportController.exportResources(
        request: nil, progress: nil
    )
    #expect(result.exportProperties.uncompressedBytes > 0)

    // Import the exported data
    try await tester.importController.importResources(
        from: result.extractedResourcesURL,
        importRequest: nil, progress: nil
    )
    try await tester.importController.registerImportCompletion(with: .success)
}

DeviceToDeviceExportProperties on the result exposes uncompressedBytes,
compressedBytes (nil if not compressible), sizeEstimate, and version.

See references/appmigrationkit-patterns.md for additional test patterns.

Common Mistakes

DON'T: Catch cancellation errors from ResourcesArchiver

// WRONG -- system kills the extension if cancellation is swallowed
func exportResources(to archiver: sending ResourcesArchiver, request: ...) async throws {
    do {
        try await archiver.appendItem(at: fileURL)
    } catch is CancellationError {
        // Swallowing this causes termination
    }
}

// CORRECT -- let cancellation propagate
func exportResources(to archiver: sending ResourcesArchiver, request: ...) async throws {
    try await archiver.appendItem(at: fileURL)
}

DON'T: Leave long gaps between archiver append calls

// WRONG -- system may assume the extension is hung and terminate it
func exportResources(to archiver: sending ResourcesArchiver, request: ...) async throws {
    let allFiles = gatherAllFiles()  // Takes 30 seconds
    for file in allFiles {
        try await archiver.appendItem(at: file)
    }
}

// CORRECT -- interleave file preparation with archiving
func exportResources(to archiver: sending ResourcesArchiver, request: ...) async throws {
    for file in knownFilePaths() {
        try await archiver.appendItem(at: file)
    }
}

DON'T: Convert files to intermediate format during export

// WRONG -- may exhaust disk space creating temporary copies
func exportResources(to archiver: sending ResourcesArchiver, request: ...) async throws {
    let converted = try convertToJSON(originalDatabase)  // Doubles disk usage
    try await archiver.appendItem(at: converted)
}

// CORRECT -- export files as-is, convert on import side if needed
func exportResources(to archiver: sending ResourcesArchiver, request: ...) async throws {
    try await archiver.appendItem(at: originalDatabase)
}

DON'T: Ignore app group containers during import error recovery

// WRONG -- system clears app container but not app groups on error
func importResources(at url: URL, request: ResourcesImportRequest) async throws {
    try writeToAppGroup(data)
    try writeToAppContainer(data)  // If this throws, app group has stale data
}

// CORRECT -- clear app group data before importing
func importResources(at url: URL, request: ResourcesImportRequest) async throws {
    try clearAppGroupData()
    try writeToAppGroup(data)
    try writeToAppContainer(data)
}

DON'T: Forget to clear import status after handling it

// WRONG -- migration UI shows every launch
if let status = MigrationStatus.importStatus {
    showMigrationResult(status)
    // Missing clearImportStatus()
}

// CORRECT
if let status = MigrationStatus.importStatus {
    showMigrationResult(status)
    MigrationStatus.clearImportStatus()
}

Review Checklist

 Extension target added with com.apple.developer.app-migration.data-container-access entitlement

 Entitlement array contains exactly one string: the containing app's bundle identifier

 Extension conforms to ResourcesExportingWithOptions or ResourcesExporting for export

 Extension conforms to ResourcesImporting for import

 resourcesSizeEstimate returns a reasonable byte estimate

 resourcesVersion is set and will be checked on import for format compatibility

 Export calls appendItem incrementally without long pauses

 Cancellation errors from ResourcesArchiver are not caught

 Import clears app group containers before writing new data

 Containing app checks MigrationStatus.importStatus on first launch

 clearImportStatus() called after handling the migration result

 AppMigrationTester used in unit tests to validate export and import

 Files are exported as-is without intermediate format conversion on the export side

 sourceVersion from import request used to handle versioned data formats

References

Extended patterns (combined extension, versioned migration, file enumeration, error recovery): references/appmigrationkit-patterns.md

AppMigrationKit framework

AppMigrationExtension

ResourcesExportingWithOptions

ResourcesImporting

ResourcesArchiver

MigrationStatus

MigrationDataContainer

AppMigrationTester

Data container entitlement