Appmigrationkit

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:

1. App extension -- An AppMigrationExtension conforming type that the system invokes during migration. It handles data export and import.
2. System orchestration -- The OS manages the device-to-device session, transport, and scheduling. The extension does not control when it runs.
3. Containing app -- After migration completes, the app checks MigrationStatus.importStatus on first launch to determine whether migration occurred and whether it succeeded.

Key types:

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