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

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/
}

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
    }
}

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")
}

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

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")
            )
        }
    }
}

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)
}

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
}

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
}

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)
}

// 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)
}

// 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)
    }
}

// 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)
}

// 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)
}

// 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()
}