Modern Concurrency: Beyond the Basics

Oct 20 2022 · Swift 5.5, iOS 15, Xcode 13.4

Part 2: Concurrent Code

19. Challenge: Using a GlobalActor

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Previous episode: 18. Using a GlobalActor Next episode: 20. Conclusion

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In this challenge, you’ll finish the debugging toolbar by connecting the second counter to display on-disk cache hits and making the last toolbar button clear the disk cache.

@MainActor private(set) var onDiskAccess: AsyncStream<Int>?
private var onDiskAccessCounter = 0 {
  didSet { onDiskAcccessContinuation?.yield(onDiskAccessCounter) }
private var onDiskAcccessContinuation: AsyncStream<Int>.Continuation?
func setUp() async throws {
  storage = await DiskStorage()
  for fileURL in try await storage.persistedFiles() {
  await imageLoader.setUp()
  let accessStream = AsyncStream<Int> { continuation in
    onDiskAcccessContinuation = continuation
  await { onDiskAccess = accessStream }
deinit {
onDiskAccessCounter += 1
onDiskAccessCounter = 0
print("Cleared disk cache.")
.task {
  guard let diskAccessSequence = ImageDatabase.shared.onDiskAccess else {
  for await count in diskAccessSequence {
    onDiskAccessCount = count
Button(action: {
  // Clear on-disk cache
  Task {
    await ImageDatabase.shared.clear()
}, label: {
  Image(systemName: "folder.badge.minus")