iOS Concurrency with GCD & Operations

Sep 12 2023 · Swift 5.8, macOS 13, iOS 16, Xcode 14.3

Part 1: Grand Central Dispatch

10. Use a DispatchSemaphore

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Previous episode: 09. Challenge: Download a Group of Images Next episode: 11. Concurrency Problems

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A serial queue runs on a single thread, restricting access to a shared resource to one task at a time. Sometimes, you want to allow more threads, but not an unlimited number of threads. If you’re downloading data from the network, you might want to limit how many downloads happen at once, because you know the data is large and resource-heavy to process.

let semaphore = DispatchSemaphore(value: 4)
  for i in 1...10 { 
  queue.async(group: group) {
defer { semaphore.signal() }
print("Downloading image \(i)")
// Simulate a network wait
Thread.sleep(forTimeInterval: 3) 
print("Image \(i) downloaded")
Downloading image 1
Downloading image 2
Downloading image 3
Downloading image 4
Image 4 downloaded
Image 2 downloaded
Image 1 downloaded
Image 3 downloaded
Downloading image 5
Downloading image 6
Downloading image 8
Downloading image 7
Image 6 downloaded
Image 5 downloaded
Image 8 downloaded
Image 7 downloaded
Downloading image 10
Downloading image 9
Image 9 downloaded
Image 10 downloaded
func dataTask_Group_Semaphore(with url: URL,
  group: DispatchGroup, semaphore: DispatchSemaphore,  // add semaphore here
  completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) {
group.enter()  // existing code
defer {
  group.leave()  // existing code
dataTask_Group_Semaphore(with: url, group: group, semaphore: semaphore) { data, _, error in
  if error == nil, let data, let image = UIImage(data: data) {