iOS Concurrency with GCD & Operations
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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) {
}
}
semaphore.wait()
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) {
semaphore.wait()
group.enter() // existing code
defer {
group.leave() // existing code
semaphore.signal()
}
dataTask_Group_Semaphore(with: url, group: group, semaphore: semaphore) { data, _, error in
if error == nil, let data, let image = UIImage(data: data) {
images.append(image)
}
}