[swift 3] code snippet using GCD's semaphore to protect accessing critical resources from multiple threads where wait and signal may not be the same thread.

gcd_semaphore.swift

class MyObject {
	let semaphore = DispatchSemaphore(value: 1)
	let session = URLSession(configuration: URLSessionConfiguration.default)

	func update1() {
		// don't call from mail thread
		print("update1 - begin")
		semaphore.wait()
		let url = URL(string: "https://www.apple.com/")!
		let task = self.session.dataTask(with: url) { data, response, error in
			defer { self.semaphore.signal() }

			// update some critical resources
			print("update1 - end")
		}
		task.resume()
	}

	func update2() {
		// don't call from mail thread
		print("update2 - begin")
		semaphore.wait()
		let url = URL(string: "https://www.apple.com/")!
		self.session.dataTask(with: url) { data, response, error in
			defer { self.semaphore.signal() }

			// update some critical resources
			print("update2 - end")
		}
	}
}