Cancelling Dispatch Blocks

• You can only cancel DispatchWorkItem before it reaches the head of a queue and starts executing.

let workItem = DispatchWorkItem(flags: .inheritQoS) {
    let url = URL(string: "")!
    let task = URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in
        // Update UI
    })
    task.resume()
}

DispatchQueue.global().async(execute: workItem)

workItem.cancel()

Grouping and chaining tasks with DispatchGroup