almaleh

// Solution A: Unwind the recursion using DFS

func printXY(_ n: Int) -> [String] {
    
    var explored = Set<Int>()
    var stack = [n]
    var output = [String]()
    
    while let last = stack.last {
        

almaleh

class ViewControllerB: UIViewController {
    
    var workItem: DispatchWorkItem?
    
    override func viewDidLoad() {
        let view = self.view
        let workItem = DispatchWorkItem {
            UIView.animate(withDuration: 1.0) { [weak self] in // this leaks
                view?.backgroundColor = .red
            }

almaleh

let view = self.view
let workItem = DispatchWorkItem {
        UIView.animate(withDuration: 1.0) { // adding [weak self] here will introduce a cycle
            view?.backgroundColor = .red
        }
}
self.workItem = workItem

almaleh

let workItem = DispatchWorkItem { [weak self] in
        UIView.animate(withDuration: 1.0) {
            self?.view.backgroundColor = .red
        }
    }
self.workItem = workItem

almaleh

let workItem = DispatchWorkItem { // <-- first closure
        UIView.animate(withDuration: 1.0) { // <-- second closure
            self.view.backgroundColor = .red
        }
    }
self.workItem = workItem

almaleh

// Can you tell which version has a retain cycle?

// Version A
class ViewControllerA: UIViewController {
    
    var workItem: DispatchWorkItem?
    
    override func viewDidLoad() {
        let workItem = DispatchWorkItem {
            UIView.animate(withDuration: 1.0) {

almaleh

let concurrent = DispatchQueue(label: "com.besher.concurrent", attributes: .concurrent)

concurrent.sync {
    for _ in 0..<5 { print("🔵") }
}

concurrent.async {
    for _ in 0..<5 { print("🔴") }
}

almaleh

let concurrent = DispatchQueue(label: "com.besher.concurrent", attributes: .concurrent)

concurrent.async {
    for _ in 0..<5 { print("🔵") }
}

concurrent.async {
    for _ in 0..<5 { print("🔴") }
}

almaleh

let serial1 = DispatchQueue(label: "com.besher.serial1")
let serial2 = DispatchQueue(label: "com.besher.serial2")

serial1.sync { // <---- we changed this to 'sync'
    for _ in 0..<5 { print("🔵") }
}
// we don't get here until first loop terminates
serial2.async {
    for _ in 0..<5 { print("🔴") }
}

almaleh

let serial = DispatchQueue(label: "com.besher.serial")

serial.async {
    for _ in 0..<5 { print("🔵") }
}

serial.async {
    for _ in 0..<5 { print("🔴") }
}