If I have a serial queue, how can I, from the main thread, tell it to immediately stop execution and cancel all of its tasks?
How to stop the execution of tasks in a dispatch queue?
Asked Answered
I answered it here with example, you can take a look of it. enter link description here –
Cleaner
If you're using Swift the DispatchWorkItem class allows works units to be cancelled individually.
Work items allow you to configure properties of individual units of work directly. They also allow you to address individual work units for the purposes of waiting for their completion, getting notified about their completion, and/or canceling them. ( available for use in iOS 8.0+ macOS 10.10+ ).
DispatchWorkItem encapsulates work that can be performed. A work item can be dispatched onto a DispatchQueue and within a DispatchGroup. A DispatchWorkItem can also be set as a DispatchSource event, registration, or cancel handler.
↳ https://developer.apple.com/reference/dispatch/dispatchworkitem
nice idea to use arrow:) –
Florilegium
Thank you, this is exactly what I was looking for! Any idea why the docs for that class are so minimal? –
Gratifying
DispatchWorkItem will not cancel its work item if it has already begun execution. Cancelling one will only stop future execution if the DispatchQueue has yet to execute it. –
Belgrade @shoe: The
DispatchWorkItem would be used as a cancel handler within a DispatchGroup. This allows the operation that is currently executing to continually check its status and cancelled per the handier, which essentially puts it in a finished state, stopping further execution. –
Aconite The OP asked for a way to stop a currently executing task. You posted an answer to their question, but it is not a solution to their problem. So in this context your answer is misleading.
DispatchWorkItem will not cancel an executing task. That functionality is not provided by DispatchWorkItem, even if it is used with DispatchGroup. –
Belgrade @shoe, When I answered this question four years ago it worked fine, so perhaps Apple changed something along the way or you aren’t understanding how to implement it correctly. –
Aconite
There is no way to empty pending tasks from a dispatch queue without implementing non-trivial logic yourself as of iOS 9 / OS X 10.11.
If you have a need to cancel a dispatch queue, you might be better off using NSOperationQueue which offers this and more. For example, here's how you "cancel" a queue:
NSOperationQueue* queue = [NSOperationQueue new];
queue.maxConcurrentOperationCount = 1; // make it a serial queue
...
[queue addOperationWithBlock:...]; // add operations to it
...
// Cleanup logic. At this point _do not_ add more operations to the queue
queue.suspended = YES; // halts execution of the queue
[queue cancelAllOperations]; // notify all pending operations to terminate
queue.suspended = NO; // let it go.
queue=nil; // discard object
There's some new apis, check out
dispatch_block_t, you can use dispatch_block_cancel to cancel a block –
Benito This is a pretty common question, and one I've answered before:
The short answer is that GCD doesn't have a cancellation API; you have to implement your cancellation code yourself. In my answer, above, I show basically how that can be done.
Details
- Xcode Version 10.2 (10E125), Swift 5
Way 1. OperationQueue
Canceling an operation object leaves the object in the queue but notifies the object that it should stop its task as quickly as possible. For currently executing operations, this means that the operation object’s work code must check the cancellation state, stop what it is doing, and mark itself as finished
Solution
class ViewController: UIViewController {
private lazy var queue = OperationQueue()
override func viewDidLoad() {
super.viewDidLoad()
queue.addOperation(SimpleOperation(title: "Task1", counter: 50, delayInUsec: 100_000))
queue.addOperation(SimpleOperation(title: "Task2", counter: 10, delayInUsec: 500_000))
DispatchQueue .global(qos: .background)
.asyncAfter(deadline: .now() + .seconds(3)) { [weak self] in
guard let self = self else { return }
self.queue.cancelAllOperations()
print("Cancel tasks")
}
}
}
class SimpleOperation: Operation {
private let title: String
private var counter: Int
private let delayInUsec: useconds_t
init(title: String, counter: Int, delayInUsec: useconds_t) {
self.title = title
self.counter = counter
self.delayInUsec = delayInUsec
}
override func main() {
if isCancelled { return }
while counter > 0 {
print("\(title), counter: \(counter)")
counter -= 1
usleep(delayInUsec)
if isCancelled { return }
}
}
}
Way 2.1 DispatchWorkItemController
Solution
protocol DispatchWorkItemControllerDelegate: class {
func workСompleted(delegatedFrom controller: DispatchWorkItemController)
}
class DispatchWorkItemController {
weak var delegate: DispatchWorkItemControllerDelegate?
private(set) var workItem: DispatchWorkItem?
private var semaphore = DispatchSemaphore(value: 1)
var needToStop: Bool {
get {
semaphore.wait(); defer { semaphore.signal() }
return workItem?.isCancelled ?? true
}
}
init (block: @escaping (_ needToStop: ()->Bool) -> Void) {
let workItem = DispatchWorkItem { [weak self] in
block { return self?.needToStop ?? true }
}
self.workItem = workItem
workItem.notify(queue: DispatchQueue.global(qos: .utility)) { [weak self] in
guard let self = self else { return }
self.semaphore.wait(); defer { self.semaphore.signal() }
self.workItem = nil
self.delegate?.workСompleted(delegatedFrom: self)
}
}
func setNeedsStop() { workItem?.cancel() }
func setNeedsStopAndWait() { setNeedsStop(); workItem?.wait() }
}
Usage of base solution (full sample)
class ViewController: UIViewController {
lazy var workItemController1 = { self.createWorkItemController(title: "Task1", counter: 50, delayInUsec: 100_000) }()
lazy var workItemController2 = { self.createWorkItemController(title: "Task2", counter: 10, delayInUsec: 500_000) }()
override func viewDidLoad() {
super.viewDidLoad()
DispatchQueue.global(qos: .default).async(execute: workItemController1.workItem!)
DispatchQueue.global(qos: .default).async(execute: workItemController2.workItem!)
DispatchQueue .global(qos: .background)
.asyncAfter(deadline: .now() + .seconds(3)) { [weak self] in
guard let self = self else { return }
self.workItemController1.setNeedsStop()
self.workItemController2.setNeedsStop()
print("tasks canceled")
}
}
private func createWorkItemController(title: String, counter: Int, delayInUsec: useconds_t) -> DispatchWorkItemController {
let controller = DispatchWorkItemController { needToStop in
var counter = counter
while counter > 0 {
print("\(title), counter: \(counter)")
counter -= 1
usleep(delayInUsec)
if needToStop() { print("canceled"); return }
}
}
controller.delegate = self
return controller
}
}
extension ViewController: DispatchWorkItemControllerDelegate {
func workСompleted(delegatedFrom controller: DispatchWorkItemController) {
print("-- work completed")
}
}
Way 2.2 QueueController
add code of DispatchWorkItemController here
protocol QueueControllerDelegate: class {
func tasksСompleted(delegatedFrom controller: QueueController)
}
class QueueController {
weak var delegate: QueueControllerDelegate?
private var queue: DispatchQueue
private var workItemControllers = [DispatchWorkItemController]()
private var semaphore = DispatchSemaphore(value: 1)
var runningTasksCount: Int {
semaphore.wait(); defer { semaphore.signal() }
return workItemControllers.filter { $0.workItem != nil } .count
}
func setNeedsStopTasks() {
semaphore.wait(); defer { semaphore.signal() }
workItemControllers.forEach { $0.setNeedsStop() }
}
func setNeedsStopTasksAndWait() {
semaphore.wait(); defer { semaphore.signal() }
workItemControllers.forEach { $0.setNeedsStopAndWait() }
}
init(queue: DispatchQueue) { self.queue = queue }
func async(block: @escaping (_ needToStop: ()->Bool) -> Void) {
queue.async(execute: initWorkItem(block: block))
}
private func initWorkItem(block: @escaping (_ needToStop: ()->Bool) -> Void) -> DispatchWorkItem {
semaphore.wait(); defer { semaphore.signal() }
workItemControllers = workItemControllers.filter { $0.workItem != nil }
let workItemController = DispatchWorkItemController(block: block)
workItemController.delegate = self
workItemControllers.append(workItemController)
return workItemController.workItem!
}
}
extension QueueController: DispatchWorkItemControllerDelegate {
func workСompleted(delegatedFrom controller: DispatchWorkItemController) {
semaphore.wait(); defer { semaphore.signal() }
if let index = self.workItemControllers.firstIndex (where: { $0.workItem === controller.workItem }) {
workItemControllers.remove(at: index)
}
if workItemControllers.isEmpty { delegate?.tasksСompleted(delegatedFrom: self) }
}
}
Usage of QueueController (full sample)
class ViewController: UIViewController {
let queue = QueueController(queue: DispatchQueue(label: "queue", qos: .utility,
attributes: [.concurrent],
autoreleaseFrequency: .workItem,
target: nil))
override func viewDidLoad() {
super.viewDidLoad()
queue.delegate = self
runTestLoop(title: "Task1", counter: 50, delayInUsec: 100_000)
runTestLoop(title: "Task2", counter: 10, delayInUsec: 500_000)
DispatchQueue .global(qos: .background)
.asyncAfter(deadline: .now() + .seconds(3)) { [weak self] in
guard let self = self else { return }
print("Running tasks count: \(self.queue.runningTasksCount)")
self.queue.setNeedsStopTasksAndWait()
print("Running tasks count: \(self.queue.runningTasksCount)")
}
}
private func runTestLoop(title: String, counter: Int, delayInUsec: useconds_t) {
queue.async { needToStop in
var counter = counter
while counter > 0 {
print("\(title), counter: \(counter)")
counter -= 1
usleep(delayInUsec)
if needToStop() { print("-- \(title) canceled"); return }
}
}
}
}
extension ViewController: QueueControllerDelegate {
func tasksСompleted(delegatedFrom controller: QueueController) {
print("-- all tasks completed")
}
}
I'm not sure if you can stop a current block that is executing, but you can call dispatch_suspend to prevent the queue from executing any new queue items. You can then call dispatch_resume to restart execution (but it doesn't sound like that is what you want to do).
See cancelAllOperations on NSOperationQueue. It's still up to you to make sure your operations handle the cancel message correctly.
I have found a fun solution to this type of problem when trying to solve my own similar issue. The basic concept is that whatever class calls the dispatch, it has a id property that tracks the current execution of some method, for me, it was opening an alert view. The method that calls the dispatch then holds a local variable of a generated id. If the id has not be changed, then I know not to cancel my callback. If it has been changed, then take no action because some other alert has taken control:
class AlertData: ObservableObject {
static var shared = AlertData()
@Published var alertOpen = false
@Published var alertMessage = ""
@Published var alertTitle = ""
var id: UUID = UUID()
func openAlert() {
// ID is used to only dismiss the most recent version of alert within timeout.
let myID = UUID()
self.id = myID
withAnimation {
self.alertOpen = true
}
DispatchQueue.main.asyncAfter(deadline: (.now() + 2), execute: {
// Only dismiss if another alert has not appeared and taken control
if self.id == myID {
withAnimation {
self.alertOpen = false
}
}
})
}
func closeAlert() {
withAnimation {
self.alertOpen = false
}
}
}
Another solution is to throw away the old queue and create a new one. It works for me. It's like to delete an array, you can delete every element on it or you can simply create a new one to replace the old one.
but how do you do that ?? –
Metzler
I don't know how you do this, but I don't think that will work. You have to release the queue, but each block on the queue retains a reference to it, so it won't actually be cleared from memory until all the blocks complete. tl;dr this will cause massive memory leaks. –
Operand
Was working around a similar problem earlier today where I wanted to abandon a task involved in loading the data for a view controller if the user were to navigate away before it finished. Basically, the approach I ended up settling on was to use weak references to the controller in the closure being executed by DispatchQueue and wrote the code to fail gracefully should it disappear.
My two cents: I use the below to cancel.
Create a dummy static variable:
static bool flushStatus;
Set the flush status, by using the above static variable as an unique key:
dispatch_queue_set_specific(queue, &flushStatus, (void*)1, NULL);
Within each of the queue block, use the static variable's value to get the flush status:
dispatch_async(queue, ^(){
if(dispatch_get_specific(&flushStatus))
return;
// Do your work here
});
Note that if block had already crossed the "dispatch_get_specific", you will not be able to stop from running.
The advantage is, it doesn't need to access any variable/object to know its cancellation state, just the queue - all by itself - is enough.
Edit: To address the above note - "Note that if block had already crossed the "dispatch_get_specific", you will not be able to stop from running.", one can place the dispatch_queue_set_specific(queue, &flushStatus, (void*)1, NULL); followed by dispatch_barrier_sync, possibly in the destructor or in any terminating position
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