I have a method that has this block inside:
func a(_ foo: () -> ()) { ... }
func b(_ foo: () -> ()) { ... }
func abc() {
a {
// some processing
b {
// some asynchronous work
}
}
}
When a button is tapped:
I call method abc()
It connects to the internet
The point is that it takes time to do so
I am looking for a way to cancel the previous block, and run the current block if tapped twice.
There is no direct option to cancel you're block. And block will be executed when it is called only one way to set condition in the body of the block and do not execute part that is inside.
In you're case try to use Operation and OperationQueue this approach will give you flexible solution to mange operation execution and gives opportunity to cancel operations.
Small example:
let operationQueue = OperationQueue.main
let operation = Operation()
operationQueue.addOperation(operation)
//Cancel operation that is executing
operation.cancel()
Related
I have a function that is a async wrapper around a synchronous function.
The synchronous function is like:
class Foo {
class func bar() -> [Int] {
return [1,2,3]
}
class func asyncBar(completion: #escaping ([Int]) -> Void) {
DispatchQueue.global(qos: .userInitiated).async {
let intArray = bar()
DispatchQueue.main.async {
completion(intArray)
}
}
}
}
When I call it from a XCTestCase completion does not run.
Is there some sort of perverse interaction between the way unit tests are done in XCode and the main thread?
I can find no documentation on the Web about this.
I have to use the main thread for the callback as it interacts with the Gui.
My test case looks something like:
func testAsyncBar() throws {
var run = true
func stopThisThing(ints: [Int]) {
run = false
}
Foo.asyncBar(completion: stopThisThing)
while run {
print ("Running...")
usleep(100000)
}
}
The busy loop at the end never stops.
Your test’s while loop will block the main thread (if the test runs on the main thread). As such, the closure dispatched via DispatchQueue.main.async can never run, and your run Boolean will never get reset. This results in a deadlock. You can confirm this by printing Thread.isMainThread or adding a test like dispatchPrecondition(condition: .onQueue(.main)).
Fortunately, unit tests have a simple mechanism that avoids this deadlock.
If you want unit test to wait for some asynchronous process, use an XCTestExpectation:
func testAsyncBar() throws {
let e = expectation(description: "asyncBar")
func stopThisThing(ints: [Int]) {
e.fulfill()
}
Foo.asyncBar(completion: stopThisThing)
waitForExpectations(timeout: 5)
}
This avoids problems introduced by the while loop that otherwise blocked the thread.
See Testing Asynchronous Operations with Expectations.
I have a 3rd-party SDK where architecture is like this:
I call a method of the service (lets call it TheirService)
Once that method completes, a delegate of that service (lets call it TheirServiceDelegate will receive a notification onSuccess if call succeeds or onFailure if it fails.
The problem with that is that methods of TheirService are dependant on each other, so I end up having a "chain reaction", where each next method of TheirService is called from previous callback.
Note that TheirService is single threaded, and is really picky about previous method to be complete before I can start the next one.
Currently my interactions with the service look like this:
protocol MyClientListener {
notifyOnFailure()
notifyOnResult(result: SomeObject)
}
class MyClient: TheirServiceDelegate {
static let instance = MyClient()
let myService = TheirService()
var myResult: SomeObject?
var resultListener: MyClientListener
private init() { }
func start(resultListener: MyClientListener) {
self.resultListener = resultListener
myService.initialize()
}
// TheirServiceDelegate method
func onInitializeSuccess() {
myService.bootstrap()
}
// TheirServiceDelegate method
func onBootstrapSuccess() {
myService.login(user, password)
}
// TheirServiceDelegate method
func onLoginSuccess() {
myService.doSomethingUsefulStep1()
}
// TheirServiceDelegate method
func onDoSomethingUsefulStep1Success() {
myService.doSomethingUsefulStep2()
}
// TheirServiceDelegate method
func onDoSomethingUsefulStep2Success(result: SomeObject) {
// ah, look, now I have some object I actually wanted!
resultListener.notifyOnResult(result)
}
}
I also have to deal with failure case for each of them. And I cannot skip or change the order of steps, which creates some sort of awkward state machine.
Instead i would like to interact with service via logical functions, that complete certain stages of the process from end to end, waiting for results between the steps:
class MyClient {
static let instance = MyClient()
let myService = MyService()
private init() { }
func connect() throws {
myService.initialize()
// wait for success or failure, throw on failure
myService.bootstrap()
// wait for success or failure, throw on failure
myService.login(user, password)
// wait for success or failure, throw on failure
}
func doSomethingUseful() -> SomeObject {
myService.doSomethingUsefulStep1()
// wait for success or failure, throw on failure
myService.doSomethingUsefulStep2()
// wait for success or failure, throw on failure
// on success, it will get an object it could return
}
Called like this:
try MyClient.instance.connect()
let x = try MyClient.instance.doSomethingUseful()
So is there any way to turn "wait for success or failure" comment into actual code that waits for that single-threaded service to call back? And where a delegate would fit in that case?
I have a function that does some work and call completion. Something like this
func doStuff(completion: (Bool) -> ()) {
performWork()
completion(true)
}
The problem is that performWork triggers some process that receives result in other method. And depending on this result I need call completion with success or not based on data from previous method.
Is there any possible solution ? Method doStuff can not be modified and I don't have access to performWork() its third party, I can only call it.
You should save completion in a variable in the scope of your class and execute it in the delegate method of your API.
var doStuffCompletion: (Bool) -> ()!
func doStuff(completion: (Bool) -> ()) {
performWork()
doStuffCompletion = completion
}
func apiStuffFinished(success: Bool) {
doStuffCompletion(success)
}
I have two tasks : task1 and task2. I want to execute task2 after task1 finishes.
let globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)
dispatch_sync(globalQueueDefault){
self.activityIndicatorView.hidden = false
self.activityIndicatorView.startAnimating()
task1()
sleep(6)
dispatch_sync(globalQueueDefault) { () -> Void in
task2()
}
}
I searched in internet, I find NSLock,NSConditionLock and objc_sync_enter...I have try them, but it doesn't work...
let lock = NSLock()
let globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)
dispatch_sync(globalQueueDefault){
self.activityIndicatorView.hidden = false
self.activityIndicatorView.startAnimating()
self.lock.lock()
task1()
self.lock.unlock()
sleep(6)
dispatch_sync(globalQueueDefault) { () -> Void in
self.lock.lock()
task2()
self.lock.unlock()
}
}
I also tried NSConditionLock and objc_sync_enter...It doesn't work. How I can use lock in swift ? Could you give me a example base on my code? Thank you.
PS: I don't want to use callback here...because I have tried it, I think multithread is more closer to my answer, Thank you.
I'm going out on a limp and making some guesses about your program structures. The first problem with your code is that it's trying to access a view on a background thread. GUI elements should always be accessed on the main thread. The second problem is sleep: don't use it to write concurrent code. It makes assumptions about how the asynchronous task is going take. You should treat that time as unknown and use a sync pattern or a call back.
Since you mentioned that task1() download JSON, it's likely asynchronous. Here's how I'd do it:
func task1(finish: () -> Void) {
// Set up your connection to the website
let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {
// Handle the response, parse the json, etc
...
// Now call the completion handler
finish()
}
}
func task2() {
// Do whatever here
}
// In the function that triggers the JSON download
func downloadJSON() {
self.activityIndicatorView.hidden = false
self.activityIndicatorView.startAnimating()
task1(task2)
}
I have this code to add a NSOperation instance to a queue
let operation = NSBlockOperation()
operation.addExecutionBlock({
self.asyncMethod() { (result, error) in
if operation.cancelled {
return
}
// etc
}
})
operationQueue.addOperation(operation)
When user leaves the view that triggered this above code I cancel operation doing
operationQueue.cancelAllOperations()
When testing cancelation, I'm 100% sure cancel is executing before async method returns so I expect operation.cancelled to be true. Unfortunately this is not happening and I'm not able to realize why
I'm executing cancellation on viewWillDisappear
EDIT
asyncMethod contains a network operation that runs in a different thread. That's why the callback is there: to handle network operation returns. The network operation is performed deep into the class hierarchy but I want to handle NSOperations at root level.
Calling the cancel method of this object sets the value of this
property to YES. Once canceled, an operation must move to the finished
state.
Canceling an operation does not actively stop the receiver’s code from
executing. An operation object is responsible for calling this method
periodically and stopping itself if the method returns YES.
You should always check the value of this property before doing any
work towards accomplishing the operation’s task, which typically means
checking it at the beginning of your custom main method. It is
possible for an operation to be cancelled before it begins executing
or at any time while it is executing. Therefore, checking the value at
the beginning of your main method (and periodically throughout that
method) lets you exit as quickly as possible when an operation is
cancelled.
import Foundation
let operation1 = NSBlockOperation()
let operation2 = NSBlockOperation()
let queue = NSOperationQueue()
operation1.addExecutionBlock { () -> Void in
repeat {
usleep(10000)
print(".", terminator: "")
} while !operation1.cancelled
}
operation2.addExecutionBlock { () -> Void in
repeat {
usleep(15000)
print("-", terminator: "")
} while !operation2.cancelled
}
queue.addOperation(operation1)
queue.addOperation(operation2)
sleep(1)
queue.cancelAllOperations()
try this simple example in playground.
if it is really important to run another asynchronous code, try this
operation.addExecutionBlock({
if operation.cancelled {
return
}
self.asyncMethod() { (result, error) in
// etc
}
})
it's because you doing work wrong. You cancel operation after it executed.
Check this code, block executed in one background thread. Before execution start – operation cancel, remove first block from queue.
Swift 4
let operationQueue = OperationQueue()
operationQueue.qualityOfService = .background
let ob1 = BlockOperation {
print("ExecutionBlock 1. Executed!")
}
let ob2 = BlockOperation {
print("ExecutionBlock 2. Executed!")
}
operationQueue.addOperation(ob1)
operationQueue.addOperation(ob2)
ob1.cancel()
// ExecutionBlock 2. Executed!
Swift 2
let operationQueue = NSOperationQueue()
operationQueue.qualityOfService = .Background
let ob1 = NSBlockOperation()
ob1.addExecutionBlock {
print("ExecutionBlock 1. Executed!")
}
let ob2 = NSBlockOperation()
ob2.addExecutionBlock {
print("ExecutionBlock 2. Executed!")
}
operationQueue.addOperation(ob1)
operationQueue.addOperation(ob2)
ob1.cancel()
// ExecutionBlock 2. Executed!
The Operation does not wait for your asyncMethod to be finished. Therefore, it immediately returns if you add it to the Queue. And this is because you wrap your async network operation in an async NSOperation.
NSOperation is designed to give a more advanced async handling instead for just calling performSelectorInBackground. This means that NSOperation is used to bring complex and long running operations in background and not block the main thread. A good article of a typically used NSOperation can be found here:
http://www.raywenderlich.com/19788/how-to-use-nsoperations-and-nsoperationqueues
For your particular use case, it does not make sense to use an NSOperation here, instead you should just cancel your running network request.
It does not make sense to put an asynchronous function into a block with NSBlockOperation. What you probably want is a proper subclass of NSOperation as a concurrent operation which executes an asynchronous work load. Subclassing an NSOperation correctly is however not that easy as it should.
You may take a look here reusable subclass for NSOperation for an example implementation.
I am not 100% sure what you are looking for, but maybe what you need is to pass the operation, as parameter, into the asyncMethod() and test for cancelled state in there?
operation.addExecutionBlock({
asyncMethod(operation) { (result, error) in
// Result code
}
})
operationQueue.addOperation(operation)
func asyncMethod(operation: NSBlockOperation, fun: ((Any, Any)->Void)) {
// Do stuff...
if operation.cancelled {
// Do something...
return // <- Or whatever makes senes
}
}