For example
UIView.animate(withDuration: 0.5, animations: {
self.datePickerView.center.y = self.view.frame.height + (self.datePickerView.frame.height/2)
self.datePickerOverlay.alpha = 0.0
}) { (true) in
self.view.sendSubview(toBack: self.datePickerOverlay)
self.view.sendSubview(toBack: self.datePickerView)
}
}
I understand the completion block is a closure where you perform tasks you want to happen after the animation. What I don't get is the function of the bool. As you can see I put true where the bool should be.
However, I'm wondering what that bool is suppose to stand for and who calls that bool in UIView.aniamteWithDuration().
Is the bool suppose to stand for finished (in the sense that once the animation complete finished will be true)? Or is the bool suppose to stand for success (in the sense that if the animation completes successfully success will be true)?
Thanks!
The Developer Reference says:
A block object to be executed when the animation sequence ends. This
block has no return value and takes a single Boolean argument that
indicates whether or not the animations actually finished before the
completion handler was called. If the duration of the animation is 0,
this block is performed at the beginning of the next run loop cycle.
This parameter may be NULL.
Related
I have a value type for which a relatively long calculation is require to produce it (> 1s). I wrap this value type in an enumeration that expresses whether it is currently being calculated or is available:
enum Calculatable<T> {
case calculated(T), calculating
}
The problem is this value is persisted. This means after the long running calculation, when I have the updated value, I try to persist it first before changing the property visible to the business logic. If persistence succeeds, the property is updated, if it fails, I want to wait and then try to persist again; with a catch — if any other values have changed that the calculated one depends on, we throw away the previously calculated value — stop trying to persist it — and submit a new Calculation operation to the queue — starting all over again. My initial attempt at that looked something like this:
/// Wraps a value that takes a long time to be calculated.
public class CalculatedValueWrapper<T> {
/// The last submitted `Calculation` to the queue.
private weak var latestCalculation: Optional<Operation>
/// The long running calculation that produces the up to date, wrapped value.
private let longCalculation: (_ cancelIf: () -> Bool) -> T
/// Calculation queue.
private let queue: OperationQueue
/// The calculated value.
private var wrappedValue: Calculatable<T>
/// Update the wrapped value.
public func update() {
// Don't set if already being calculated.
if case .calculated(_)=self.wrappedValue { self.wrappedValue = .calculating }
// Initiate new calculation.
self.latestCalculation?.cancel()
self.latestCalculation={
let calc: Calculation = .init(self, self.longCalculation)
self.queue.addOperation(calc)
return calc
}()
}
}
/// Executes a long running calculation and persists the result once complete.
class Calculation<T>: Operation {
/// The calculation.
private let calculation: (_ cancelIf: () -> Bool) -> T
/// The owner of the wrapped value we're calculating.
private weak var owner: Optional<CalculatedValueWrapper<T>>
func main() {
let result=self.calculation(cancelIf: { [unowned self] in self.isCancelled })
let persist: () -> Bool={
// In case the persistence fails.
var didPersist=false
// Persist the result on the main thread.
DispatchQueue.main.sync { [unowned self] in
// The owner may have been deallocated between the time this dispatch item was submitted and the time it began executing.
guard let owner=self.owner else { self.cancel(); return }
// May have been cancelled.
guard !self.isCancelled else { return }
// Attempt to persist the calculated result.
if let _=try? owner.persist(result) {
didPersist=true
}
}
// Done.
return didPersist
}
// Persist the new result. If it fails, and we're not cancelled, keep trying until it succeeds.
while !self.isCancelled && !self.persist() {
usleep(500_000)
}
}
}
I would have stuck with this, but after further research I noticed a prevailing sentiment that it was bad practice to sleep the thread in DispatchQueue items and Operations. The alternative that seems to be considered better, in the case of DispatchQueue for example, was to initiate another DispatchQueue item using asyncAfter(deadline:execute:).
That solution appears more complicated in my case as being able to cancel a single operation that encapsulates everything that needs to be done makes cancelling easy. I can hold a reference to the last Calculation operation executed, and if another one is submitted on the main thread while one is in progress, I cancel the old one, add the new one, and with that I know the old value won't be persisted because it is done on the main thread after cancellation has already been performed; and a Calculation must not be cancelled in order for it to persist its result.
Is there something about DispatchQueues or OperationQueues that would make the alternative solution more straightforward to implement in this case? Or is sleeping here totally fine?
Make a RetryPersist NSOperation. Just before adding it to the queue, set its isReady to false. Have it do a dispatchAfter weakly capturing itself to set its isReady to true. Now you can cancel any RetryPersist operations in the queue, if the count of them where not ready, not cancelled is not zero you know there’s one waiting to go etc.
I have a process which runs for a long time and which I would like the ability to interrupt.
func longProcess (shouldAbort: #escaping ()->Bool) {
// Runs a long loop and periodically checks shouldAbort(),
// returning early if shouldAbort() returns true
}
Here's my class which uses it:
class Example {
private var abortFlag: NSObject? = .init()
private var dispatchQueue: DispatchQueue = .init(label: "Example")
func startProcess () {
let shouldAbort: ()->Bool = { [weak abortFlag] in
return abortFlag == nil
}
dispatchQueue.async {
longProcess(shouldAbort: shouldAbort)
}
}
func abortProcess () {
self.abortFlag = nil
}
}
The shouldAbort closure captures a weak reference to abortFlag, and checks whether that reference points to nil or to an NSObject. Since the reference is weak, if the original NSObject is deallocated then the reference that is captured by the closure will suddenly be nil and the closure will start returning true. The closure will be called repeatedly during the longProcess function, which is occurring on the private dispatchQueue. The abortProcess method on the Example class will be externally called from some other queue. What if someone calls abortProcess(), thereby deallocating abortFlag, at the exact same time that longProcess is trying to perform the check to see if abortFlag has been deallocated yet? Is checking myWeakReference == nil a thread-safe operation?
You can create the dispatched task as a DispatchWorkItem, which has a thread-safe isCancelled property already. You can then dispatch that DispatchWorkItem to a queue and have it periodically check its isCancelled. You can then just cancel the dispatched as such point you want to stop it.
Alternatively, when trying to wrap some work in an object, we’d often use Operation, instead, which encapsulates the task in its own class quite nicely:
class SomeLongOperation: Operation {
override func main() {
// Runs a long loop and periodically checks `isCancelled`
while !isCancelled {
Thread.sleep(forTimeInterval: 0.1)
print("tick")
}
}
}
And to create queue and add the operation to that queue:
let queue = OperationQueue()
let operation = SomeLongOperation()
queue.addOperation(operation)
And to cancel the operation:
operation.cancel()
Or
queue.cancelAllOperations()
Bottom line, whether you use Operation (which is, frankly, the “go-to” solution for wrapping some task in its own object) or roll-your-own with DispatchWorkItem, the idea is the same, namely that you don’t need to have your own state property to detect cancellation of the task. Both dispatch queues and operation queues already have nice mechanisms to simplify this process for you.
I saw this bug (Weak properties are not thread safe when reading SR-192) indicating that weak reference reads weren't thread safe, but it has been fixed, which suggests that (absent any bugs in the runtime), weak reference reads are intended to be thread safe.
Also interesting: Friday Q&A 2017-09-22: Swift 4 Weak References by Mike Ash
I feel that I've always misunderstood that when reference cycles are created. Before I use to think that almost any where that you have a block and the compiler is forcing you to write .self then it's a sign that I'm creating a reference cycle and I need to use [weak self] in.
But the following setup doesn't create a reference cycle.
import Foundation
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution
class UsingQueue {
var property : Int = 5
var queue : DispatchQueue? = DispatchQueue(label: "myQueue")
func enqueue3() {
print("enqueued")
queue?.asyncAfter(deadline: .now() + 3) {
print(self.property)
}
}
deinit {
print("UsingQueue deinited")
}
}
var u : UsingQueue? = UsingQueue()
u?.enqueue3()
u = nil
The block only retains self for 3 seconds. Then releases it. If I use async instead of asyncAfter then it's almost immediate.
From what I understand the setup here is:
self ---> queue
self <--- block
The queue is merely a shell/wrapper for the block. Which is why even if I nil the queue, the block will continue its execution. They’re independent.
So is there any setup that only uses queues and creates reference cycles?
From what I understand [weak self] is only to be used for reasons other than reference cycles ie to control the flow of the block. e.g.
Do you want to retain the object and run your block and then release it? A real scenario would be to finish this transaction even though the view has been removed from the screen...
Or you want to use [weak self] in so that you can exit early if your object has been deallocated. e.g. some purely UI like stopping a loading spinner is no longer needed
FWIW I understand that if I use a closure then things are different ie if I do:
import PlaygroundSupport
import Foundation
PlaygroundPage.current.needsIndefiniteExecution
class UsingClosure {
var property : Int = 5
var closure : (() -> Void)?
func closing() {
closure = {
print(self.property)
}
}
func execute() {
closure!()
}
func release() {
closure = nil
}
deinit {
print("UsingClosure deinited")
}
}
var cc : UsingClosure? = UsingClosure()
cc?.closing()
cc?.execute()
cc?.release() // Either this needs to be called or I need to use [weak self] for the closure otherwise there is a reference cycle
cc = nil
In the closure example the setup is more like:
self ----> block
self <--- block
Hence it's a reference cycle and doesn't deallocate unless I set block to capturing to nil.
EDIT:
class C {
var item: DispatchWorkItem!
var name: String = "Alpha"
func assignItem() {
item = DispatchWorkItem { // Oops!
print(self.name)
}
}
func execute() {
DispatchQueue.main.asyncAfter(deadline: .now() + 1, execute: item)
}
deinit {
print("deinit hit!")
}
}
With the following code, I was able to create a leak ie in Xcode's memory graph I see a cycle, not a straight line. I get the purple indicators. I think this setup is very much like how a stored closure creates leaks. And this is different from your two examples, where execution is never finished. In this example execution is finished, but because of the references it remains in memory.
I think the reference is something like this:
┌─────────┐─────────────self.item──────────────▶┌────────┐
│ self │ │workItem│
└─────────┘◀︎────item = DispatchWorkItem {...}───└────────┘
You say:
From what I understand the setup here is:
self ---> queue
self <--- block
The queue is merely a shell/wrapper for the block. Which is why even if I nil the queue, the block will continue its execution. They’re independent.
The fact that self happens to have a strong reference to the queue is inconsequential. A better way of thinking about it is that a GCD, itself, keeps a reference to all dispatch queues on which there is anything queued. (It’s analogous to a custom URLSession instance that won’t be deallocated until all tasks on that session are done.)
So, GCD keeps reference to the queue with dispatched tasks. The queue keeps a strong reference to the dispatched blocks/items. The queued block keeps a strong reference to any reference types they capture. When the dispatched task finishes, it resolves any strong references to any captured reference types and is removed from the queue (unless you keep your own reference to it elsewhere.), generally thereby resolving any strong reference cycles.
Setting that aside, where the absence of [weak self] can get you into trouble is where GCD keeps a reference to the block for some reason, such as dispatch sources. The classic example is the repeating timer:
class Ticker {
private var timer: DispatchSourceTimer?
func startTicker() {
let queue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".ticker")
timer = DispatchSource.makeTimerSource(queue: queue)
timer!.schedule(deadline: .now(), repeating: 1)
timer!.setEventHandler { // whoops; missing `[weak self]`
self.tick()
}
timer!.resume()
}
func tick() { ... }
}
Even if the view controller in which I started the above timer is dismissed, GCD keeps firing this timer and Ticker won’t be released. As the “Debug Memory Graph” feature shows, the block, created in the startTicker routine, is keeping a persistent strong reference to the Ticker object:
This is obviously resolved if I use [weak self] in that block used as the event handler for the timer scheduled on that dispatch queue.
Other scenarios include a slow (or indefinite length) dispatched task, where you want to cancel it (e.g., in the deinit):
class Calculator {
private var item: DispatchWorkItem!
deinit {
item?.cancel()
item = nil
}
func startCalculation() {
let queue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".calcs")
item = DispatchWorkItem { // whoops; missing `[weak self]`
while true {
if self.item?.isCancelled ?? true { break }
self.calculateNextDataPoint()
}
self.item = nil
}
queue.async(execute: item)
}
func calculateNextDataPoint() {
// some intense calculation here
}
}
All of that having been said, in the vast majority of GCD use-cases, the choice of [weak self] is not one of strong reference cycles, but rather merely whether we mind if strong reference to self persists until the task is done or not.
If we’re just going to update the the UI when the task is done, there’s no need to keep the view controller and its views in the hierarchy waiting some UI update if the view controller has been dismissed.
If we need to update the data store when the task is done, then we definitely don’t want to use [weak self] if we want to make sure that update happens.
Frequently, the dispatched tasks aren’t consequential enough to worry about the lifespan of self. For example, you might have a URLSession completion handler dispatch UI update back to the main queue when the request is done. Sure, we theoretically would want [weak self] (as there’s no reason to keep the view hierarchy around for a view controller that’s been dismissed), but then again that adds noise to our code, often with little material benefit.
Unrelated, but playgrounds are a horrible place to test memory behavior because they have their own idiosyncrasies. It’s much better to do it in an actual app. Plus, in an actual app, you then have the “Debug Memory Graph” feature where you can see the actual strong references. See https://stackoverflow.com/a/30993476/1271826.
I am trying to understand 'Closure' of Swift more precisely.
But #escaping and Completion Handler are too difficult to understand
I searched many Swift postings and official documents, but I felt it was still not enough.
This is the code example of official documents
var completionHandlers: [()->Void] = []
func someFunctionWithEscapingClosure(completionHandler: #escaping ()->Void){
completionHandlers.append(completionHandler)
}
func someFunctionWithNoneescapingClosure(closure: ()->Void){
closure()
}
class SomeClass{
var x:Int = 10
func doSomething(){
someFunctionWithEscapingClosure {
self.x = 100
//not excute yet
}
someFunctionWithNoneescapingClosure {
x = 200
}
}
}
let instance = SomeClass()
instance.doSomething()
print(instance.x)
completionHandlers.first?()
print(instance.x)
I heard that there are two ways and reasons using #escaping
First is for storing a closure, second is for Async operating purposes.
The following are my questions:
First, if doSomething executes then someFunctionWithEscapingClosure will executing with closure parameter and that closure will be saved in global variable array.
I think that closure is {self.x = 100}
How self in {self.x = 100} that saved in global variable completionHandlers can connect to instance that object of SomeClass ?
Second, I understanding someFunctionWithEscapingClosure like this.
To store local variable closure completionHandler to global variable 'completionHandlerswe using#escaping` keyword!
without #escaping keyword someFunctionWithEscapingClosure returns, local variable completionHandler will remove from memory
#escaping is keep that closure in the memory
Is this right?
Lastly, I just wonder about the existence of this grammar.
Maybe this is a very rudimentary question.
If we want some function to execute after some specific function. Why don't we just call some function after a specific function call?
What are the differences between using the above pattern and using an escaping callback function?
Swift Completion Handler Escaping & Non-Escaping:
Assume the user is updating an app while using it. You definitely want
to notify the user when it is done. You possibly want to pop up a box
that says, “Congratulations, now, you may fully enjoy!”
So, how do you run a block of code only after the download has been
completed? Further, how do you animate certain objects only after a
view controller has been moved to the next? Well, we are going to find
out how to design one like a boss.
Based on my expansive vocabulary list, completion handlers stand for
Do stuff when things have been done
Bob’s post provides clarity about completion handlers (from a developer point of view it exactly defines what we need to understand).
#escaping closures:
When one passes a closure in function arguments, using it after the function’s body gets executed and returns the compiler back. When the function ends, the scope of the passed closure exist and have existence in memory, till the closure gets executed.
There are several ways to escaping the closure in containing function:
Storage: When you need to store the closure in the global variable, property or any other storage that exist in the memory past of the calling function get executed and return the compiler back.
Asynchronous execution: When you are executing the closure asynchronously on despatch queue, the queue will hold the closure in memory for you, can be used in future. In this case you have no idea when the closure will get executed.
When you try to use the closure in these scenarios the Swift compiler will show the error:
For more clarity about this topic you can check out this post on Medium.
Adding one more points , which every ios developer needs to understand :
Escaping Closure : An escaping closure is a closure that’s called after the function it was passed to returns. In other words,
it outlives the function it was passed to.
Non-escaping closure : A closure that’s called within the function it was passed into, i.e. before it returns.
Here's a small class of examples I use to remind myself how #escaping works.
class EscapingExamples: NSObject {
var closure: (() -> Void)?
func storageExample(with completion: (() -> Void)) {
//This will produce a compile-time error because `closure` is outside the scope of this
//function - it's a class-instance level variable - and so it could be called by any other method at
//any time, even after this function has completed. We need to tell `completion` that it may remain in memory, i.e. `escape` the scope of this
//function.
closure = completion
//Run some function that may call `closure` at some point, but not necessary for the error to show up.
//runOperation()
}
func asyncExample(with completion: (() -> Void)) {
//This will produce a compile-time error because the completion closure may be called at any time
//due to the async nature of the call which precedes/encloses it. We need to tell `completion` that it should
//stay in memory, i.e.`escape` the scope of this function.
DispatchQueue.global().async {
completion()
}
}
func asyncExample2(with completion: (() -> Void)) {
//The same as the above method - the compiler sees the `#escaping` nature of the
//closure required by `runAsyncTask()` and tells us we need to allow our own completion
//closure to be #escaping too. `runAsyncTask`'s completion block will be retained in memory until
//it is executed, so our completion closure must explicitly do the same.
runAsyncTask {
completion()
}
}
func runAsyncTask(completion: #escaping (() -> Void)) {
DispatchQueue.global().async {
completion()
}
}
}
/*the long story short is that #escaping means that don't terminate the function life time until the #escaping closure has finished execution in the opposite of nonEscaping closure the function can be terminated before the closure finishes execution Ex:
*/
func fillData(completion: #escaping: () -> Void){
/// toDo
completion()
}
//___________________________
//The call for this function can be in either way's #escaping or nonEscaping :
fillData{
/// toDo
}
/* again the deference between the two is that the function can be terminated before finish of execution nonEscaping closure in the other hand the #escaping closure guarantees that the function execution will not be terminated before the end of #escaping closure execution. Hope that helps ***#(NOTE THAT THE CLOSURE CAN BE OF ANY SWIFT DATA TYPE EVEN IT CAN BE TYPEALIAS)*/
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
}
}