Here is my method to fetch some data from the network:
func fetchProducts(parameters: [String: Any],
success: #escaping ([Product]) -> Void)
As you noticed, it has escaping closure. Here is how I call above method in ViewModel:
service.fetchProducts(parameters: params, success: { response in
self.isLoading?(false)
/// doing something with response
})
The question is should I capture self weakly or strongly? Why? I think I can capture it strongly. Because, fetchProducts is a function which has closure as a parameter. But, I might be wrong. But, from other perspective, I think it should be weak. Because, ViewModel has strong reference to service, service has strong reference to success closure which has strong reference to self (which is ViewModel). It creates retain cycle. But deinit of ViewModel is called anyway, after ViewController which owns ViewModel is deinitialized. It means that there was no retain cycle. Why?
As long as your viewmodel is a class, you have to capture self weakly, otherwise you'll have a strong reference cycle. Since fetchProducts is asynchronous, its success closure might be executed after your viewmodel has already been deallocated - or would have been deallocated if the closure wasn't holding a strong reference to it. The strong reference in the async closure will block the viewmodel from being deallocated.
If you call service.fetchProducts in a class AND access self inside the async closure, you do need [weak self]. If you were to do this in a value type (struct or enum) OR if you didn't access self inside the closure, you don't need [weak self] - in a value type, you cannot even do [weak self].
service.fetchProducts(parameters: params, success: { [weak self] response in
self?.isLoading?(false)
/// doing something with response
})
Related
Okay so we all know that in traditional concurrency in Swift, if you are performing (for example) a network request inside a class, and in the completion of that request you reference a function that belongs to that class, you must pass [weak self] in, like this:
func performRequest() {
apiClient.performRequest { [weak self] result in
self?.handleResult(result)
}
}
This is to stop us strongly capturing self in the closure and causing unnecessary retention/inadvertently referencing other entities that have dropped out of memory already.
How about in async/await? I'm seeing conflicting things online so I'm just going to post two examples to the community and see what you think about both:
class AsyncClass {
func function1() async {
let result = await performNetworkRequestAsync()
self.printSomething()
}
func function2() {
Task { [weak self] in
let result = await performNetworkRequestAsync()
self?.printSomething()
}
}
func function3() {
apiClient.performRequest { [weak self] result in
self?.printSomething()
}
}
func printSomething() {
print("Something")
}
}
function3 is straightforward - old fashioned concurrency means using [weak self].
function2 I think is right, because we're still capturing things in a closure so we should use [weak self].
function1 is this just handled by Swift, or should I be doing something special here?
Bottom line, there is often little point in using [weak self] capture lists with Task objects. Use cancelation patterns instead.
A few detailed considerations:
Weak capture lists are not required.
You said:
in traditional concurrency in Swift, if you are performing (for example) a network request inside a class, and in the completion of that request you reference a function that belongs to that class, you must pass [weak self] …
This is not true. Yes, it may be prudent or advisable to use the [weak self] capture list, but it is not required. The only time you “must” use a weak reference to self is when there is a persistent strong reference cycle.
For well-written asynchronous patterns (where the called routine releases the closure as soon as it is done with it), there is no persistent strong reference cycle risk. The [weak self] is not required.
Nonetheless, weak capture lists are useful.
Using [weak self] in these traditional escaping closure patterns still has utility. Specifically, in the absence of the weak reference to self, the closure will keep a strong reference to self until the asynchronous process finishes.
A common example is when you initiate a network request to show some information in a scene. If you dismiss the scene while some asynchronous network request is in progress, there is no point in keeping the view controller in memory, waiting for a network request that merely updates the associated views that are long gone.
Needless to say, the weak reference to self is really only part of the solution. If there’s no point in retaining self to wait for the result of the asynchronous call, there is often no point in having the asynchronous call continue, either. E.g., we might marry a weak reference to self with a deinit that cancels the pending asynchronous process.
Weak capture lists are less useful in Swift concurrency.
Consider this permutation of your function2:
func function2() {
Task { [weak self] in
let result = await apiClient.performNetworkRequestAsync()
self?.printSomething()
}
}
This looks like it should not keep a strong reference to self while performNetworkRequestAsync is in progress. But the reference to a property, apiClient, will introduce a strong reference, without any warning or error message. E.g., below, I let AsyncClass fall out of scope at the red signpost, but despite the [weak self] capture list, it was not released until the asynchronous process finished:
The [weak self] capture list accomplishes very little in this case. Remember that in Swift concurrency there is a lot going on behind the scenes (e.g., code after the “suspension point” is a “continuation”, etc.). It is not the same as a simple GCD dispatch. See Swift concurrency: Behind the scenes.
If, however, you make all property references weak, too, then it will work as expected:
func function2() {
Task { [weak self] in
let result = await self?.apiClient.performNetworkRequestAsync()
self?.printSomething()
}
}
Hopefully, future compiler versions will warn us of this hidden strong reference to self.
Make tasks cancelable.
Rather than worrying about whether you should use weak reference to self, one could consider simply supporting cancelation:
var task: Task<Void, Never>?
func function2() {
task = Task {
let result = await apiClient.performNetworkRequestAsync()
printSomething()
task = nil
}
}
And then,
#IBAction func didTapDismiss(_ sender: Any) {
task?.cancel()
dismiss(animated: true)
}
Now, obviously, that assumes that your task supports cancelation. Most of the Apple async API does. (But if you have written your own withUnsafeContinuation-style implementation, then you will want to periodically check Task.isCancelled or wrap your call in a withTaskCancellationHandler or other similar mechanism to add cancelation support. But this is beyond the scope of this question.)
if you are performing (for example) a network request inside a class, and in the completion of that request you reference a function that belongs to that class, you must pass [weak self] in, like this
This isn't quite true. When you create a closure in Swift, the variables that the closure references, or "closes over", are retained by default, to ensure that those objects are valid to use when the closure is called. This includes self, when self is referenced inside of the closure.
The typical retain cycle that you want to avoid requires two things:
The closure retains self, and
self retains the closure back
The retain cycle happens if self holds on to the closure strongly, and the closure holds on to self strongly — by default ARC rules with no further intervention, neither object can be released (because something has retained it), so the memory will never be freed.
There are two ways to break this cycle:
Explicitly break a link between the closure and self when you're done calling the closure, e.g. if self.action is a closure which references self, assign nil to self.action once it's called, e.g.
self.action = { /* Strongly retaining `self`! */
self.doSomething()
// Explicitly break up the cycle.
self.action = nil
}
This isn't usually applicable because it makes self.action one-shot, and you also have a retain cycle until you call self.action(). Alternatively,
Have either object not retain the other. Typically, this is done by deciding which object is the owner of the other in a parent-child relationship, and typically, self ends up retaining the closure strongly, while the closure references self weakly via weak self, to avoid retaining it
These rules are true regardless of what self is, and what the closure does: whether network calls, animation callbacks, etc.
With your original code, you only actually have a retain cycle if apiClient is a member of self, and holds on to the closure for the duration of the network request:
func performRequest() {
apiClient.performRequest { [weak self] result in
self?.handleResult(result)
}
}
If the closure is actually dispatched elsewhere (e.g., apiClient does not retain the closure directly), then you don't actually need [weak self], because there was never a cycle to begin with!
The rules are exactly the same with Swift concurrency and Task:
The closure you pass into a Task to initialize it with retains the objects it references by default (unless you use [weak ...])
Task holds on to the closure for the duration of the task (i.e., while it's executing)
You will have a retain cycle if self holds on to the Task for the duration of the execution
In the case of function2(), the Task is spun up and dispatched asynchronously, but self does not hold on to the resulting Task object, which means that there's no need for [weak self]. If instead, function2() stored the created Task, then you would have a potential retain cycle which you'd need to break up:
class AsyncClass {
var runningTask: Task?
func function4() {
// We retain `runningTask` by default.
runningTask = Task {
// Oops, the closure retains `self`!
self.printSomething()
}
}
}
If you need to hold on to the task (e.g. so you can cancel it), you'll want to avoid having the task retain self back (Task { [weak self] ... }).
I am trying to clear my code form memory leaks and I am not sure in some situations. I am adding capture lists to all my closures to make them stop capturing and making retain cycles, but not sure about functions passed to closure form arguments... onInternetFailed gets to closure and gets strongly captured.
Situation like this:
public func send<Data>(_ operation: CSOperation<Data>, _ title: String, _ isProgress: Bool,
_ canCancel: Bool, _ isFailedDialog: Bool, _ onInternetFailed: (() -> Void)?,
_ onSuccess: ((Data) -> Void)?) -> CSOperation<Data> {
let process = operation.send(listenOnFailed: false).process!
if isProgress {
let cancelAction = canCancel ? CSDialogAction(title: .cs_dialog_cancel) { [unowned operation] in
operation.cancel()
} : nil
let progress = show(progress: title, cancel: cancelAction)
process.onDone { [unowned progress] _ in progress.hideDialog() }
}
//TODO : does function get captured strongly in closure ?
process.onFailed { [unowned self, unowned operation] failed in
onProcessFailed(operation, failed, title, isProgress, isFailedDialog, onInternetFailed, onSuccess)
}
onSuccess.notNil { [unowned process] in process.onSuccess($0) }
return operation
}
Closures (functions) have reference semantics and will always be captured strongly. In fact, you cannot change the capture mode to weak or unowned. If you think about it, it wouldn't make sense either.
When you deal with completion handlers, the best practice you can follow is to ensure that the completion handlers will be called eventually. This ensures, the closure is released (actually the objects it references).
It's a common programmer error to forget to call a completion handler, or to call it twice. A completion handler must be called once (eventually) and only once. For example, check CSOperation if it actually calls either onFailed or onSuccess when the task completes, when it bails out early, or in any other possible case.
Update
When analysing your code, the object operation returns an object process (presumably holding a strong reference itself).
This process value has a closure value onFailed which will be assigned a closure which imports unowned self, unowned operation and two other closures onInternetFailed and onSuccess.
(I omit the other details).
When you now look at it, it's the value operation that is responsible to hold everything together.
Note also, that there is nowhere a "completion handler" pattern *). Instead, your handlers are kept in instance variables. If these get called, they remain allocated.
So, even if your operation completes, and calls onFailed eventually - nothing gets deallocated.
It's your responsibility to set the "completion handlers" to nil after they have been called. Alternatively, set process to nil, alternatively set operation to `nil.
IMHO, the design should be made more simple and more easy to comprehend.
What I do generally, is to avoid storing "completion" handlers in instance variables. This opens a host of potential errors (due to reference cycles) which you cannot avoid in the code itself, but must be avoided by the caller by enforcing a convention and following strict rules which you have to document, which in turn leads to "leaking implementation details", ...
But you can alleviate the problems by ensuring your "completion handler" will be set to nil once it has been called.
Even, better avoid storing completion handlers in instance variables and apply the "completion handler pattern".
Completion handler pattern
The handler will not be stored in an object as an instance variable:
func doWorkAsync(completion: #escaping (Result) -> Void) {
self.workerQueue.async {
// work
completion(result)
}
}
"Operation Style" variant which clears the completion handler after completion:
class MyOperation {
var completion: ((Result) -> Void)?
init(completion: (Result) -> Void) {
self.completion = completion
}
func start() {
assert(self.completion != nil)
doWorkAsync { result in
let completion = self.completion
self.completion = nil
completion?(result)
}
}
}
Note that - in certain perspective - a Closure is nothing else than an Operation, and an operation can be represented as a Closure. In other words, it's possible to refactor code using Operations and replace it with pure Closures, thus avoiding any issues stemming from using Operations.
We implemented and extension to NSData that asynchronously persists the data to a URL.
Here is a short version of the function.
extension NSData {
func writeToURL1(url:NSURL, completion: () -> Void) {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), { [weak self] in
guard let strongSelf = self else { return }
strongSelf.writeToURL(url, atomically: true)
completion()
})
}
}
Here is how we use it:
var imageData = UIImageJPEGRepresentation(image, 0.8)
imageData?.writeToURL(someURL) { in ... }
The problem of course is if imageData gets deallocated before the write operation is completed strongSelf will be nil and the completion handler will never be called.
There are two solutions to that problem.
remove [weak self] (i.e. have a strong reference to self in writeToURL1.
reference imageData in the completion block (i.e. imageData?.writeToURL(someURL) { in imageData = nil ... })
What approach is more Swift friendly and what approach should we choose?
Thank you!
You should use a strong reference if you don't want the object to go away from under your feet (and doing so wouldn't create a reference cycle); a weak reference if you don't care if the object goes away from under your feet; an unowned reference if you need the object to be around, and have a strong guarantee that the object won't go away from under your feet, but a strong reference would create a cycle.
In your case, you care that the object might go away, because that means that you won't be able to save it. As you have no guarantee that something else will keep the object alive until that task is completed, you should use a strong reference.
Strong references to self in closures are a problem when you carry the closure around because it's easy to end up with a non-obvious reference cycle, but you know for a fact that the reference will be dropped right after the closure is executed, so it's not a problem here.
It sounds like you just want to strongly capture the reference to the NSData object, so removing the [weak self] is the easiest and best approach in my opinion. Usually, weak references are used to avoid retain cycles when capturing in a closure. However, you're not actually creating a retain cycle, simply a one way retain by capturing self in the block. The block is not retained by self, it's simply passed down the call stack into dispatch_async, where it is ultimately invoked and deallocated. So there is no retain cycle to avoid by using weak self, you just have a retain occur via the closure which is desirable. That is, you want to keep the data in memory until that closure is invoked.
I am trying to grasp how I can recognize when a strong retain cycle is possible and requires me to use [weak/unowned self]. I've been burned by unnecessarily using [weak/unowned self] and the self was deallocated immediately before giving me a chance to use it.
For example, below is an async network call that refers to self in the closure. Can a memory leak happen here since the network call is made without storing the call it self into a variable?
NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: url)!) {
(data, response, error) in
self.data = data
)
Here's another example using the NSNotificationCenter, where a call can be made later asynchronously:
NSNotificationCenter.defaultCenter().addObserverForName(
UIApplicationSignificantTimeChangeNotification, object: nil, queue: nil) {
[unowned self] _ in
self.refresh()
}
My question is in what cases is a strong retain cycle possible? If I am making an asynchronous call or static call that references self in a closure, does that make it a candidate for [weak/unowned self]? Thanks for shedding any light on this.
A retain cycle is a situation when two objects has a strong reference to each other.
You are working with static variables NSURLSession.sharedSession() & NSNotificationCenter.defaultCenter() and as you may remember:
A singleton object provides a global point of access to the resources
of its class...You obtain the global instance from a singleton class
through a factory method. The class lazily creates its sole instance
the first time it is requested and thereafter ensures that no other
instance can be created. A singleton class also prevents callers from
copying, retaining, or releasing the instance.
https://developer.apple.com/library/ios/documentation/General/Conceptual/DevPedia-CocoaCore/Singleton.html
Your "self" instance (like the others) doesn't have a strong reference to singletons objects and its closures too, that's why you don't have to worry about retain cycle in your case.
Check this great article for more details:
https://digitalleaves.com/blog/2015/05/demystifying-retain-cycles-in-arc/
In a nutshell:
The retain cycle can happen in two cases.
Case 1:
When two instances hold a strong reference to each other. You have to solve this by marking one of them as weak.
Case 2: (Which is related to your questions)
If you assign a closure to a property of a class instance and the body of that closure captures the instance.
In your two examples, no need to use weak self at all as NSNotificationCenter nor NSURLSession are properties to your class instance. (Or in other meaning, you don't have strong references to them)
Check this example where I have to use weak self:
[self.mm_drawerController setDrawerVisualStateBlock:^(MMDrawerController *drawerController, MMDrawerSide drawerSide, CGFloat percentVisible) {
if (drawerSide == MMDrawerSideRight && percentVisible == 1.0) {
[weakself showOverlayBgWithCloseButton:YES];
}
else{
[weakself hideOverlayBg];
}
}];
I have a strong reference to mm_drawerController and I assign a closure to it right?. inside this closure I want to capture self. So the closure will have a strong reference to self !! which is a disaster. In that case you will have a retain cycle. To break this cycle, use weak self inside the closure.
Many posts seem to advise against notifications when trying to synchronize functions, but there are also other posts which caution against closure callbacks because of the potential to inadvertently retain objects and cause memory issues.
Assume inside a custom view controller is a function, foo, that uses the Bar class to get data from the server.
class CustomViewController : UIViewController {
function foo() {
// Do other stuff
// Use Bar to get data from server
Bar.getServerData()
}
}
Option 1: Define getServerData to accept a callback. Define the callback as a closure inside CustomViewController.
Option 2: Use NSNotifications instead of a callback. Inside of getServerData, post a NSNotification when the server returns data, and ensure CustomViewController is registered for the notification.
Option 1 seems desirable for all the reasons people caution against NSNotification (e.g., compiler checks, traceability), but doesn't using a callback create a potential issue where CustomViewController is unnecessarily retained and therefore potentially creating memory issues?
If so, is the right way to mitigate the risk by using a callback, but not using a closure? In other words, define a function inside CustomViewController with a signature matching the getServerData callback, and pass the pointer to this function to getServerData?
I'm always going with Option 1 you just need to remember of using [weak self] or whatever you need to 'weakify' in order to avoid memory problems.
Real world example:
filterRepository.getFiltersForType(filterType) { [weak self] (categories) in
guard let strongSelf = self, categories = categories else { return }
strongSelf.dataSource = categories
strongSelf.filteredDataSource = strongSelf.dataSource
strongSelf.tableView?.reloadData()
}
So in this example you can see that I pass reference to self to the completion closure, but as weak reference. Then I'm checking if the object still exists - if it wasn't released already, using guard statement and unwrapping weak value.
Definition of network call with completion closure:
class func getFiltersForType(type: FilterType, callback: ([FilterCategory]?) -> ()) {
connection.getFiltersCategories(type.id).response { (json, error) in
if let data = json {
callback(data.arrayValue.map { FilterCategory(attributes: $0) } )
} else {
callback(nil)
}
}
}
I'm standing for closures in that case. To avoid unnecessary retains you just need to ensure closure has proper capture list defined.