I am implementing download request from a remote source and i ran into the notion of #escaping function. As Apple says:
A closure is said to escape a function when the closure is passed as
an argument to the function, but is called after the function returns.
But I actually noticed (with the breakpoint tool) that it calls and implements before return statement.
static func fetchFeaturedApps(completionHandler: #escaping ([AppCategory]) -> ()) {
let urlString = "https://api.letsbuildthatapp.com/appstore/featured"
URLSession.shared.dataTask(with: URL(string: urlString)!) {
(data, response,error) -> Void in
if error != nil {
print(error?.localizedDescription)
return
}
do {
let json = try(JSONSerialization.jsonObject(with: data!, options: .mutableContainers)) as! Dictionary<String, Any>
var appCategories = [AppCategory]()
// invokes before return [![enter image description here][1]][1]
completionHandler(appCategories)
for dict in json["categories"] as! [[String: Any]] {
let appCategory = AppCategory()
appCategory.setValuesForKeys(dict)
appCategories.append(appCategory)
}
print(appCategories)
DispatchQueue.main.async {
// completionHandler(appCategories)
}
} catch let error as NSError {
print(error.localizedDescription)
}
}.resume()
}
Then and of course after it deals with "completionHandler" it goes on implementing function further, as if i send it plain closure. It turns out #escaping closure call before return statement, strictly in the place where I call it in function body.
But I think maybe I am wrong? Maybe Apple keeps in mind another scenario? Please how do I need to understand #escaping notation with Apples quote about calling them after return? Actually in the example it calls before return , why?
You said:
It turns out #escaping closure [is called] strictly in the place where I call it in function body.
Yep, that's exactly what happens. It's called wherever you place it in your code. If you happen to call it before you return from the method, that's what it's going to do.
As others have pointed out, the fact that it is declared as #escaping means that it can be called later, not that it will necessarily be called later.
In fact, this pattern of calling an #escaping closure synchronously (i.e. before the method returns) is not uncommon. For example, you'll see this if you're dealing with network requests where responses can be cached. In that scenario, you might check your cache and call the closure immediately if the resource has already been retrieved, but call the closure asynchronously if it wasn't previously cached and you have to now retrieve the resource asynchronously from the web. E.g., you might have something like:
func fetchImage(for identifier: String, completion: #escaping (UIImage?) -> Void) {
if let image = cache.retrieveImage(for: identifier) {
completion(image)
return
}
webService.fetchImageAsynchronously(for: identifier) { image in
completion(image)
}
}
Note, just because the closure is designated as #escaping, that doesn't mean that my code path is required to call it asynchronously, regardless. I can call the closure either synchronously or asynchronously, whatever makes sense.
That having been said, if you have a method where you know that you will always call the closure synchronously, you would not use the #escaping designation. Not only would gratuitous use of #escaping in non-escaping scenarios make one's code unclear, but the #escaping designation prevents the compiler from performing certain types of optimizations. So we only use #escaping where it's needed, i.e. in those cases that we know it will or can be called asynchronously.
As it is said in documentation:
A closure is said to escape a function when the closure is passed as an argument to the function, but is called after the function returns. When you declare a function that takes a closure as one of its parameters, you can write #escaping before the parameter’s type to indicate that the closure is allowed to escape.
Declaring a closure #escaping does not make it execute after your function returns. It just means that it may or may not execute after function returns, which completely depends on your code.
Concerning your code, your closure is directly called in the function context, so it is no wonder that closure is executed before the function returns. If you would like to make it execute after the function returns, you may want to use some multithreading mechanism, like GCD or OperationQueue.
Related
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.
I am using Firebase to observe event and then setting an image inside completion handler
FirebaseRef.observeSingleEvent(of: .value, with: { (snapshot) in
if let _ = snapshot.value as? NSNull {
self.img = UIImage(named:"Some-image")!
} else {
self.img = UIImage(named: "some-other-image")!
}
})
However I am getting this error
Closure cannot implicitly capture a mutating self parameter
I am not sure what this error is about and searching for solutions hasn't helped
The short version
The type owning your call to FirebaseRef.observeSingleEvent(of:with:) is most likely a value type (a struct?), in which case a mutating context may not explicitly capture self in an #escaping closure.
The simple solution is to update your owning type to a reference once (class).
The longer version
The observeSingleEvent(of:with:) method of Firebase is declared as follows
func observeSingleEvent(of eventType: FIRDataEventType,
with block: #escaping (FIRDataSnapshot) -> Void)
The block closure is marked with the #escaping parameter attribute, which means it may escape the body of its function, and even the lifetime of self (in your context). Using this knowledge, we construct a more minimal example which we may analyze:
struct Foo {
private func bar(with block: #escaping () -> ()) { block() }
mutating func bax() {
bar { print(self) } // this closure may outlive 'self'
/* error: closure cannot implicitly capture a
mutating self parameter */
}
}
Now, the error message becomes more telling, and we turn to the following evolution proposal was implemented in Swift 3:
SE-0035: Limiting inout capture to #noescape contexts
Stating [emphasis mine]:
Capturing an inout parameter, including self in a mutating
method, becomes an error in an escapable closure literal, unless the
capture is made explicit (and thereby immutable).
Now, this is a key point. For a value type (e.g. struct), which I believe is also the case for the type that owns the call to observeSingleEvent(...) in your example, such an explicit capture is not possible, afaik (since we are working with a value type, and not a reference one).
The simplest solution to this issue would be making the type owning the observeSingleEvent(...) a reference type, e.g. a class, rather than a struct:
class Foo {
init() {}
private func bar(with block: #escaping () -> ()) { block() }
func bax() {
bar { print(self) }
}
}
Just beware that this will capture self by a strong reference; depending on your context (I haven't used Firebase myself, so I wouldn't know), you might want to explicitly capture self weakly, e.g.
FirebaseRef.observeSingleEvent(of: .value, with: { [weak self] (snapshot) in ...
Sync Solution
If you need to mutate a value type (struct) in a closure, that may only work synchronously, but not for async calls, if you write it like this:
struct Banana {
var isPeeled = false
mutating func peel() {
var result = self
SomeService.synchronousClosure { foo in
result.isPeeled = foo.peelingSuccess
}
self = result
}
}
You cannot otherwise capture a "mutating self" with value types except by providing a mutable (hence var) copy.
Why not Async?
The reason this does not work in async contexts is: you can still mutate result without compiler error, but you cannot assign the mutated result back to self. Still, there'll be no error, but self will never change because the method (peel()) exits before the closure is even dispatched.
To circumvent this, you may try to change your code to change the async call to synchronous execution by waiting for it to finish. While technically possible, this probably defeats the purpose of the async API you're interacting with, and you'd be better off changing your approach.
Changing struct to class is a technically sound option, but doesn't address the real problem. In our example, now being a class Banana, its property can be changed asynchronously who-knows-when. That will cause trouble because it's hard to understand. You're better off writing an API handler outside the model itself and upon finished execution fetch and change the model object. Without more context, it is hard to give a fitting example. (I assume this is model code because self.img is mutated in the OP's code.)
Adding "async anti-corruption" objects may help
I'm thinking about something among the lines of this:
a BananaNetworkRequestHandler executes requests asynchronously and then reports the resulting BananaPeelingResult back to a BananaStore
The BananaStore then takes the appropriate Banana from its inside by looking for peelingResult.bananaID
Having found an object with banana.bananaID == peelingResult.bananaID, it then sets banana.isPeeled = peelingResult.isPeeled,
finally replacing the original object with the mutated instance.
You see, from the quest to find a simple fix it can become quite involved easily, especially if the necessary changes include changing the architecture of the app.
If someone is stumbling upon this page (from search) and you are defining a protocol / protocol extension, then it might help if you declare your protocol as class bound. Like this:
protocol MyProtocol: class {
...
}
You can try this! I hope to help you.
struct Mutating {
var name = "Sen Wang"
mutating func changeName(com : #escaping () -> Void) {
var muating = self {
didSet {
print("didSet")
self = muating
}
}
execute {
DispatchQueue.global(qos: .background).asyncAfter(deadline: .now() + 15, execute: {
muating.name = "Wang Sen"
com()
})
}
}
func execute(with closure: #escaping () -> ()) { closure() }
}
var m = Mutating()
print(m.name) /// Sen Wang
m.changeName {
print(m.name) /// Wang Sen
}
Another solution is to explicitly capture self (since in my case, I was in a mutating function of a protocol extension so I couldn't easily specify that this was a reference type).
So instead of this:
functionWithClosure(completion: { _ in
self.property = newValue
})
I have this:
var closureSelf = self
functionWithClosure(completion: { _ in
closureSelf.property = newValue
})
Which seems to have silenced the warning.
Note this does not work for value types so if self is a value type you need to be using a reference type wrapper in order for this solution to work.
So, I am trying to do a Alamofire request, then, I'd take the information I need from the JSON data and put it into a global variable, here's my code.
struct myVariables {
static var variableOne = ""
}
func function() {
Alamofire.request(.GET, "API URL").responseJSON { response in
if let rawJSON = response.result.value {
// Here I just take the JSON and put it into dictionaries and parse the data.
myVariables.variableOne = String("data")
}
}
}
Ok, so basically, I am trying to access variableOne's data from another Swift file. Let's say I made two Swift files and in one of those files I had a function that edited the value of global variable, in the other file, if I attempted to print that global variable, I'd see the edited value. But whenever I use Alamofire, when I try to edit a global variable, the other Swift file doesn't see the changed value. So if I tried to edit the global variable within the Alamofire request block of code, I don't see the change whenever I print the variable from another file.
If anyone knows a better way to phrase that, please do correct it.
I suspect the problem isn't that you're not seeing the value change, but rather an issue arising from the fact that you're dealing with an asynchronous method. For example, when you call function, it returns immediately, but your variableOne may not be updated immediately, but rather later. I bet you're checking it before this asynchronous response closure had a chance to be called.
You wouldn't have this problem if, rather than using this "global" (which is a bad idea, anyway), you instead adopted the completion handler pattern yourself.
func performRequest(completionHandler: #escaping (String?, Error?) -> Void) {
Alamofire.request("API URL").responseJSON { response in
switch response.result {
case .failure(let error):
completionHandler(nil, error)
case .success(let responseObject):
let dictionary = responseObject as? [String: Any]
let string = dictionary?["someKey"] as? String
completionHandler(string, nil)
}
}
}
An you'd call this like so:
performRequest() { string, error in
guard let string = string, error == nil else {
// handle error here
return
}
// use `string` here
}
// but not here, because the above closure runs asynchronously (i.e. later)
By using this completion handler pattern, we solve the "how do I know when the asynchronous method is done" problem. And by passing the necessary data back as a parameter of the closure, we can excise the use of globals, keeping the scope of our data as narrow as possible.
Clearly, you should change the parameter of the closure to match whatever is appropriate in your case. But hopefully this illustrates the basic idea.
See previous revision of this answer for Swift 2/Alamofire 3 answer.
I'm trying to pass a completionHandler as a parameter in a function (no problem here).
My problem is that I have multiple precise Types possible that I can recieve in my completionHandler function.
So I thought, "Let's use templates", and I tried.
This is the scheme I want to use:
FuncA(completionHandler as MyType?)
-> FuncB(..){completionHandler(Mappable?)}
-> FuncC(sender: T?){performSegueWithIdentifier("segue", sender)}
Problem:
Func A is printing me an error
Func B seems to be ok
Func C seems to be ok
Do you guy know how to do that, I'm not used to templates yet ??
Thanks for any help :)
I don't believe you can cast completionHandler like that in a method signature. You're going to need to do your typecasting inside the method body. e.g.
typealias handler = () -> Array<AnyObject>
funcA(handler)
func funcA<T>(completion: T?) -> funcB {
if let completion = completion as? handler {
let array = completion()
//do whatever you want here
}
}
brand new to Swift here. Trying to figure out how to do a very simple call back block from any asynchronous function I write.
For example:
func downloadData(completion: (success: Bool) -> Void){
let success: Bool
//Some asynchronous task here
success = true
//Asynchronous task finished
//Now I want to pass this back
completion(success)
}
I want to be able to call this function and get the value of the success variable in the block. However I'm getting an error "Missing argument label success in call". Don't understand what is going on here. Why would I need to include the argument label? Any pointers on this would be greatly appreciated!
You have the choice:
Either you add the label in the call
completion(success: success)
or you omit the label in the declaration
func downloadData(completion: (Bool) -> Void){
The rule is: all declared labels must be passed.