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Closure use of non-escaping parameter may allow it to escape
This is due to a change in the default behaviour for parameters of function type. Prior to Swift 3 (specifically the build that ships with …
Source: stackoverflow.com
Date Published: 4/18/2021
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Escaping closure captures non-esca… – Apple Developer
I get the error “Escaping closure captures non-escaping parameter ‘action'” on lines 2 and 4. What am I doing wrong and how can I fix this?
Source: developer.apple.com
Date Published: 7/11/2022
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Escaping and Non-Escaping Closures in Swift | by Kiran
It says escaping closure captures non-escaping parameter ‘completion . What does it mean? Let’s ask the Swift documentation.
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Date Published: 12/23/2021
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In Swift 1.x and Swift 2.x, closure parameter was @escaping by default, means that closure can be escape during the function body execution. if don’t want to …
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Swift: Escaping closure captures non-escaping parameter ‘onCompletion’ I have a problem with my swift. I am trying an …
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Escaping Closures in Swift Explained – AppyPie
In Swift, closures are non-escaping by default. This means that the closure can’t outlive the function it was passed into as a parameter. If you …
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Swift – Closure use of non-escaping parameter may allow it to …
Swift – Closure use of non-escaping parameter may allow it to escape. closuresswiftswift3xcode. I have a protocol: enum DataFetchResult { case success(data: …
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주제에 대한 기사 평가 escaping closure captures non-escaping parameter
- Author: Knowledge Base
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- Date Published: 2022. 1. 24.
- Video Url link: https://www.youtube.com/watch?v=KckbyD-sYO8
What is escaping and non-escaping closures?
Escaping closures are different from non-escaping closures since we can preserve escaping closures to execute them later on. Meanwhile, the function body can execute and returns the compiler back. The scope of the escaping closure exists and has existence in memory as well until it gets executed.
What is an 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. A non-escaping closure is a closure that’s called within the function it was passed into, i.e. before it returns.
What does @escaping do Swift?
In Swift, a closure marked with @escaping means the closure can outlive the scope of the block of code it is passed into. In a sense, @escaping tells the closure to “stay up even after the surrounding function is gone”.
What are closures in Swift?
In Swift, a closure is a special type of function without the function name. For example, { print(“Hello World”) } Here, we have created a closure that prints Hello World . Before you learn about closures, make sure to know about Swift Functions.
What is capture value in Swift?
If you use any external values inside your closure, Swift captures them – stores them alongside the closure, so they can be modified even if they don’t exist any more.
What is capture list in closure?
Capture lists: The list of values that you want to remain unchanged at the time the closure is created. Capturing values: If you use external values inside your closure, Swift stores them alongside the closure. This way they persist even when the external part of the code no longer exists.
How do closures capture references to variables by default?
In Swift, closures capture the variables they reference: variables declared outside of the closure but that you use inside the closure are retained by the closure by default, to ensure they are still alive when the closure is executed.
What is difference between closure and completion handler Swift?
As an example, many functions that start an asynchronous operation take a closure argument as a completion handler. The function returns after it starts the operation, but the closure isn’t called until the operation is completed—the closure needs to escape, to be called later.
What is a trailing closure in Swift?
If the last parameter to a function is a closure, Swift lets you use special syntax called trailing closure syntax. Rather than pass in your closure as a parameter, you pass it directly after the function inside braces.
What is completion handler in IOS?
A completion handler in Swift is a function that calls back when a task completes. This is why it is also called a callback function. A callback function is passed as an argument into another function.
How do I pass a completion handler in Swift?
Make a Completion handler :
Create a completion handler closure and than pass it to function. Here we create an completion handler closure which type is () → Void . Now We will create a function which can take this closure .. Here we create a function which take a parameter of type () →Void .
Why closures are reference types?
Closures are reference types. If you assign a closure to a variable and you copy the variable into another variable, you also copy a reference to the same closure and its capture list.
What is difference between function and closure?
Difference between Function and Closure
Function is declared using func keyword whereas Closure doesn’t have func keyword. Function has always name but Closure doesn’t have. Function doesn’t have in keyword but closure has in the keyword.
How many types of closures are there in IOS Swift?
As shown in the above table, there are three types of closures in Swift, namely global functions, nested functions, and closure expressions. They differ in several aspects, including their use scopes, their names, and whether they capture values, which will be discussed more in a later section.
What is the difference between closure and function in Swift?
Difference between Function and Closure
Function is declared using func keyword whereas Closure doesn’t have func keyword. Function has always name but Closure doesn’t have. Function doesn’t have in keyword but closure has in the keyword.
Why we use closures in Swift?
Closures can capture and store references to any constants and variables from the context in which they’re defined. This is known as closing over those constants and variables. Swift handles all of the memory management of capturing for you.
Are closures value or reference types?
The Basics. In Swift, structs, enums and tuples are all value types, while classes and closures are reference types. In a nutshell, a value type contains data and a reference type contains the location in memory where data lives.
What is a completion handler in Swift?
A completion handler in Swift is a function that calls back when a task completes. This is why it is also called a callback function. A callback function is passed as an argument into another function. When this function completes running a task, it executes the callback function.
Closure use of non-escaping parameter may allow it to escape
This is due to a change in the default behaviour for parameters of function type. Prior to Swift 3 (specifically the build that ships with Xcode 8 beta 6), they would default to being escaping – you would have to mark them @noescape in order to prevent them from being stored or captured, which guarantees they won’t outlive the duration of the function call.
However, now @noescape is the default for function-typed parameters. If you want to store or capture such functions, you now need to mark them @escaping :
protocol DataServiceType { func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void) func cachedData(location: String) -> Data? }
func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void) { // … }
See the Swift Evolution proposal for more info about this change.
Escaping closure captures non-esca…
Had a reply all typed out, switched tabs, switched back and it’s all gone. !@$* new forums.
You should be able to fix it by marking that action parameter as @escaping. It has to be marked as such because the parameter goes out of scope before the closure is executed.
Escaping and Non-Escaping Closures in Swift
next → ← prev Escaping and Non-Escaping Closures in Swift In swift, closures can be defined as the self-contained block of code that can be passed in methods or used in our code. However, we can define two types of closures, i.e., escaping and non-escaping closures. In swift 5, closure parameters are non-escaping by default. Closures can also be executed within the function body; if we require escaping closure, we can mark it as @escaping. Closures can capture and store references to any constants and variables from the context in which they’re defined. This is known as closing over those constants and variables. In this article, we will discuss escaping and non-escaping closures. We will also discuss why we should use escaping closures in our code. Non-Escaping Closures When we pass a closure as an argument to the function, the function executes the closure and returns the compiler. Here, as soon as the closure gets executed and execution ends, the passed closure goes out of the scope, and therefore, it has no more existence in the memory. The non-escaping closures transitions into the following states in their lifecycle span. Closure gets passed as the function argument in the function call. Function body gets executed. The closure gets executed along with the function body. The function returns the compiler back. Example Let’s consider the following example in which the function accepts a non-escaping closure as the argument. func calculate_Sum(_ array:Array , performSum:((Int) -> Void)){ var sum = 0 for i in array{ sum = sum + i } performSum(sum) } func calculation(){ var array = [0,1,2,3,4,5,6,7,8,9] calculate_Sum(array) { (sum) in debugPrint(sum) } } In the above example, we passed the closure in a function that prints the sum of the array being passed as other argument in the same function. Here, the function calculate_Sum performs the closure execution at the end of the body. Here, we are not escaping the execution of the closure. Therefore, the closure has no existence in the memory once it gets executed. Escaping Closures Escaping closures are different from non-escaping closures since we can preserve escaping closures to execute them later on. Meanwhile, the function body can execute and returns the compiler back. The scope of the escaping closure exists and has existence in memory as well until it gets executed. We can use the escaping closures according to our requirements. There are the following ways to escape the closure. 1. Storage One way to escape closure is to preserve the closure in the variable defined outside of the function and execute it later. This approach is used in the scenario where we need to use the function that starts an asynchronous operation and takes closure argument as the completion handler. The function returns after it starts then operation, but the closure argument is preserved to be called later on. Consider the following example in which the calculate_Sum() accepts an escaping closure as the argument preserved in completionHandler, which is a similar closure type variable. Here, the completionHandler is called after the function gets executed. var complitionHandler: ((Int)->Void)? var sum = 0 func calculate_Sum(_ array:Array , performSum: @escaping ((Int) -> Void)){ for i in array{ sum = sum + i } complitionHandler = performSum } func calculation(){ sum = 0 var array = [0,1,2,3,4,5,6,7,8,9] calculate_Sum(array) { (sum) in debugPrint(sum) } } calculation() complitionHandler!(sum) Asynchronous Execution When we execute closures asynchronously on DispatchQueue, the closure is preserved in the memory by the queue to use in the future. In this case, we cannot determine when the closure will get executed. Consider the following example. func calculate_Sum(_ array:Array , performSum: @escaping ((Int) -> Void)){ var sum = 0 for i in array{ sum = sum + i } DispatchQueue.global().asyncAfter(deadline: .now() + 0.5) { performSum(sum) } } func calculation(){ let array = [0,1,2,3,4,5,6,7,8,9] calculate_Sum(array) { (sum) in debugPrint(sum) } } calculation() Next Topic #
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Escaping Closures in Swift Explained
Escaping Closures in Swift Explained
Aasif Khan | Last Updated on December 3rd, 2021 10:37 am | 5-min read By
A closure is said to “escape” a function when it’s called after that function returns. In Swift, closures are non-escaping by default. What are escaping closures, anyway? And what do you use @escaping for? Let’s find out!
This tutorial discusses closures, escaping vs. non-escaping, capture lists and retain cycles, and what that means for practical iOS development.
What we’ll get into:
What escaping closures are, and why you need them
How and when to use the @escaping keyword
Risks of escaping closures, such as strong reference cycles
Fixing a strong reference cycle in an escaping closure
Quick Recap: What’s a Closure?
Closures are awesome! You probably already know this, but a closure is a block of code you can pass around and use elsewhere in your code.
It’s kinda like a function you can assign to a variable. You can move the closure around in your code, and call it at a later point. This makes closures exceptionally useful as completion handlers, i.e. the code you call when a long-running task has finished.
Check this out:
let makeGreeting: (String) -> String = { name in
return “Hello, \(name)!”
}
let message = makeGreeting(“Bob”)
print(message)
// Output: Hello, Bob!
In the example above the constant makeGreeting contains a closure. It has one parameter of type String, and it’s returning a value of String as well. It works the same way as a function.
When the closure is called in the above example, with makeGreeting(“Bob”), its value is assigned to constant message and printed out.
A common use-case for a closure is a so-called completion handler. Say you’re downloading an image from the web that you want to display in a view controller. Your code could look something like this:
let task = URLSession.shared.dataTask(with: URL(string: “http://example.com/cats.jpg”)!, completionHandler: { data, response, error in
// Do something with image data…
})
In the above code, we’re starting a data task with URLSession; making a HTTP request to download some cats.jpg file. The argument for the completionHandler parameter of the dataTask() function is a closure. It’s called when the image has been downloaded, hence the name completion handler.
Closures are powerful. In the example above, we’re writing the response to the data task in the same “place” in the code as the initial request. That means we can start the request and handle it, as if both actions follow each other immediately.
There’s a bit of time between the start of the request and its completion, but we can code that as if it’s immediate. That makes reading the code and reasoning about it much easier!
The code between the squiggly brackets { } is passed into the dataTask(with:completionHandler:) function. Passing around closures is a core principle, and it’s what escaping closures are about, too. Closures that outlive the context that declared them can potentially cause a memory leak, as you’ll soon see!
Escaping Closures in Swift
Let’s take a look at the declaration of that dataTask(with:completionHandler:) function, as found in the Swift Foundation framework’s source code:
func dataTask(with url: URL,
completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask
Right there, you can see that this function has 2 parameters:
url of type URL – the URL resource we’re going to download/request completionHandler – a closure
You can also see that the closure itself has 3 parameters, and returns Void.
Data? – the data that the request itself returns (i.e., the image) URLResponse? – an object that contains information about the response Error? – an optional value that contains an error object or nil
The function declaration also contains a special keyword: @escaping. This means that the closure that’s passed in as an argument for this parameter can escape the dataTask(with:completionHandler:) function. In other words, the closure is called after the function dataTask() finishes executing.
Here’s the official definition of an escaping closure:
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.
It can be helpful to think of an escaping closure as code that outlives the function it was passed into. Think of that function as a jail, and the closure as a bandit who’s escaping the jail.
An example of an escaping closure is the completion handler. Depending on its implementation, a completion handler is typically executed in the future. A length task completes long after it was initiated, which means that the closure escapes and outlives the function it was created in.
If we’ve got escaping closures, we can also have non-escaping closures:
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.
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. A non-escaping closure is a closure that’s called within the function it was passed into, i.e. before it returns. This closure never passes the bounds of the function it was passed into. .
In Swift, a closure is non-escaping by default. If a closure can escape the function, you’ll need to annotate its function parameter with the @escaping keyword. This is what happens in the code at the top of this section.
Here’s a quick code example of an escaping closure:
class TaskManager
{
var onTaskFinished:(() -> Void)?
func startLengthyTask(completionHandler: @escaping () -> Void)
{
// Store completion handler for later
onTaskFinished = completionHandler
// Do lengthy task
// …
}
func onLengthyTaskFinished() {
onTaskFinished?() // Call completion handler
}
}
let task = TaskManager()
task.startLengthyTask(completionHandler: {
// Do this when task has finished…
})
What’s going on in the code? We’ve created a class called TaskManager, with a function called startLengthyTask(completionHandler:). At the end of the code, we’re calling this function and pass a closure into it.
The closure for the completionHandler parameter is passed into the function. Inside the function, the closure is assigned to the onTaskFinished property of the TaskManager class (of the same closure type). It’s temporarily stored there.
At some point in the future, the function onLengthyTaskFinished() is called by the TaskManager class. This will invoke the previously stored completion handler, which will execute the closure we passed into it.
First, note that the closure type for completionHandler is marked with that @escaping property. This is an escaping closure. But how does it escape? With this line:
onTaskFinished = completionHandler
The closure that’s passed into the function is now assigned to a property on the TaskManager class. The closure will continue to exist, until after the startLengthyTask() function returns.
Think of the closure as a number – you can use that number after the function exits, because it’s now assigned to that property. Beyond the startLengthyTask() function, you cannot use its completionHandler parameter.
But because its value – the closure – is stored in the onTaskFinished property, you can use it later on. The closure has escaped the function it was passed into!
Why Use @Escaping?
What’s the problem with escaping closures? The code we’ve used so far runs OK. Why would we need to mark a closure as @escaping anyway?
In short, an escaping closure can cause a strong reference cycle if you use self inside the closure. Safety features like non-default, explicit @escaping are built into the Swift programming language, so you can remind yourself to check if you haven’t inadvertently caused a retain cycle.
The @escaping keyword isn’t a feature, it’s a warning sign! You’re letting the closure escape anyway, and once Swift finds out, it’ll help you avoid problems by forcing you to mark the closure as @escaping.
Before we move on, think back to the semantics of an “escaping” closure. In the previous example, you’ve seen that a closure can literally escape the function it was passed into by getting stored in a property outside that function.
When that escaping closure references self, or a strongly retained property, it will capture that reference strongly. If the escaping closure isn’t property released, you’ve created a strong reference cycle between self and the closure.
Here’s a quick shorthand:
A non-escaping closure can refer to self implicitly
An escaping closure must refer to self explicitly, or use a capture list
It’s impossible to create a strong reference cycle with a closure that’s non-escaping, because that closure doesn’t outlive the function it was passed into. The closure is released by the end of the function, and so are the values it captures.
It’s worth noting here that escaping closures work differently inside structs and classes. An escaping closure passed into a struct’s function cannot capture a mutable reference to self, because structs are value types. You cannot change a struct outside (“escaping”) a function marked with mutating; but only inside it.
It’s not that @escaping avoids a strong reference cycle; it just makes your code more explicit. It increases the safety of your code, just as with optionals, which decreases the likelyhood of accidentally creating bugs.
Strong Reference Cycles in Escaping Closures
Let’s take a look at a comprehensive example. We’re going to create an escaping closure that’ll reference self and causes a strong reference cycle. We’ll also discuss 2 potential solutions.
First, some code:
class NetworkManager
{
var onFinishRequest: ((String) -> Void)?
func makeRequest(completionHandler: @escaping (String) -> Void)
{
onFinishRequest = completionHandler
// Do lengthy task…
self.finishRequest()
}
func finishRequest() {
onFinishRequest?(“some data”)
}
}
The NetworkManager class is responsible for managing a lengthy task, like an HTTP request. You can best compare it with a bare-bones URLSession, for example.
In the above code, you can see a function makeRequest(). You can pass a closure into this function, which is called when a lengthy task completes. The closure escapes the function, because it’s assigned to the onFinishRequest property. After the lengthy task completes, finishRequest() will invoke the closure and pass some string data with it.
Next up, a class that uses the above code:
class API
{
var manager = NetworkManager()
var data = “”
init() {
print(“init API”)
}
func getData()
{
manager.makeRequest(completionHandler: { data in
self.data = data
print(“called completion handler with data: \(data)”)
})
}
deinit {
print(“deinit API”)
}
}
This API class has a function getData(), which calls the makeRequest() function on the NetworkManager object. It provides a closure for the completionHandler parameter of that function.
We’ve also marked the init and deinit functions. Especially the deinit function is important, because it’ll tell us if an instance of API is deallocated. When a strong reference cycle occurs, the API object won’t get deallocated.
What’s important to note here is that the closure strongly captures self. This happens with the self.data = data line of code. Why is that?
In Swift, references are strong by default The completionHandler closure in makeRequest() is escaping Closures will hold onto reference values you use inside them; this is called capturing or “closing over” (which is where closures get their name!)
Differently said, for the closure to hold onto self, so it can assign the closure’s data to the data property of self, a strong reference needs to be created. Otherwise, self would be gone by the time the lengthy task finishes.
If you’re a bit unclear on ARC, strong references and closures, check out my 3-part series about them, starting with: Automatic Reference Counting (ARC) in Swift
Next up, we’re going to try out 3 scenarios:
Run the above code and attempt to deallocate the API object Run the above code, and make sure self is referenced weakly .
Run the above code, and manually deallocate the closure
Let’s run the code, and see what happens. Here’s what we’re executing:
var api:API? = API()
api?.getData()
api = nil
// Output:
// init API
// called completion handler with data: some data
Would you look at that!? Even though we’re explicitly setting the api variable to nil, its deinit function is not called and the API object is not deallocated. This is a problem!
Let’s back up for a sec. What’s really going on here? Well, if we remove the call to getData(), you get the following output:
init API
deinit API
This tells us that setting a variable to nil will prompt the deallocation of its value. That’s how Automatic Reference Counting works; no one is holding onto the data, so it’s removed from memory to make space.
The culprit is getData(), but why? Inside that function, we’ve got a closure that captures a reference to self – the current class instance. A strong reference is created between self and the closure. This closure is retained strongly when it’s assigned to the onFinishRequest property.
Because the closure and self hold strongly onto each other, they cannot get deallocated. This is a strong reference cycle, which causes a memory leak. Less memory for other apps. That’s bad!
This reference cycle typically resolves itself if the closure doesn’t escape, but ours does! When a non-escaping closure is passed into a function, it’s automatically deallocated by the end of that function. This resolves the strong reference.
OK, how do you fix this? You’ve got 2 options:
Make the weak capture of self explicit with a capture list
Manually deallocate the closure when done, to break the cycle
First, the capture list. Check this out:
manager.makeRequest(completionHandler: { [weak self] data in
self?.data = data
})
The capture list [weak self] creates a weak reference between the closure and self. This prevents the strong reference cycle, because a weak reference doesn’t increase an object’s retain count. It does make the reference to self of an optional type, so you’ll need to unwrap self. (An alternative is unowned.)
When we run the code with the above change, here’s the output you get:
init API
called completion handler with data: some data
deinit API
Awesome. Crisis averted!
An alternative is to manually deallocate the closure. The trick is to break the cycle, and you can do that at both ends. Check this out:
class NetworkManager
{
func finishRequest() {
onFinishRequest?(“some data”)
onFinishRequest = nil
}
}
We’ve changed the finishRequest() function of the NetworkManager class. Right after invoking the closure that’s stored in the onFinishRequest property, we’re setting that property to nil. This deallocates that closure, which releases any strong references (if any) that the closure holds.
Resolving the strong reference cycle isn’t always viable this way, because it depends on the implementation of the NetworkManager code. It’ll need to hold onto that closure for some time, during which the strong reference persists.
Technically, making the onFinishRequest property weak would also break the strong reference cycle. It’ll also make the closure downright unusable, because the whole point is holding onto it until the lengthy task completes. You could also deallocate the NetworkManager instance, or overwrite the onFinishRequest property with something else. Of these alternatives, a capture list is the clearest and most reliable way to break the cycle.
Wrapping Up
In Swift, closures are non-escaping by default. This means that the closure can’t outlive the function it was passed into as a parameter. If you need to hold onto that closure after the function it was passed into returns, you’ll need to mark the closure with the keyword @escaping.
Escaping closures have an inherent risk: strong reference cycles. This happens when the closure strongly holds onto a reference, like self, and the closure itself is also retained. You can avoid a strong reference cycle by using a capture list, like [weak self].
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What Is @escaping in Swift [Updated in 2022]
In Swift, a closure marked with @escaping means the closure can outlive the scope of the block of code it is passed into.
In a sense, @escaping tells the closure to “stay up even after the surrounding function is gone”.
But why is this useful?
Let’s say you are performing an (asynchronous) network request such that:
Function f() fetches information from a web page.
fetches information from a web page. After completion, a closure function g() runs inside f().
The only way to make this possible is to pass the function g() as an escaping closure into the function f().
This tells the Swift compiler after calling f() to keep g() alive to wait for the network request to complete and the response to arrive.
In this guide, we are going to take a look at non-escaping closures and escaping closures in detail.
Also, you are going to see a demonstrative example to support understanding.
Closures in Swift
In Swift, closures are non-escaping by default.
If you pass a non-escaping closure into a function, it is called right away.
The life of the non-escaping closure ends when the function call finishes.
As a result, there will be no trace of that closure.
This is the default behavior.
Let’s see a simple example of a non-escaping closure and its behavior.
Please repeat the actions in this guide in your own code editor to truly understand what happens. ×
Let’s write a function add that takes: Integers a and b A closure called action
that takes: Right after summing the numbers up, the function add calls the action.
func add(a: Int, b: Int, _ action: (Int) -> Void) { var sum = a + b action(sum) }
Let’s call this function with integers 10 and 20 and a closure that prints the result into the console:
add(a: 10, b: 20, { res in print(res) })
Output:
30
As you can see, the result of adding 10 and 20 gets printed to the console right away.
However, this default behavior of a closure can be an issue when dealing with asynchronous code.
In an asynchronous function, you might not want to execute the closure right away. Instead, you want to wait for the asynchronous task to complete before calling a function.
This is where escaping comes in handy.
@escaping: Escaping Closures in Swift
Escaping makes it possible for the closure to outlive the function surrounding it.
This means the closure can stay around to catch a network request-response that arrives later.
To make a closure escaping, use the @escaping keyword modifier in front of the closure type.
Let’s see an example of how using a non-escaping closure fails in the context of an async function, and how to fix it using an escaping closure.
Example
Say you are using an app to message your friend and you ask them “How are you?”.
As you know, it always takes a while to get a response.
For the sake of demonstration, let’s say it takes 2 seconds for your friend to reply back.
Let’s simulate this conversation in Swift code.
To mimic this behavior, you need a function that takes a response handler closure as a parameter.
This response handler is simply responsible for taking care of handling your friend’s response that arrives 2 seconds later.
Here is how it looks in code:
func howAreYou(_ responseHandler: (String) -> Void) { print(“Hey, how are you?”) // you ask how are you // Simulating how it takes 2 seconds for your friend to answer: DispatchQueue.main.asyncAfter(deadline: .now() + 2, execute: { responseHandler(“Hi, I’m doing really good.”) }) print(“Responding takes a while…”) } howAreYou({ friendResponse in print(friendResponse) // print the response that arrives later })
But this code causes a compiler error.
This is because you are trying to execute the responseHandler function 2 seconds after the howAreYou function has executed.
This is a problem because 2 seconds after calling the howAreYou, there is no trace of that function or the responseHandler closure.
Thus the responseHandler never has a chance to handle your friend’s response. It gets destroyed before the response even arrives.
The Swift compiler is clever. It knows this is about to happen. Thus it shows you an error and does not run the code.
To make it work, you should make the responseHandler closure outlive the context of the howAreYou function to see the response arrive.
To do this, you need to turn it into an escaping closure:
func howAreYou(_ responseHandler: @escaping (String) -> Void) { print(“Hey, how are you?”) // you ask how are you // It takes 2 seconds for your friend to answer: DispatchQueue.main.asyncAfter(deadline: .now() + 2, execute: { responseHandler(“Hi, I’m doing really good.”) }) print(“Responding takes a while…”) } howAreYou({ friendResponse in print(friendResponse) // print the response that arrives later })
This way you tell the program that the responseHandler is an asynchronous closure and you want it to stay alive even after howAreYou call completes.
Now the compiler error is gone and the code works as intended.
It is as simple as that!
Now your responseHandler closure is able to show your friend’s response as it can out-live the howAreYou function’s scope!
But this example is quite arbitrary.
The next logical question is when is an escaping closure actually useful.
Let’s take a look.
When Are Escaping Closures Actually Useful
Escaping closures are useful whenever you want the closure to be able to outlive the function’s scope from where you are calling it.
A great example is when dealing with network requests.
When fetching data over a server, it takes a while for a network request to complete, and thus, the response to arrive.
To perform actions on the response, you need to wait for it to arrive.
To make a closure wait for the data, it has to be able to outlive the function’s scope where it is called.
To do this, you need to use an escaping closure.
To perform actions on the response, you need to
Conclusion
Today you learned what is @escaping in Swift.
In short, @escaping is a keyword modifier that turns a closure into an escaping closure.
An escaping closure is useful in asynchronous tasks when the closure has to wait for the function to complete.
When you use an escaping closure, the closure outlives the function in which it is called. In other words, when the function completes, the closure stays up and keeps waiting for the response.
Thanks for reading. Happy coding!
Further Reading
50 Swift Interview Questions
Swift Closures (With Examples)
In Swift, a closure is a special type of function without the function name. For example,
{ print(“Hello World”) }
Here, we have created a closure that prints Hello World .
Before you learn about closures, make sure to know about Swift Functions.
Swift Closure Declaration
We don’t use the func keyword to create closure. Here’s the syntax to declare a closure:
{ (parameters) -> returnType in // statements }
Here,
parameters – any value passed to closure returnType – specifies the type of value returned by the closure in (optional) – used to separate parameters/returnType from closure body
Let’s see an example,
var greet = { print(“Hello, World!”) }
Here, we have defined a closure and assigned it to the variable named greet . Statement enclosed inside the {} is the closure body.
To execute this closure, we need to call it. Here’s how we can call the closure
// call the closure greet()
The closure simply prints the text Hello World .
Note: This closure doesn’t have any parameters and return type.
Example: Swift Closure
// declare a closure var greet = { print(“Hello, World!”) } // call the closure greet()
Output
Hello, World!
In the above example, we have defined a closure and assigned it to the variable named greet .
When we call the closure, the print() statement inside the closure is executed.
Closure Parameters
Similar to functions, a closure can also accept parameters. For example,
// closure that accepts one parameter let greetUser = { (name: String) in print(“Hey there, \(name).”) } // closure call greetUser(“Delilah”)
Output
Hey there, Delilah.
In the above example, we have assigned a closure to the greetUser variable.
Inside the closure, (name: String) specifies that the closure accepts the String type parameter named. Notice that we have used in to separate closure parameter with body.
Also, notice the call of closure
greetUser(“Delilah”)
Here, we have passed a string value “Delilah” to our closure.
And finally, the statement inside the closure is executed.
Note: Unlike function, we call the closure without mentioning the parameter name.
Closure That Returns Value
A Swift closure may or may not return a value. If we want our closure to return some value, we need to mention it’s return type and use the return statement. For example,
// closure definition var findSquare = { (num: Int) -> (Int) in var square = num * num return square } // closure call var result = findSquare(3) print(“Square:”,result)
Output
Square: 9
In the above example, we have defined a closure that returns the square of a number. Notice the closure definition,
var findSquare = { (num: Int) -> (Int) in … return square }
Here, (num: Int) -> (Int) indicates that the closure
(num: Int) – indicates the parameter of integer type
– indicates the parameter of integer type -> (Int) – represents the closure return value of Int type
– represents the closure return value of Int type return square – return statement inside the closure
The returned value is stored in the result variable.
Closures as Function Parameter
In Swift, we can create a function that accepts closure as its parameter.
// define a function func grabLunch(search: () -> ()) { … // closure call search() }
Here,
search – function parameter
– function parameter () -> () – represents the type of the closure
– represents the type of the closure search() – call closure from the inside of the function.
Now, to call this function, we need to pass a closure as its argument
// function call grabLunch(search: { print(“Alfredo’s Pizza: 2 miles away”) })
Example: Closure as function parameter
// define a function and pass closure func grabLunch(search: ()->()) { print(“Let’s go out for lunch”) // closure call search() } // pass closure as a parameter grabLunch(search: { print(“Alfredo’s Pizza: 2 miles away”) })
Output
Let’s go out for food Alfredo’s Pizza: 2 miles away
Trailing Closure
In trailing closure, if a function accepts a closure as its last parameter,
// function definition func grabLunch(message: String, search: ()->()) { … }
We can call the function by passing closure as a function body without mentioning the name of the parameter. For example,
// calling the function grabLunch(message:”Let’s go out for lunch”) { // closure body }
Here, everything inside the {…} is a closure body.
Example: Trailing Closure
func grabLunch(message: String, search: ()->()) { print(message) search() } // use of trailing closure grabLunch(message:”Let’s go out for lunch”) { print(“Alfredo’s Pizza: 2 miles away”) }
Output
Let’s go out for lunch Alfredo’s Pizza: 2 miles away
In the above example, the grabLunch() function accepts a closure search: () -> () . Here, closure is the last parameter of the function.
Hence, we have called the function by passing the closure argument as function definition.
grabLunch(message:”Let’s go out for lunch”) { print(“Alfredo’s Pizza: 2 miles away”) }
Autoclosure
While calling a function, we can also pass the closure without using the braces {} .For example,
// using {} display(greet:{ print(“Hello World!”) } // without using {} display(greet: print(“Hello World!”))
To pass the closure argument without using braces, we must use the @autoclosure keyword in function definition. For example,
func display(greet: @autoclosure () -> ()) { … }
Here, the @autoclosure automatically adds curly braces.
Example: Autoclosure
// define a function with automatic closure func display(greet: @autoclosure () -> ()) { greet() } // pass closure without {} display(greet: print(“Hello World!”))
Output
Hello World!
Swift Escaping Closure Captures Non Escaping Parameter 39oncompletion39
Tags: ios , swift , get , api Answers: 1 | Viewed 9,832 times
I have a problem with my swift. I am trying to send an API request and then retrieve data but I get the following error message:
“Swift: Escaping closure captures non-escaping parameter ‘onCompletion'”.
Does anyone know how I can solve this? thanks in advance
Code:
Swift: Escaping closure captures non-escaping p…anycodings
I have a problem with my swift. I am trying anycodings_ios to send an API request and then retrieve anycodings_ios data but I get the following error message:
“Swift: Escaping closure captures anycodings_ios non-escaping parameter ‘onCompletion'”.
Does anyone know how I can solve this? anycodings_ios thanks in advance
Code:
Escaping Closures in Swift Explained
Escaping Closures in Swift Explained
Aasif Khan | Last Updated on December 3rd, 2021 10:37 am | 5-min read By
A closure is said to “escape” a function when it’s called after that function returns. In Swift, closures are non-escaping by default. What are escaping closures, anyway? And what do you use @escaping for? Let’s find out!
This tutorial discusses closures, escaping vs. non-escaping, capture lists and retain cycles, and what that means for practical iOS development.
What we’ll get into:
What escaping closures are, and why you need them
How and when to use the @escaping keyword
Risks of escaping closures, such as strong reference cycles
Fixing a strong reference cycle in an escaping closure
Quick Recap: What’s a Closure?
Closures are awesome! You probably already know this, but a closure is a block of code you can pass around and use elsewhere in your code.
It’s kinda like a function you can assign to a variable. You can move the closure around in your code, and call it at a later point. This makes closures exceptionally useful as completion handlers, i.e. the code you call when a long-running task has finished.
Check this out:
let makeGreeting: (String) -> String = { name in
return “Hello, \(name)!”
}
let message = makeGreeting(“Bob”)
print(message)
// Output: Hello, Bob!
In the example above the constant makeGreeting contains a closure. It has one parameter of type String, and it’s returning a value of String as well. It works the same way as a function.
When the closure is called in the above example, with makeGreeting(“Bob”), its value is assigned to constant message and printed out.
A common use-case for a closure is a so-called completion handler. Say you’re downloading an image from the web that you want to display in a view controller. Your code could look something like this:
let task = URLSession.shared.dataTask(with: URL(string: “http://example.com/cats.jpg”)!, completionHandler: { data, response, error in
// Do something with image data…
})
In the above code, we’re starting a data task with URLSession; making a HTTP request to download some cats.jpg file. The argument for the completionHandler parameter of the dataTask() function is a closure. It’s called when the image has been downloaded, hence the name completion handler.
Closures are powerful. In the example above, we’re writing the response to the data task in the same “place” in the code as the initial request. That means we can start the request and handle it, as if both actions follow each other immediately.
There’s a bit of time between the start of the request and its completion, but we can code that as if it’s immediate. That makes reading the code and reasoning about it much easier!
The code between the squiggly brackets { } is passed into the dataTask(with:completionHandler:) function. Passing around closures is a core principle, and it’s what escaping closures are about, too. Closures that outlive the context that declared them can potentially cause a memory leak, as you’ll soon see!
Escaping Closures in Swift
Let’s take a look at the declaration of that dataTask(with:completionHandler:) function, as found in the Swift Foundation framework’s source code:
func dataTask(with url: URL,
completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask
Right there, you can see that this function has 2 parameters:
url of type URL – the URL resource we’re going to download/request completionHandler – a closure
You can also see that the closure itself has 3 parameters, and returns Void.
Data? – the data that the request itself returns (i.e., the image) URLResponse? – an object that contains information about the response Error? – an optional value that contains an error object or nil
The function declaration also contains a special keyword: @escaping. This means that the closure that’s passed in as an argument for this parameter can escape the dataTask(with:completionHandler:) function. In other words, the closure is called after the function dataTask() finishes executing.
Here’s the official definition of an escaping closure:
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.
It can be helpful to think of an escaping closure as code that outlives the function it was passed into. Think of that function as a jail, and the closure as a bandit who’s escaping the jail.
An example of an escaping closure is the completion handler. Depending on its implementation, a completion handler is typically executed in the future. A length task completes long after it was initiated, which means that the closure escapes and outlives the function it was created in.
If we’ve got escaping closures, we can also have non-escaping closures:
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.
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. A non-escaping closure is a closure that’s called within the function it was passed into, i.e. before it returns. This closure never passes the bounds of the function it was passed into. .
In Swift, a closure is non-escaping by default. If a closure can escape the function, you’ll need to annotate its function parameter with the @escaping keyword. This is what happens in the code at the top of this section.
Here’s a quick code example of an escaping closure:
class TaskManager
{
var onTaskFinished:(() -> Void)?
func startLengthyTask(completionHandler: @escaping () -> Void)
{
// Store completion handler for later
onTaskFinished = completionHandler
// Do lengthy task
// …
}
func onLengthyTaskFinished() {
onTaskFinished?() // Call completion handler
}
}
let task = TaskManager()
task.startLengthyTask(completionHandler: {
// Do this when task has finished…
})
What’s going on in the code? We’ve created a class called TaskManager, with a function called startLengthyTask(completionHandler:). At the end of the code, we’re calling this function and pass a closure into it.
The closure for the completionHandler parameter is passed into the function. Inside the function, the closure is assigned to the onTaskFinished property of the TaskManager class (of the same closure type). It’s temporarily stored there.
At some point in the future, the function onLengthyTaskFinished() is called by the TaskManager class. This will invoke the previously stored completion handler, which will execute the closure we passed into it.
First, note that the closure type for completionHandler is marked with that @escaping property. This is an escaping closure. But how does it escape? With this line:
onTaskFinished = completionHandler
The closure that’s passed into the function is now assigned to a property on the TaskManager class. The closure will continue to exist, until after the startLengthyTask() function returns.
Think of the closure as a number – you can use that number after the function exits, because it’s now assigned to that property. Beyond the startLengthyTask() function, you cannot use its completionHandler parameter.
But because its value – the closure – is stored in the onTaskFinished property, you can use it later on. The closure has escaped the function it was passed into!
Why Use @Escaping?
What’s the problem with escaping closures? The code we’ve used so far runs OK. Why would we need to mark a closure as @escaping anyway?
In short, an escaping closure can cause a strong reference cycle if you use self inside the closure. Safety features like non-default, explicit @escaping are built into the Swift programming language, so you can remind yourself to check if you haven’t inadvertently caused a retain cycle.
The @escaping keyword isn’t a feature, it’s a warning sign! You’re letting the closure escape anyway, and once Swift finds out, it’ll help you avoid problems by forcing you to mark the closure as @escaping.
Before we move on, think back to the semantics of an “escaping” closure. In the previous example, you’ve seen that a closure can literally escape the function it was passed into by getting stored in a property outside that function.
When that escaping closure references self, or a strongly retained property, it will capture that reference strongly. If the escaping closure isn’t property released, you’ve created a strong reference cycle between self and the closure.
Here’s a quick shorthand:
A non-escaping closure can refer to self implicitly
An escaping closure must refer to self explicitly, or use a capture list
It’s impossible to create a strong reference cycle with a closure that’s non-escaping, because that closure doesn’t outlive the function it was passed into. The closure is released by the end of the function, and so are the values it captures.
It’s worth noting here that escaping closures work differently inside structs and classes. An escaping closure passed into a struct’s function cannot capture a mutable reference to self, because structs are value types. You cannot change a struct outside (“escaping”) a function marked with mutating; but only inside it.
It’s not that @escaping avoids a strong reference cycle; it just makes your code more explicit. It increases the safety of your code, just as with optionals, which decreases the likelyhood of accidentally creating bugs.
Strong Reference Cycles in Escaping Closures
Let’s take a look at a comprehensive example. We’re going to create an escaping closure that’ll reference self and causes a strong reference cycle. We’ll also discuss 2 potential solutions.
First, some code:
class NetworkManager
{
var onFinishRequest: ((String) -> Void)?
func makeRequest(completionHandler: @escaping (String) -> Void)
{
onFinishRequest = completionHandler
// Do lengthy task…
self.finishRequest()
}
func finishRequest() {
onFinishRequest?(“some data”)
}
}
The NetworkManager class is responsible for managing a lengthy task, like an HTTP request. You can best compare it with a bare-bones URLSession, for example.
In the above code, you can see a function makeRequest(). You can pass a closure into this function, which is called when a lengthy task completes. The closure escapes the function, because it’s assigned to the onFinishRequest property. After the lengthy task completes, finishRequest() will invoke the closure and pass some string data with it.
Next up, a class that uses the above code:
class API
{
var manager = NetworkManager()
var data = “”
init() {
print(“init API”)
}
func getData()
{
manager.makeRequest(completionHandler: { data in
self.data = data
print(“called completion handler with data: \(data)”)
})
}
deinit {
print(“deinit API”)
}
}
This API class has a function getData(), which calls the makeRequest() function on the NetworkManager object. It provides a closure for the completionHandler parameter of that function.
We’ve also marked the init and deinit functions. Especially the deinit function is important, because it’ll tell us if an instance of API is deallocated. When a strong reference cycle occurs, the API object won’t get deallocated.
What’s important to note here is that the closure strongly captures self. This happens with the self.data = data line of code. Why is that?
In Swift, references are strong by default The completionHandler closure in makeRequest() is escaping Closures will hold onto reference values you use inside them; this is called capturing or “closing over” (which is where closures get their name!)
Differently said, for the closure to hold onto self, so it can assign the closure’s data to the data property of self, a strong reference needs to be created. Otherwise, self would be gone by the time the lengthy task finishes.
If you’re a bit unclear on ARC, strong references and closures, check out my 3-part series about them, starting with: Automatic Reference Counting (ARC) in Swift
Next up, we’re going to try out 3 scenarios:
Run the above code and attempt to deallocate the API object Run the above code, and make sure self is referenced weakly .
Run the above code, and manually deallocate the closure
Let’s run the code, and see what happens. Here’s what we’re executing:
var api:API? = API()
api?.getData()
api = nil
// Output:
// init API
// called completion handler with data: some data
Would you look at that!? Even though we’re explicitly setting the api variable to nil, its deinit function is not called and the API object is not deallocated. This is a problem!
Let’s back up for a sec. What’s really going on here? Well, if we remove the call to getData(), you get the following output:
init API
deinit API
This tells us that setting a variable to nil will prompt the deallocation of its value. That’s how Automatic Reference Counting works; no one is holding onto the data, so it’s removed from memory to make space.
The culprit is getData(), but why? Inside that function, we’ve got a closure that captures a reference to self – the current class instance. A strong reference is created between self and the closure. This closure is retained strongly when it’s assigned to the onFinishRequest property.
Because the closure and self hold strongly onto each other, they cannot get deallocated. This is a strong reference cycle, which causes a memory leak. Less memory for other apps. That’s bad!
This reference cycle typically resolves itself if the closure doesn’t escape, but ours does! When a non-escaping closure is passed into a function, it’s automatically deallocated by the end of that function. This resolves the strong reference.
OK, how do you fix this? You’ve got 2 options:
Make the weak capture of self explicit with a capture list
Manually deallocate the closure when done, to break the cycle
First, the capture list. Check this out:
manager.makeRequest(completionHandler: { [weak self] data in
self?.data = data
})
The capture list [weak self] creates a weak reference between the closure and self. This prevents the strong reference cycle, because a weak reference doesn’t increase an object’s retain count. It does make the reference to self of an optional type, so you’ll need to unwrap self. (An alternative is unowned.)
When we run the code with the above change, here’s the output you get:
init API
called completion handler with data: some data
deinit API
Awesome. Crisis averted!
An alternative is to manually deallocate the closure. The trick is to break the cycle, and you can do that at both ends. Check this out:
class NetworkManager
{
func finishRequest() {
onFinishRequest?(“some data”)
onFinishRequest = nil
}
}
We’ve changed the finishRequest() function of the NetworkManager class. Right after invoking the closure that’s stored in the onFinishRequest property, we’re setting that property to nil. This deallocates that closure, which releases any strong references (if any) that the closure holds.
Resolving the strong reference cycle isn’t always viable this way, because it depends on the implementation of the NetworkManager code. It’ll need to hold onto that closure for some time, during which the strong reference persists.
Technically, making the onFinishRequest property weak would also break the strong reference cycle. It’ll also make the closure downright unusable, because the whole point is holding onto it until the lengthy task completes. You could also deallocate the NetworkManager instance, or overwrite the onFinishRequest property with something else. Of these alternatives, a capture list is the clearest and most reliable way to break the cycle.
Wrapping Up
In Swift, closures are non-escaping by default. This means that the closure can’t outlive the function it was passed into as a parameter. If you need to hold onto that closure after the function it was passed into returns, you’ll need to mark the closure with the keyword @escaping.
Escaping closures have an inherent risk: strong reference cycles. This happens when the closure strongly holds onto a reference, like self, and the closure itself is also retained. You can avoid a strong reference cycle by using a capture list, like [weak self].
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Swift – Closure use of non-escaping parameter may allow it to escape – iTecNote
closuresswiftswift3xcode
I have a protocol:
enum DataFetchResult { case success(data: Data) case failure } protocol DataServiceType { func fetchData(location: String, completion: (DataFetchResult) -> (Void)) func cachedData(location: String) -> Data? }
With an example implementation:
/// An implementation of DataServiceType protocol returning predefined results using arbitrary queue for asynchronyous mechanisms. /// Dedicated to be used in various tests (Unit Tests). class DataMockService: DataServiceType { var result : DataFetchResult var async : Bool = true var queue : DispatchQueue = DispatchQueue.global(qos: .background) var cachedData : Data? = nil init(result : DataFetchResult) { self.result = result } func cachedData(location: String) -> Data? { switch self.result { case .success(let data): return data default: return nil } } func fetchData(location: String, completion: (DataFetchResult) -> (Void)) { // Returning result on arbitrary queue should be tested, // so we can check if client can work with any (even worse) implementation: if async == true { queue.async { [weak self ] in guard let weakSelf = self else { return } // This line produces compiler error: // “Closure use of non-escaping parameter ‘completion’ may allow it to escape” completion(weakSelf.result) } } else { completion(self.result) } } }
The code above compiled and worked in Swift3 (Xcode8-beta5) but does not work with beta 6 anymore. Can you point me to the underlying cause?
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