My brief understanding of the difference between escaping and non-escaping closures:
Escaping closures are invoked after the function returns. From what I can tell, this is similar to the way callbacks are handled in node.js — the function returns immediately, they execute asynchronously, and the result of the long-running operation inside the function is handled when the closure/completion block/callback is called.
Non-escaping closures appear to be execute synchronously, where the function itself doesn't return until the long-running operation is complete.
(Did I get that right?)
Reading the change log and justification for moving from Swift 2's #noescape to Swift 3's #escaping (in SE-0103), it appears you can asynchronously dispatch non-escaping closures.
Is this true? Possible?
Can you dispatch a non-escaping closure function asynchronously? If so, how?
Bonus points — I really want to understand why both escaping and non-escaping closures exist:
Is there a practical use-case for asynchronously dispatching a non-escaping closure in place of an escaping closure?
In the design proposal, it suggests that functional programming benefits from non-escaping (read: synchronous?) being the default. Is this true? Why?
Is there any similarity between non-escaping closures and a promise in node.js?
If I'm wrong about all of this, why not just call it #async instead of #escaping?
I can't conceive of how an asynchronously dispatched function can be non-escaping. Is there a specific statement in the proposal that suggests that?
Regarding your additional questions:
Is there a practical use-case for asynchronously dispatching a non-escaping closure in place of an escaping closure?
This shouldn't be possible, so there wouldn't be a use-case for it :)
In the design proposal, it suggests that functional programming benefits from non-escaping (read: synchronous?) being the default. Is this true? Why?
The proposal says that functional programming would benefit from having less boilerplate (i.e. having to mark all parameters as #noescape). Since functional programming usually entails many functions-as-parameters that are predominantly non-escaping, this new default allows functional-style algorithms to have cleaner, less cluttered signatures and reduces typing the the #noescape boilerplate everywhere.
Is there any similarity between non-escaping closures and a promise in node.js?
No, promises are asynchronous and would be implemented in Swift using escaping closures, not non-escaping closures.
If I'm wrong about all of this, why not just call it #async instead of #escaping?
Because they are not exactly the same concept. An escaping closure may execute synchronously, but be saved off for execution at a later time. #async describes the nature of execution, but #escaping means only that the closure will be executed after the function it was passed to returns. When an escaping closure does execute, it might do so either synchronously or asynchronously at that time.
Related
When using collectionview.performBatchUpdates, the updates block does not need explicit self like below image.
The method's docs are here. https://developer.apple.com/documentation/uikit/uicollectionview/1618045-performbatchupdates
My guesss is that performBatchUpdates's updates block life cycle is garanteed shorter than collectionView. And the updates block called using withoutActuallyEscaping.
Is there anyone have clear answer?
The answer is simple. Because it's not a escaping closure
The animate closure is escaping, while the performBatchUpdates closure is not.
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
An escaping closure that refers to self needs special consideration if self refers to an instance of a class. Capturing self in an escaping closure makes it easy to accidentally create a strong reference cycle. For information about reference cycles, see Automatic Reference Counting.
Normally, a closure captures variables implicitly by using them in the body of the closure, but in this case you need to be explicit. If you want to capture self, write self explicitly when you use it, or include self in the closure’s capture list. Writing self explicitly lets you express your intent, and reminds you to confirm that there isn’t a reference cycle.
I am new to swift and trying to learn when I saw apple doc in it there is a definition about escaping closure like:
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.
# other hand I saw a blog where blogger explained about completion handler like:
There are times you put a closure as one of the parameters, but you only want to execute the closure after the function returns. For example, what if you want to print “Hello, world” only after you’ve completely switched to the next ViewController?
So I want to know if completion handler and escaping closure are same or what?
I want to know that completion handler and escaping closure are same
or what?
Completion and Error Handlers are related to a process that should be done after executing -and returning- a chunk of code (function):
Completion handlers are callbacks that allow a client to perform some
action when a framework method or function completes its task.
https://developer.apple.com/library/content/featuredarticles/Short_Practical_Guide_Blocks/
which are -logically- representation of escaping closures.
So, referring to the escaping closures definition, a completion handler is by default is an escaping closures.
For reviewing example(s), you might want to check this answer. Also, if you think that you need more description about what is the escaping closure, you might wan to check this Q&A.
Completion handlers are one example where escaping closures are passed, but they are not the only one.
In fact, the concept of a completion handler works on a different level than the concept of an escaping closure.
Escaping Closure
The closure is stored, and may be invoked at some time in the future.
This is in contrast to a non-escaping closure, where the closure gets called immediately or not at all.
The difference between escaping and non-escaping closure is important for memory management. For non-escaping closures, the compiler knows that any resources used by the closure aren't used anymore after the call.
Completion Handler
A completion handler is a closure that gets called once an operation is finished.
Operations that use completion handlers often are long-running and work on different threads. While such an operation is running, your program can continue with its control flow, thereby making the program more responsive. For example, if you call URLSession.downloadTask(withResumeData:completionHandler:), you will schedule a new download task in URLSession. Downloading may take a while, depending on your connectivity status and other factors, and only once the download was finished your completion handler gets called.
The way such operations are implemented is usually, that they store the completion handler somewhere, perform the operation, and then call the completion handler. Therefore, completion handlers are usually escaping closures.
I am learning objective c and swift. i didn't get what is difference between block and method or function in Objective c or in Swift.
int mutiplier=10;
int (^myBlock)(void)=^{
return 10 *3;
};
NSLog(#"%d",myBlock());
Or can write method/function like this
-(int)function:(int)num{
return num* 10;
}
Blockquote
Short and simple:
Block of code - just a block of code. You can declare it, define types of blocks(then make an instances), call blocks one by one, etc. Blocks can take parameters, can return something, It`s quite convenient to use them along with a Grand Central Dispatch. Blocks can declared right in the middle of the code, as an instance variable or as a property. They can also be passed as an arguments to a method/function call. After block has done its work you can call the 'completion' part to run some specific code, which is convenient in some cases. In a swift language similar to blocks (but not equal) thing is a closure. Would like to add that there is a block-based enumeration approach available in Objective-c which is almost as fast as Fast enumeration. Would recommend fast enumeration for most cases, but sometimes (rare) block enumeration is better. Other loops usually is not as fast as this two ones. One more important thing we should keep in mind is that the blocks are Objective-C objects while functions and methods are not. Blocks can capture values from the variables from the enclosing scope while to get the same values inside of a function/method you need to pass these variables as an arguments. Using blocks you can even change these variables using f.e.
__block int anInteger = 123;
syntax before the block call.
Keep in mind you avoid strong reference to a self when capturing it inside of a block to avoid retain cycles. Use weakSelf in that case.
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/WorkingwithBlocks/WorkingwithBlocks.html
Function should have name, return something or be void.
Method is a function of a class.
Blocks are a way of wrapping up a piece of code and effectively storing it for later use. A block is commonly used in place of a call back function. Newer APIs in the iPhone SDK use blocks this way. The API will take a "block" of code that it will run on completion. And Blocks are presented in swift as a Closure.
In swift it's little tough syntax to learn closure. but trust me it's very handy way when you start to use it.
It saves you having to create your own threads and maintain the state of each thread, manage locks, setup autorelease pools etc.
Also, when used with the Grand Central Dispatch (GCD) API blocks can be run on queues and entire portions of code can be made to run asynchronously with very little effort, but still keeping the robustness that is required for multithreaded code.
I am converting code to Swift using Xcode 8, but compiler warns to add #escape in some of the nested functions already created in Swift 2.3, with closure syntax. I have found some other keywords also #noescape and #autoclosure, but I have some question regarding this :
What is functional need of this keyword?
What is impact of writing #escaping ?
Is it necessary to write ?
Is #autoclosure behave same as #escape?
When to use which keyword and why ?
Here is Swift-evolution document, but not getting much from it.
The most important difference is between #escaping and #noescaping (there is no such keyword in Swift 3!). When a closure is marked as #noescape, you can be sure that the closure won't be retained by the method (e.g. to perform an asynchronous call), therefore you don't have to worry about ownership cycles (there are some other minor benefits).
#escaping closures can be saved or called sometimes in the future therefore you have to be sure to handle ownership (e.g. [weak self]) correctly.
For #autoclosure see How to use Swift #autoclosure . In short, it allows you to skip braces around the closure in some situations.
The default (when not specified) is #noescaping in Swift 3 (see rationale). They keyword does not actually exist anymore. There is only #escaping.
#escape means that your function can be called after method ends.
#noescape means that your closure will be called before the end of function it passed to.
#autoclosure means that you can omit writing closure braces, but it automatically becomes #noescape.
#autoclosure creates an automatic closure around the expression. So when the caller writes an expression like 2 > 1, it's automatically wrapped into a closure to become {2 > 1} before it is passed to some function.
2 - Easier for compiler to perform optimisations.
3 - beter to write, to give compiler(or anybody that using your function) information how it behaves with your closure
4 - no
5 - described on the top of the answer
If I have some code that takes a closure, and self is implicitly captured, can I make it cause a compiler warning? Is there some flag I can add to the compiler command for this?
Not exactly. But there is something that addresses what's presumably the reason you'd want this warning — to prevent situation where a closure capturing self can cause a retain cycle...
The potential retain cycle from capturing self in a closure comes up when the closure is passed as a parameter to a function but is used after the function returns — that is, the lifetime of the closure (and anything captured in it) "escapes" the context in which it's written. When the closure escapes, it has to be sure it still has access to the things it uses (like self and any properties or methods thereof), so we get into memory management and possible retain cycles, [weak self]/strongSelf, etc.
With #noescape, you can declare that closures passed as parameters to a function will not escape the calling context, and as such don't require the extra memory management overhead for closures that do. And the compiler can apply some other optimizations that make running your function and its closure faster.
func someFunctionWithNoescapeClosure(#noescape closure: () -> Void) {
closure()
}
Within a #noescape closure, you can then access methods/properties on self without explicitly writing self., because you're not capturing self anymore. And, once you label a parameter #noescape, if you try to do anything that would permit it to escape (say, take the closure from the parameter and assign it to a stored property), you'd get a compiler error (not just a warning).
For the full writeup from Apple, see Nonescaping Closures in The Swift Programming Language. And this post provides an alternate perspective that might explain things further.
No, the only thing I can think of which can do anything similar is linting.
SwiftLint might be able to do something like that, and if not you can probably write your own rule for just such a thing.