I was checking through the Alamofire documentation and found the below code
Alamofire.request("https://httpbin.org/get").responseJSON { response in
debugPrint(response)
if let json = response.result.value {
print("JSON: \(json)")
}
}
It seems as if the function is written as
Class.Function().Function{
}
Is this called a nested function? If so how would I create a simple nested function to know the syntax
this is not a nested function but a chain of functions.
class Alamofire {
static func request(_ url: String)->Request {
//... code ...
return aRequest
}
}
class Request {
func responseJson(completion: (response: Response)->()) {
//code
}
}
So this is what is happening.
The request function of Alamofire is returning a Request object which has the responseJson function that accepts a closure as parameter.
In swift if the closure parameter is the last one, the function call can be synthesized by removing the parameter name and define the closure as a function so to do
Alamofire.request("whatever").responseJson(completion: { (response) in
//whatever
})
is exactly the same as doing
Alamofire.request("whatever").responseJson(){ (response) in
//whatever
}
This is a trailing closure. You can find more info about it here in the "Trailing Closure" paragraph.
Hope it helps
It's Method chaining and last one is the syntax of trailing closures.
Closures are self-contained blocks of functionality that can be passed
around and used in your code.
func someFunctionThatTakesAClosure(closure: () -> Void) {
// function body goes here
}
// Here's how you call this function without using a trailing closure:
someFunctionThatTakesAClosure(closure: {
// closure's body goes here
})
// Here's how you call this function with a trailing closure instead:
someFunctionThatTakesAClosure() {
// trailing closure's body goes here
}
For more information about closures read it
Related
I'm trying to declare an argument in Swift that takes an optional closure. The function I have declared looks like this:
class Promise {
func then(onFulfilled: ()->(), onReject: ()->()?){
if let callableRjector = onReject {
// do stuff!
}
}
}
But Swift complains that "Bound value in a conditional must be an Optional type" where the "if let" is declared.
You should enclose the optional closure in parentheses. This will properly scope the ? operator.
func then(onFulfilled: ()->(), onReject: (()->())?){
if let callableRjector = onReject {
// do stuff!
}
}
To make the code even shorter we can use nil as default value for onReject parameter and optional chaining ?() when calling it:
func then(onFulfilled: ()->(), onReject: (()->())? = nil) {
onReject?()
}
This way we can omit onReject parameter when we call then function.
then({ /* on fulfilled */ })
We can also use trailing closure syntax to pass onReject parameter into then function:
then({ /* on fulfilled */ }) {
// ... on reject
}
Here is a blog post about it.
Since I assume, that this "optional" closure should simply do nothing, you could use a parameter with an empty closure as default value:
func then(onFulfilled: ()->(), onReject: ()->() = {}){
// now you can call your closures
onFulfilled()
onReject()
}
this function can now be called with or without the onReject callback
then({ ... })
then({ ... }, onReject: { ... })
No need for Swift's awesome Optionals? here!
Maybe it's a cleaner way. Specially when the closure has complicated parameters.
typealias SimpleCallBack = () -> ()
class Promise {
func then(onFulfilled: SimpleCallBack, onReject: SimpleCallBack?){
if let callableRjector = onReject {
// do stuff!
}
}
}
As an alternative to creating a TypeAlias or using well-placed parentheses, there's always Swift's Optional type itself.
It can be used just like your typical Java (or in this case Swift) Generic Array syntax Optional<() -> ()>
OR in context:
func callAClosure(firstClosure: () -> (), secondClosure: Optional<() -> ()> {
if let secondClosure = secondClosure {
secondClosure()
}
else { firstClosure() }
}
I find this to be fairly clean. As a bonus, in the context of SwiftUI where generics can be common, like struct CommonList<T: View>: View then you don't have to create a typealias that only gets used once (commonly the init function for that struct). Instead, you make one simple optional closure parameter, and you're all done!
Hopefully this helps anyone that runs into the issue and happy coding!
I'm new to Swift and looking at how the dispatch_async function works. The API doc shows dispatch_async having two parameters. However, I'm able to pass in one argument and it's okay.
dispatch_async(dispatch_get_main_queue()) {
}
How come I don't need to pass in two arguments?
Thank you,
API Doc:
It is a trailing closure syntax
func someFunctionThatTakesAClosure(closure: () -> ()) {
// function body goes here
}
// here's how you call this function without using a trailing closure:
someFunctionThatTakesAClosure({
// closure's body goes here
})
// here's how you call this function with a trailing closure instead:
someFunctionThatTakesAClosure() {
// trailing closure's body goes here
}
https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html
This is how the dispatch_async looks like..
dispatch_async(dispatch_get_main_queue(), ^{
});
this part
^{
}
is the second parameter to your function, which is an anonymous code block used for callBack implementation.
I need to translate such a func from Objective-C to Swift language. But can't find an example and can't get how to send 2 closures into func in Swift.
For example, original function in Objective-C:
- (void)getForDemoDataWithToken:(Token *)token
onSuccess:(void(^)(NSArray *demoData))success
onFailure:(void(^)(NSError *error))failure {
}
I know to send 1 closure as param:
getForDemoDataWithToken(token) {(success: String) -> Void in
// some code here
print(success)
}
But, how to send two closures?
Thank you
What about this?
Declaration
func getForDemoDataWithToken(
token: Token,
onSuccess: (demoData:NSArray?) -> (),
onFailure: (error:NSError?) -> ()) {
}
Invocation
getForDemoDataWithToken(Token(),
onSuccess: { (demoData) -> () in
},
onFailure: { (demoData) -> () in
}
)
A more Swifty approach
I usually see Swift code where only one closure is used. So instead of 2 distinct onSuccess and onFailure closures you could have just completion.
Next we should remember that NSArray is compatible with Swift but it's not the Swiftest way to use an array.
Let's see an example where the 2 concepts above are applied.
func getForDemoData(token:Token, completion:(data:[Foo]?, error:NSError?) -> ()) {
}
You can invoke it with the trailing closure syntax.
getForDemoData(Token()) { (data, error) -> () in
if let data = data where error == nil {
// success!
} else {
// oh no... something wrong here
}
}
You should pass the closures as normal parameters, like this:
func acceptsTwoClosures(
onSuccess onSuccess: (success: String) -> Void,
onFailure: (failure: String) -> Void) {
onSuccess(success: "Ook")
onFailure(failure: "Eek")
}
acceptsTwoClosures(
onSuccess: { print("Success: \($0)") },
onFailure: { print("Failure: \($0)") }
)
// In the playground the above prints:
//
// Success: Ook
// Failure: Eek
The way that you used in the question is called trailing closure, and it only works for the closures that are the last arguments in the function signature.
From the documentation:
If you need to pass a closure expression to a function as the function’s final argument and the closure expression is long, it can be useful to write it as a trailing closure instead. A trailing closure is a closure expression that is written outside of (and after) the parentheses of the function call it supports.
For example, you could also re-write my suggested snippet from above like this:
acceptsTwoClosures(onSuccess: { print("Success: \($0)") }) {
print("Failure: \($0)")
}
.. as you can see, I can pass the second (i.e. the last) closure outside of acceptsTwoClosures call as a trailing closure, but I still have to pass the first one as a normal parameter.
I have come across this code:
class WebServerTests: XCTestCase {
let webServer: GCDWebServer = GCDWebServer()
var webServerBase: String!
/// Setup a basic web server that binds to a random port and that has one default handler on /hello
private func setupWebServer() {
webServer.addHandlerForMethod("GET", path: "/hello", requestClass: GCDWebServerRequest.self) { (request) -> GCDWebServerResponse! in
return GCDWebServerDataResponse(HTML: "<html><body><p>Hello World</p></body></html>")
}
I am confused by the webServer.addHandlerForMethod part. It seems to me it is already a complete function call (webServer.addHandlerForMethod("GET", path: "/hello", requestClass: GCDWebServerRequest.self)). Therefore I do not understand why it is followed by a closure ( {(request) -> ... )
EDIT: Clarify what I do not understand
According to the documentation on https://github.com/swisspol/GCDWebServer, the function signature in obj-c is:
[webServer addDefaultHandlerForMethod:#"GET"
requestClass:[GCDWebServerRequest class]
asyncProcessBlock:^(GCDWebServerRequest* request, GCDWebServerCompletionBlock completionBlock) {
Therefore I expect its swift counterpart will be called somewhat like this:
webServer.addHandlerForMethod("GET", path: "/hello", requestClass: GCDWebServerRequest.self, { (request) -> GCDWebServerResponse! in
return GCDWebServerDataResponse(HTML: "<html><body><p>Hello World</p></body></html>")
})
i.e. the handling of the incoming request is passed as the third parameter. But since the closure comes after the closing ')', it does not look like part of the function call at all.
Why the function signature is mapped from obj-c to swift this way?
In Swift, you can use this syntax if the last argument to a function is a closure. Here's the example from the Swift language guide section about closures (scroll down to Trailing Closures):
func someFunctionThatTakesAClosure(closure: () -> ()) {
// function body goes here
}
// here's how you call this function without using a trailing closure:
someFunctionThatTakesAClosure({
// closure's body goes here
})
// here's how you call this function with a trailing closure instead:
someFunctionThatTakesAClosure() {
// trailing closure's body goes here
}
And then there's also this note:
If a closure expression is provided as the function’s only argument and you provide that expression as a trailing closure, you do not need to write a pair of parentheses () after the function’s name when you call the function.
This means it's also legal to write this:
someFunctionThatTakesAClosure {
// closure body
}
… which helps provide a nice meta-programming syntax. For example:
let lock = NSLock()
func locked(closure: () -> ()) {
lock.lock();
closure()
lock.unlock();
}
locked {
NSLog("Hello, world!")
}
The closure is where the handling of the incoming request is done. It tells the server to run the closure's code when a GET method that requests /hello path comes.
In the code you posted code in the closure creates a response that the server returns.
I'm trying to declare an argument in Swift that takes an optional closure. The function I have declared looks like this:
class Promise {
func then(onFulfilled: ()->(), onReject: ()->()?){
if let callableRjector = onReject {
// do stuff!
}
}
}
But Swift complains that "Bound value in a conditional must be an Optional type" where the "if let" is declared.
You should enclose the optional closure in parentheses. This will properly scope the ? operator.
func then(onFulfilled: ()->(), onReject: (()->())?){
if let callableRjector = onReject {
// do stuff!
}
}
To make the code even shorter we can use nil as default value for onReject parameter and optional chaining ?() when calling it:
func then(onFulfilled: ()->(), onReject: (()->())? = nil) {
onReject?()
}
This way we can omit onReject parameter when we call then function.
then({ /* on fulfilled */ })
We can also use trailing closure syntax to pass onReject parameter into then function:
then({ /* on fulfilled */ }) {
// ... on reject
}
Here is a blog post about it.
Since I assume, that this "optional" closure should simply do nothing, you could use a parameter with an empty closure as default value:
func then(onFulfilled: ()->(), onReject: ()->() = {}){
// now you can call your closures
onFulfilled()
onReject()
}
this function can now be called with or without the onReject callback
then({ ... })
then({ ... }, onReject: { ... })
No need for Swift's awesome Optionals? here!
Maybe it's a cleaner way. Specially when the closure has complicated parameters.
typealias SimpleCallBack = () -> ()
class Promise {
func then(onFulfilled: SimpleCallBack, onReject: SimpleCallBack?){
if let callableRjector = onReject {
// do stuff!
}
}
}
As an alternative to creating a TypeAlias or using well-placed parentheses, there's always Swift's Optional type itself.
It can be used just like your typical Java (or in this case Swift) Generic Array syntax Optional<() -> ()>
OR in context:
func callAClosure(firstClosure: () -> (), secondClosure: Optional<() -> ()> {
if let secondClosure = secondClosure {
secondClosure()
}
else { firstClosure() }
}
I find this to be fairly clean. As a bonus, in the context of SwiftUI where generics can be common, like struct CommonList<T: View>: View then you don't have to create a typealias that only gets used once (commonly the init function for that struct). Instead, you make one simple optional closure parameter, and you're all done!
Hopefully this helps anyone that runs into the issue and happy coding!