I am new to swift programing development. I want to know to how to pass a closure to another closure.
Is there any difference between closure in swift and blocks in objective.
How to pass a closure to a closure?
A closure can be seen as just any other (non-closure) type. This means you can construct a closure where the argument of the closure describes another closure type.
E.g.
let sendMeAClosure: ((Int) -> String) -> () = {
print($0(42)) /* ^^^^^^^^^^^^^^^- argument of sendMeAClosure
is a closure itself */
}
let myClosure: (Int) -> String = {
return "The answer is \($0)."
}
sendMeAClosure(myClosure) // The answer is 42
Note that functions in Swift are just a special type of closure, so you might as well supply a function reference (which has a signature matching the argument type) to sendMeAClosure above.
/* sendMeAClosure as above */
func myFunc(arg: Int) -> String {
return "The answer is \(arg)."
}
sendMeAClosure(myFunc) // The answer is 42
Is there any difference between closure in swift and blocks in objective?
For your 2nd question, refer to the following Q&A
Difference between block (Objective C) and closure (Swift) in ios
Related
How it's called and where to find information in Swift what is when for reading property or invoked method it used body in brace.
Example:
Property:
mySerialQueue.sync { task("New Task") }
SomeObj someObj = obj {
name = "some name"
}
Method:
workItem.notify(queue: DispatchQueue.main) {
if let imageData = data {
eiffelImage.image = UIImage(data: imageData)
}
}
From “Trailing Closures” in The Swift Programming Language:
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 written after the function call’s parentheses, even though it is still an argument to the function. When you use the trailing closure syntax, you don’t write the argument label for the closure as part of the function call.
[…]
If a closure expression is provided as the function or method’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 or method’s name when you call the function:
I am new to iOS coding and I am stuck in closures feature of SWIFT. I have referred to many tutorials and found that closures are self written codes which can be used in many ways eg. as arguments in function call,parameters in function definition,variables. I am giving below an example below with my associated thoughts about the code & questions. Please help me if I am wrong in my understanding. I know I am wrong at many points,so please rectify me.
1.1st Part
func TEST(text1:String,text2:String,flag: (S1:String,S2:String)->Bool)//In this line,I think,I am using flag is a closure which is passed as parameter in a function. And if so why doesn't it follow the standard closure syntax?
{
if flag(S1: text1, S2: text2) == true//I want to check the return type what flag closure gets when it compares the both string during function call. Why can't I write as if flag == true as flag is the name of the closure and ultimately refers to the return type of the closure?
{
print("they are equal")
}
else
{
//
}
}
2nd Part
This part is the most troublesome part that really confuses me when I am calling the function. Here I am also using the same closure. What is happening over here? How is the closure being used? Is it capturing values or something else?
TEST("heyy", text2: "heyy") { (S1, S2) -> Bool in
S1==S2
}
Thanks for your kind consideration.
Your closure usage is ok. A closure is some code that can be passed to be executed somewhere else. In your case you can choose to pass the real test you want to the function TEST, simple string test or case-insensitive test, etc. This is one of the first usage of closure: obtain more genericity.
And yes closures capture something, it captures some part of the environnement, i.e. the context in which they are defined. Look:
var m = "foo"
func test(text1:String, text2:String, testtFunc: (s1:String, s2:String) -> Bool) {
m = "bar"
if testFunc(s1: text1, s2: text2) { print("the test is true") }
}
m = "baz"
test("heyy", text2: "heyy") { (s1, s2) -> Bool in
Swift.print("Value for m is \(m)")
return s1==s2
}
The closure captures m (a variable that is defined in the context in which you define the closure), this means that this will print bar because at the time the closure is executed, the captured m equals to bar. Comment bar-line and baz will be printed; comment baz-line and foo will be printed. The closure captures m, not its value, m by itself, and this is evaluated to the correct value when the closure is evaluated.
Your first function works like this :
arguments :
String 1
String 2
a function that takes two Strings as arguments and returns a Bool
body :
execute the function (flag) with text1 and text2 and check the result.
The function doesn't know at all what you are testing, it only knows that two pieces of text are needed and a Bool will be returned.
So this function allows you to create a general way of handling different functions that all have two Strings as input. You can check for equality or if the first pieces of text is a part of the second and so on.
This is useful for many things and not so far from how array filtering / sorting / map works.
2nd Part :
This is just how you call a function with a closure.
TEST("heyy", text2: "heyy") { (S1, S2) -> Bool in
S1 == S2
}
You can also call it like this :
func testStringEqualityFor(text:String, and:String) -> Bool {
return text == and
}
TEST("hey", text2: "hey", flag: testStringEqualityFor)
Instead of using the trailing closure syntax to pass an unnamed function, you now pass a named function as one of the arguments.
It al becomes a lot clearer when you simplify it.
This is a function that takes another function as an argument.
Now we can call/use this function inside it. The argument function takes a bool as it's argument. So we give it a true
func simpleFunctionWithClosure(closure:(success:Bool) -> Void) {
// use the closure
closure(success: true)
}
When we use the function we need to pass it a function. In Swift you have the trailing closure syntax for that, but that is only available (and even then optional) to the first function as argument.
Trailing closure syntax means that instead of passing a named function you can write:
myFunction { arguments-for-closure-as-tuple -> return-for-closure-as-tuple in
function-body
}
The closure will receive an argument of Bool and returns nothing so Void.
In the body we can handle the arguments and do stuff with them.
But it is important to remember that what is inside the closure is not called directly. It is a function declaration that will be executed by simpleFunctionWithClosure
// use the function
simpleFunctionWithClosure { (success) -> Void in
if success {
print("Yeah")
} else {
print("Ow")
}
}
Or with a named function :
func argumentFunction(success:Bool) -> Void {
if success {
print("Yeah")
} else {
print("Ow")
}
}
simpleFunctionWithClosure(argumentFunction)
Compiler wouldn't have any expectation how your closure is for. For example in your first case, it could not estimate the closure's intake parameters is always just reflect to the first and second parameters of the TEST function when we are always able to write the following code :
func Test(str1:String,str2:String,closure:(String,String)->Bool){
if closure(str[str1.startIndex...str1.startIndex.advanced(2)],str2[str2.startIndex.advanced(1)...str2.endIndex])
{ ... }else{ ... }
//Just an example, everybody know no one write their code like this.
}
The second case, I thought you've just overlooked the syntax sugar:
For a trailing closure A->B:
{ a:A -> B in a.bValue() }
is equal to :
{ a:A -> B in return a.bValue() }
Also, I think this TEST function isn't a good example when the task of it can be done without using closure. I think you can write a map function by yourself for a better understand of why and when to use closure.
It seems to me that there is a discrepancy in Swift's syntax between calling an initializer and a function with at least one paremeter.
Let's consider these two examples:
class SimpleClass {
var desc: String
init(desc: String) {
self.desc = desc
}
}
let aClass = SimpleClass(desc: "description")
and
func simpleFunc(a: Int, b:Int) -> Int {
return a + b;
}
let aVal = simpleFunc(5, b: 6)
It seems odd to me that the compiler forces you to omit the first label in a function call, otherwise you will get an error "Extraneous argument label 'a:' in call". Whereas if we want to omit the first label during initilization, you get the error "Missing argument label 'desc:' in call".
The language guide says:
When calling a function with more than one parameter, any argument after the first is labeled according to its corresponding parameter name.
Source: https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Functions.html
The arguments to the initializer are passed like a function call when
you create an instance of the class.
Source: https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/GuidedTour.html
I'm new to Swift so I hope I didn't miss something, but this seems like a syntax discrepancy, because initializers/ constructors are just kind of functions and forcing to omit the first label in a function call seems inconsistent to me.
That's because Swift focuses on readability; function calls to be able to be read like a sentence. See this, specifically the section on "Local and External Parameter Names for Methods". Your function, to comply with this style, should be more like:
func add(a: Int, to b: Int) -> Int {
return a + b
}
let c = add(1, to: 2)
As a homage to Ruby I've been playing with an extension to Int that allows me to write useful code like this:
3.times { println("I keep holding on") }
This works well and here's the extension:
extension Int {
func times(fn: () -> ()) {
for i in 1...self {
fn()
}
}
}
Now, I'd like to pass the iteration number in to the closure, so I added a 2nd times() function to extension:
extension Int {
func times(fn: (iteration: Int) -> ()) {
for i in 1...self {
fn(iteration: i)
}
}
}
Which can be invoked like this:
5.times { (i: Int) -> () in println("Year \(i)") }
Now, according to the Swift docs,
It is always possible to infer the parameter types and return type
when passing a closure to a function as an inline closure expression.
As a result, you never need to write an inline closure in its fullest
form when the closure is used a function argument.
That sounds great, because then I can omit the parameter and return types, i.e. (i: Int) -> (), and just use the following syntax instead:
5.times { i in println("Year \(i)") }
But this leads to the following error: Error: Ambiguous use of 'times'
Is this way of invoking my times() function really ambigous to the compiler?
It is ambiguous. Both .times() methods can be used with the given closure expression if the parameter type of the closure is not known.
If you just write { i in println("Year \(i)") }, it is just a closure that takes one parameter of any type. Well, EVERY function type in Swift can be viewed as taking one parameter:
What you think of as zero-parameter functions actually take one parameter of type () (a.k.a. Void), of value (), that's why the type is written as () -> something
What you think of as multiple-parameter functions actually take one parameter of tuple type, a tuple of all the "multiple arguments", that's why the type is written as (foo, bar) -> something.
So, basically, your closure expression, without specifying the type of i, can be inferred to ANY function type in Swift that returns Void. The functions taken by both .times() methods match this -- for the first .times() method, it is inferred as a function of type () -> (), i.e. i has type (); for the second .times() method, it is inferred as a function of type Int -> (), i.e. i has type Int.
it looks like the ambiguity derives from the 2 extension methods having the same name. If you comment your first version of the times function, it works fine - if you comment the 2nd instead, surprisingly, you do not get an error from the compiler.
There is no ambiguity in my opinion, because the 2 functions have different signature - () -> () is different than (iteration: Int) -> (). I think it's a bug in the compiler, specifically type inference is failing.
Making the call explicit instead it works fine
5.times { (i: Int) -> () in println("Year \(i)") }
If the first version with the parameterless closure is commented, the line above compiles correctly, if instead the 2nd overload is commented, compilation fails as expected.
To prove that something is wrong in the compiler, this seems to work, whereas I'd expect a compilation error instead:
extension Int {
func times(fn: () -> ()) {
for i in 1...self {
fn()
}
}
}
5.times { i in println("Year \(i)") }
This is the output from the playground console:
Year ()
Year ()
Year ()
Year ()
Year ()
Is it possible to use Shorthand Argument Names with a Swift function. Closures have this feature, but since a function is in itself a closure, there might be a way to access parameters with name omitted. In detail here is what my query is:
You can implement a closure with shorthand argument name like this:
someFunction(param1, { $0 > $1 })
There is no need to provide parameter names in a closure, simply use $0, $1 etc.
For a function you may define it like so:
func functionC(Int, String) {
}
Omitting the param names here does not give any compiler error. Probably this is a swift feature. So does this mean I can access the params without name. If yes, then how?
This may be a bug or half implemented feature now because I can't find anything in the documentation about it. Using a function like you described compiles and runs fine:
func functionC(Int, String) {
println("function called")
}
functionC(10, "hello")
but attempting to use the arguments with the closure syntax $0 and $1 results in the error: Anonymous closure argument not contained in a closure which pretty clearly states you aren't allowed to use anonymous arguments in a function.
I think the purpose of this then is to be able to have the same method signature of a required function even if you don't have to use all of the arguments like this example:
protocol myProtocol{
func requiredFunc(Int, String) -> Bool
}
class myClass: myProtocol{
func requiredFunc(x: Int, String) -> Bool{
return x > 10
}
}
You can use the shorthand arguments in a closure because the compiler can infer the parameters and their types. But it is not fair to say that func functionC(Int, String) is the same as a closure with no parameter list. A closure declared the same way will not work either. Just like your function declaration, this closure is not valid because the parameters were declared:
{ (Int, String) -> Bool in
return $0 > $1
}
Short hand arguments don't work for functions. According to the compiler they work only for closures.
Coming to your stated case:
func functionC(Int, String) {
}
here it doesn't report any error because you just defined the function to take arguments of Int and String but cannot be used anywhere since they're not assigned. So there is no purpose to take this case other to verify how the compiler works.