I'm trying to convert my project's source code from Swift 3 to Swift 4. One warning Xcode is giving me is about my selectors.
For instance, I add a target to a button using a regular selector like this:
button.addTarget(self, action: #selector(self.myAction), for: .touchUpInside)
This is the warning it shows:
Argument of '#selector' refers to instance method 'myAction()' in 'ViewController' that depends on '#objc' attribute inference deprecated in Swift 4
Add '#objc' to expose this instance method to Objective-C
Now, hitting Fix on the error message does this to my function:
// before
func myAction() { /* ... */ }
// after
#objc func myAction() { /* ... */ }
I don't really want to rename all of my functions to include the #objc mark and I'm assuming that's not necessary.
How do I rewrite the selector to deal with the deprecation?
Related question:
The use of Swift 3 #objc inference in Swift 4 mode is deprecated?
The fix-it is correct – there's nothing about the selector you can change in order to make the method it refers to exposed to Objective-C.
The whole reason for this warning in the first place is the result of SE-0160. Prior to Swift 4, internal or higher Objective-C compatible members of NSObject inheriting classes were inferred to be #objc and therefore exposed to Objective-C, therefore allowing them to be called using selectors (as the Obj-C runtime is required in order to lookup the method implementation for a given selector).
However in Swift 4, this is no longer the case. Only very specific declarations are now inferred to be #objc, for example, overrides of #objc methods, implementations of #objc protocol requirements and declarations with attributes that imply #objc, such as #IBOutlet.
The motivation behind this, as detailed in the above linked proposal, is firstly to prevent method overloads in NSObject inheriting classes from colliding with each other due to having identical selectors. Secondly, it helps reduce the binary size by not having to generate thunks for members that don't need to be exposed to Obj-C, and thirdly improves the speed of dynamic linking.
If you want to expose a member to Obj-C, you need to mark it as #objc, for example:
class ViewController: UIViewController {
#IBOutlet weak var button: UIButton!
override func viewDidLoad() {
super.viewDidLoad()
button.addTarget(self, action: #selector(foo), for: .touchUpInside)
}
#objc func foo() {
// ...
}
}
(the migrator should do this automatically for you with selectors when running with the "minimise inference" option selected)
To expose a group of members to Obj-C, you can use an #objc extension:
#objc extension ViewController {
// both exposed to Obj-C
func foo() {}
func bar() {}
}
This will expose all the members defined in it to Obj-C, and give an error on any members that cannot be exposed to Obj-C (unless explicitly marked as #nonobjc).
If you have a class where you need all Obj-C compatible members to be exposed to Obj-C, you can mark the class as #objcMembers:
#objcMembers
class ViewController: UIViewController {
// ...
}
Now, all members that can be inferred to be #objc will be. However, I wouldn't advise doing this unless you really need all members exposed to Obj-C, given the above mentioned downsides of having members unnecessarily exposed.
As Apple Official Documentation. you need to use #objc to call your Selector Method.
In Objective-C, a selector is a type that refers to the name of an
Objective-C method. In Swift, Objective-C selectors are represented by
the Selector structure, and can be constructed using the #selector
expression. To create a selector for a method that can be called from
Objective-C, pass the name of the method, such as
#selector(MyViewController.tappedButton(sender:)). To construct a selector for a property’s Objective-C getter or setter method, pass
the property name prefixed by the getter: or setter: label, such as
#selector(getter: MyViewController.myButton).
As of, I think Swift 4.2, all you need to do is assign #IBAction to your method and avoid the #objc annotation.
let tap = UITapGestureRecognizer(target: self, action: #selector(self.cancel))
#IBAction func cancel()
{
self.dismiss(animated: true, completion: nil)
}
As already mentioned in other answers, there is no way to avoid the #objc annotation for selectors.
But warning mentioned in the OP can be silenced by taking following steps:
Go to Build Settings
Search for keyword #objc
Set the value of Swift 3 #objc interface to Off
below is the screenshot that illustrates the above mentioned steps:
Hope this helps
If you need objective c members in your view controller just add #objcMembers at the top of the view controller. And you can avoid this by adding IBAction in your code.
#IBAction func buttonAction() {
}
Make sure to connect this outlet in storyboard.
Related
I am trying to use a selector to see if a certain protocol can perform an action. When I try it like this:
protocol Test {
func hello()
func goodBye(a: String)
}
class Tester: NSObject, Test {
override init() {}
func hello() { }
func goodBye(a: String) { }
}
let a: Test = Tester()
let result = a.responds(to: Selector("goodByeWithA:"))
In this case, result evaluates to false.
But if I add the #objc tag to the protocol, it evaluates as true.
#objc protocol Test {
func hello()
func goodBye(a: String)
}
Why is this?
On a side note, I know that it is now recommended to use the #selector syntax and to move away from using strings, but for various reasons, I have to use a string in this case.
EDIT: This only started happening once I migrated my project to Swift 4.2
By default Swift generates code that is only available to other Swift code, but if you need to interact with the Objective-C runtime – all of UIKit, for example – you need to tell Swift what to do.
That’s where the #objc attribute comes in: when you apply it to a class or method it instructs Swift to make those things available to Objective-C as well as Swift code. So, any time you want to call a method from a UIBarButtonItem or a Timer, you’ll need to mark that method using #objc so it’s exposed – both of those, and many others, are Objective-C code.
Don’t worry: if you forget to add #objc when it’s needed, your code simply won’t compile – it’s not something you can forget by accident and introduce a bug.
New to Swift. I have two snippets below:
NotificationCenter.default.addObserver(self,
selector:#selector(ViewController.notificationReceived),
name: Notification.Name(rawValue: name), object: nil)
#objc func notificationReceived(notification:Notification){
let x = notification.userInfo!
print("\(x["name"]!)")
}
and finally
let x:UITapGestureRecognizer = UITapGestureRecognizer(target: self,
action: #selector(tapped))
self.addGestureRecognizer(x)
func tapped(){
print("tapped")
self.delegate!.theViewTapped()
}
Why is it that for the notificationCenter? I am supposed to provide the #objc tag for the selector parameter but not for the UITapGestureRecognizer action parameter?
What exactly is the difference between Selector and Action in Swift?
Check this proposal for Swift 4:
SE-0160 Limiting #objc inference
According to the description in the proposal, your second code snippet also needs #objc.
In fact, Swift 4 compiler bundled with Xcode 9 beta2 generates this error for the line using #selector(tapped):
error: argument of '#selector' refers to instance method 'tapped()'
that is not exposed to Objective-C
note: add '#objc' to expose this instance method to Objective-C
Maybe your second is a little bit too old to use with Swift 4. You better think all methods invoked through selector need #objc attribute.
[NOTE This question was originally formulated under Swift 2.2. It has been revised for Swift 4, involving two important language changes: the first method parameter external is no longer automatically suppressed, and a selector must be explicitly exposed to Objective-C.]
Let's say I have these two methods in my class:
#objc func test() {}
#objc func test(_ sender:AnyObject?) {}
Now I want to use Swift 2.2's new #selector syntax to make a selector corresponding to the first of these methods, func test(). How do I do it? When I try this:
let selector = #selector(test) // error
... I get an error, "Ambiguous use of test()." But if I say this:
let selector = #selector(test(_:)) // ok, but...
... the error goes away, but I'm now referring to the wrong method, the one with a parameter. I want to refer to the one without any parameter. How do I do it?
[Note: the example is not artificial. NSObject has both Objective-C copy and copy: instance methods, Swift copy() and copy(sender:AnyObject?); so the problem can easily arise in real life.]
[NOTE This answer was originally formulated under Swift 2.2. It has been revised for Swift 4, involving two important language changes: the first method parameter external is no longer automatically suppressed, and a selector must be explicitly exposed to Objective-C.]
You can work around this problem by casting your function reference to the correct method signature:
let selector = #selector(test as () -> Void)
(However, in my opinion, you should not have to do this. I regard this situation as a bug, revealing that Swift's syntax for referring to functions is inadequate. I filed a bug report, but to no avail.)
Just to summarize the new #selector syntax:
The purpose of this syntax is to prevent the all-too-common runtime crashes (typically "unrecognized selector") that can arise when supplying a selector as a literal string. #selector() takes a function reference, and the compiler will check that the function really exists and will resolve the reference to an Objective-C selector for you. Thus, you can't readily make any mistake.
(EDIT: Okay, yes you can. You can be a complete lunkhead and set the target to an instance that doesn't implement the action message specified by the #selector. The compiler won't stop you and you'll crash just like in the good old days. Sigh...)
A function reference can appear in any of three forms:
The bare name of the function. This is sufficient if the function is unambiguous. Thus, for example:
#objc func test(_ sender:AnyObject?) {}
func makeSelector() {
let selector = #selector(test)
}
There is only one test method, so this #selector refers to it even though it takes a parameter and the #selector doesn't mention the parameter. The resolved Objective-C selector, behind the scenes, will still correctly be "test:" (with the colon, indicating a parameter).
The name of the function along with the rest of its signature. For example:
func test() {}
func test(_ sender:AnyObject?) {}
func makeSelector() {
let selector = #selector(test(_:))
}
We have two test methods, so we need to differentiate; the notation test(_:) resolves to the second one, the one with a parameter.
The name of the function with or without the rest of its signature, plus a cast to show the types of the parameters. Thus:
#objc func test(_ integer:Int) {}
#nonobjc func test(_ string:String) {}
func makeSelector() {
let selector1 = #selector(test as (Int) -> Void)
// or:
let selector2 = #selector(test(_:) as (Int) -> Void)
}
Here, we have overloaded test(_:). The overloading cannot be exposed to Objective-C, because Objective-C doesn't permit overloading, so only one of them is exposed, and we can form a selector only for the one that is exposed, because selectors are an Objective-C feature. But we must still disambiguate as far as Swift is concerned, and the cast does that.
(It is this linguistic feature that is used — misused, in my opinion — as the basis of the answer above.)
Also, you might have to help Swift resolve the function reference by telling it what class the function is in:
If the class is the same as this one, or up the superclass chain from this one, no further resolution is usually needed (as shown in the examples above); optionally, you can say self, with dot-notation (e.g. #selector(self.test), and in some situations you might have to do so.
Otherwise, you use either a reference to an instance for which the method is implemented, with dot-notation, as in this real-life example (self.mp is an MPMusicPlayerController):
let pause = UIBarButtonItem(barButtonSystemItem: .pause,
target: self.mp, action: #selector(self.mp.pause))
...or you can use the name of the class, with dot-notation:
class ClassA : NSObject {
#objc func test() {}
}
class ClassB {
func makeSelector() {
let selector = #selector(ClassA.test)
}
}
(This seems a curious notation, because it looks like you're saying test is a class method rather than an instance method, but it will be correctly resolved to a selector nonetheless, which is all that matters.)
I want to add a missing disambiguation: accessing an instance method from outside the class.
class Foo {
#objc func test() {}
#objc func test(_ sender: AnyObject?) {}
}
From the class' perspective the full signature of the test() method is (Foo) -> () -> Void, which you will need to specify in order to get the Selector.
#selector(Foo.test as (Foo) -> () -> Void)
#selector(Foo.test(_:))
Alternatively you can refer to an instance's Selectors as shown in the original answer.
let foo = Foo()
#selector(foo.test as () -> Void)
#selector(foo.test(_:))
In my case (Xcode 11.3.1) the error was only when using lldb while debugging. When running it works properly.
I'm creating a view programatically, and adding a function so the action responds to the UIControlEvents.TouchUpInside event:
button.addTarget(self, action: action, forControlEvents:
UIControlEvents.TouchUpInside)
So, by going into the documentation I've added this action as a selector:
#selector(ViewController.onRegularClick)
XCode then complaints about:
Argument of #selector refers to a method that is not exposed to
Objective-C
So I have to set up the handler function with:
#objc func onRegularClick(sender: UIButton)
Can some one please put this noob on the right direction by guiding me to the documentation, or even give a short explanation, on:
why can't I no longer pass simply the function name String to the action?
how is the proper way to implement this following the Swift Way? Using the Selector class?
why do we need to pass the #objc keyword and how it affects the function?
Thank you!
why can't I no longer pass simply the function name String to the action?
Using strings for selectors has been deprecated, and you should now write #selector(methodName)instead of "methodName". If the methodName() method doesn't exist, you'll get a compile error – another whole class of bugs eliminated at compile time. This was not possible with strings.
how is the proper way to implement this following the Swift Way? Using the Selector class?
You did it the right way:
button.addTarget(self, action: #selector(ClassName.methodName(_:)), forControlEvents: UIControlEvents.TouchUpInside)
why do we need to pass the #objc keyword and how it affects the function?
In Swift the normal approach is to bind method's calls and method's bodies at compile time (like C and C++ do). Objective C does it at run time. So in Objective C you can do some things that are not possible in Swift - for example it is possible to exchange method's implementation at run time (it is called method swizzling). Cocoa was designed to work with Objective C approach and this is why you have to inform the compiler that your Swift method should be compiled in Objective-C-like style. If your class inherits NSObject it will be compiled ObjC-like style even without #objc keyword.
Well, it is called evolution
When there are some arguments in the method, you should declare the selector as:
let selector = #selector(YourClass.selector(_:))
You can type only #selector(selector(_:)) if the selector is in the same class of the caller. _: means that accept one parameter. So, if it accept more parameters, you should do something like: (_:, _:) and so on.
I found out that the #objc is needed only when the function is declared as private or the object doesn't inherit from NSObject
1: Currently you can, but it will create a deprecated warning. In Swift
3 this will be an error, so you should fix it soon. This is done
because just using a String can not be checked by the compiler if
the function really exists and if it is a valid Objective C function
which can be resolved dynamically during runtime.
2: Do it in this way:
button.addTarget(self, action: #selector(MyViewControllerClass.buttonPressed(_:)), forControlEvents: UIControlEvents.TouchUpInside)
3: Usually you not have to use the #objc attribute. I assume your class ViewController is (for any reason) not derived from UIViewController. If it derives from UIViewController is inherits also the needed ObjC behavior for calling selectors on functions.
For swift3.0 just do like below code :
yourButton.addTarget(self, action: #selector(yourButtonPressed), for: .touchUpInside)
and yourButtonPressed method
#IBAction func yourButtonPressed(sender:UIButton) {
// Do your code here
}
Everyones's answers are perfect but I have a better approach. Hope you gonna like it.
fileprivate extension Selector {
static let buttonTapped =
#selector(ViewController.buttonTapped(_:))
}
...
button.addTarget(self, action: .buttonTapped, for: .touchUpInside)
here in this file private will help to show buttonTapped only in file.
Programmatically
button.addTarget(self, action: #selector(returnAction), for: .touchUpInside)
// MARK: - Action
#objc private func returnAction(sender: UIButton) {
print(sender.tag)
}
I'm working on an iOS application and I'm using swift in that. For more readability and organising functions I've used extensions in my swift file.
// MARK: Class Declaration
class PaymentView
{
// Some stuffs
}
// MARK: Lifecycle methods
extension PaymentView
{
// Overriden for adding gesture recogniser
override func awakeFromNib()
{
super.awakeFromNib()
// Causes a crash when tapped on view
let tapGesture = UITapGestureRecognizer(target: self, action: Selector("paymentViewSelected:"))
self.addGestureRecognizer(tapGesture)
// Works correctly !!!
paymentViewSelected(tapGesture);
}
}
// MARK: Private Methods
extension PaymentView
{
private func paymentViewSelected(sender : UITapGestureRecognizer)
{
print("Method called")
}
}
My issue is when I tap on my view the application crashes with unrecognised selector error. If I remove that private access control specifier from the method it works perfectly.
My question is, I can call the paymentViewSelected: directly from the awakeFromNib regardless the private. But why it is crashing when used as a selector ?
According to Swift AccessControl Reference
Private access restricts the use of an entity to its own defining
source file. Use private access to hide the implementation details of
a specific piece of functionality.
But my class, extension, gesture all are in same file. I think I'm missing some basic key concept here. Please help me to understand the issue.
About Your point : I can call the paymentViewSelected: directly from the awakeFromNib regardless the private. But why it is crashing when used as a selector ?
Its because methods that are marked with private can accessible inside the class, but the object of class cannot call the method marked with private.
In your tapGesture, it is called using object of class automatically, once you tap on the view.
In this case your method is not available because it is marked with private, this is the reason of crash error unrecognised selector.
If you are calling a method from a selector and it is private they cannot be called because the method is called from outside. When you call paymentViewSelected() in the awakeFromNib it is called inside the class. However when it is called via Selector, it is called from outside. like object.paymentViewSelected(). You cannot call private method from outside.