Swift function defined in MySwift.swift File:
func SomeSwift()
{
}
SomeSwift() is not defined in any Swift class, it is just a pure function.
After CMD + B to build the project, open Project-Swift.h, the SomeSwift() isn't show in there.
Does the function in Swift have to be defined in some Swift class? and with #objc marked?
like the following:
#objc class SomeSwift: NSObject {
func SomeSwift()
{
}
}
Referring to Apple Documentation about Using Swift from Objective-C:
A Swift class must be a descendant of an Objective-C class to be
accessible and usable in Objective-C
Means that your class should be #objc class SomeSwift: NSObject (You're right!), but you CANNOT access the whole thing in Swift file:
When you create a Swift class that descends from an Objective-C class,
the class and its members—properties, methods, subscripts, and
initializers—that are compatible with Objective-C are automatically
available from Objective-C. This excludes Swift-only features, such as
those listed here:
Generics
Tuples
Enumerations defined in Swift without Int raw value type
Structures defined in Swift
Top-level functions defined in Swift
Global variables defined in Swift
Typealiases defined in Swift
Swift-style variadics
Nested types
Curried functions
Reference.
So, you cannot use the SomeSwift top-level function.
Even if you tried to add #objc before its declaration, the compiler will tell that:
#objc can only used when with memebers of classes, #objc protocols,
and concrete extensions of classes.
with a suggestion to remove #objc.
Related
I read this documentation for importing swift code into objective c.
https://developer.apple.com/documentation/swift/imported_c_and_objective-c_apis/importing_swift_into_objective-c
I have a few questions.
Can I put #objc annotation for a Struct?
Do I need to inherit all the class that I want to export to obj to be child of NSobject ? I am getting error 'error: only classes that inherit from NSObject can be declared #objc'
When I export a swift class with #objc, I need to add #objc to all its parent classes, protocol and interface and also class and structure in all its methods, is that correct?
No. Objective-C cannot see a native Swift struct.
Yes. Objective-C classes must basically be derived from NSObject. Objective-C can be made aware of the existence of other classes, but it cannot do anything useful with them.
You can mark the class with #objcMembers, in which case you will give everything within it full visibility to Objective-C.
So I am slowly adding more swift files to codebase.
Suppose I have class in Objective c:
ListData<Type : CSJSONData *> : ServerData <CSListData>
I want to subclass it in swift class like this:
#objc class ArticlesData: ListData<Article>
Compiler gives me error:
Generic subclasses of '#objc' classes cannot have an explicit '#objc' because they are not directly visible from Objective-C
Replace '#objc ' with ''
But I need to have it #objc to keep available in my objc part of code ! How to do it ?
From The Swift Programming Language (4.2), Attributes:
The compiler implicitly adds the objc attribute to subclasses of any class defined in Objective-C. However, the subclass must not be generic, and must not inherit from any generic classes. You can explicitly add the objc attribute to a subclass that meets these criteria, to specify its Objective-C name as discussed below.
Unfortunately for you this is pretty definitive.
When I declare static parameter in extension of class then I have to write #nonobjc before variable like:
#nonobjc static let test = "test"
and sometimes I have to write #objc before method, so what is use of #objc and #nonobjc in Swift.
Can anyone help me for this problem?
This is explained in the Apple's official documentation about Objective-C - Swift interoperability:
When you use the #objc(name) attribute on a Swift class, the class is
made available in Objective-C without any namespacing. As a result,
this attribute can also be useful when migrating an archivable
Objective-C class to Swift. Because archived objects store the name of
their class in the archive, you should use the #objc(name) attribute
to specify the same name as your Objective-C class so that older
archives can be unarchived by your new Swift class.
Conversely, Swift also provides the #nonobjc attribute, which makes a
Swift declaration unavailable in Objective-C. You can use it to
resolve circularity for bridging methods and to allow overloading of
methods for classes imported by Objective-C. If an Objective-C method
is overridden by a Swift method that cannot be represented in
Objective-C, such as by specifying a parameter to be a variable, that
method must be marked #nonobjc.
To summarize, use #objc when you want to expose a Swift attribute to Objective-C without a namespace . Use #nonobjc if you want to keep the attribute available and accessible only in Swift code.
(Addendum/additional official details to #bontoJR well summarizing answer)
From the Swift Language Reference - Attributes [emphasis mine]:
objc
Apply this attribute to any declaration that can be represented in
Objective-C — for example, non-nested classes, protocols, nongeneric
enumerations (constrained to integer raw-value types), properties and
methods (including getters and setters) of classes and protocols,
initializers, deinitializers, and subscripts. The objc attribute tells
the compiler that a declaration is available to use in Objective-C
code.
...
nonobjc
Apply this attribute to a method, property, subscript, or initializer
declaration to suppress an implicit objc attribute. The nonobjc
attribute tells the compiler to make the declaration unavailable in
Objective-C code, even though it is possible to represent it in
Objective-C.
...
Here you can find more details in this Swift Documentation : InteractingWithObjective-C
As an answer of your question, overview from attached link is as below.
#objc : You can use attribute to change the name of a class, property, method, enumeration type, or enumeration case declaration in
your interface as it’s exposed to Objective-C code.
Example : if the name of your Swift class contains a character that isn’t supported by Objective-C, you can provide an alternative name to use in Objective-C.
#nonobjc : It makes a swift declaration unavailable in Objective-C. You can use it to resolve circularity for bridging
methods and to allow overloading of methods for classes imported by
Objective-C.
I had a Swift class declared like this:
#objc class MyHelper {
// class code
}
I had to do this to access such class from Objective-C classes, since I'm mixing languages in my project. Now that I upgraded to Xcode 7, I get this error:
Only classes that inherit from NSObject can be declared #objc
And such class is not known by my Objective-C classes anymore. I guess that then I should inherit my Swift class from NSObject, will that have any impact in the way the app was working?
Thanks
See the Apple staff SevenTenEleven's reply in the Apple Developer Forum.
He mentioned that this is because of #objc on Swift-rooted classes never quite behaved like an NSObject-rooted class, leading to various weirdness in the generated header and at runtime.
We can still treat any Swift class instance as an AnyObject, mark methods and properties on a Swift class as #objc, and conform to Objective-C protocols; the class just isn't exposed in the generated header and doesn't default to having its members available in Objective-C.
I've been looking in to the new Swift language trying to find what's the equivalent for an interface(in java) or a protocol(in objective-c) in Swift, after surfing on the internet and searching in the book provided by Apple, I still can't seem to find it.
Does any one know what's the name of this component in swift and what's its syntax?
Protocols in Swift are very similar to Objc, except you may use them not only on classes, but also on structs and enums.
protocol SomeProtocol {
var fullName: String { get } // You can require iVars
class func someTypeMethod() // ...or class methods
}
Conforming to a protocol is a bit different:
class myClass: NSObject, SomeProtocol // Specify protocol(s) after the class type
You can also extend a protocol with a default (overridable) function implementation:
extension SomeProtocol {
// Provide a default implementation:
class func someTypeMethod() {
print("This implementation will be added to objects that adhere to SomeProtocol, at compile time")
print("...unless the object overrides this default implementation.")
}
}
Note: default implementations must be added via extension, and not in the protocol definition itself - a protocol is not a concrete object, so it can't actually have method bodies attached. Think of a default implementation as a C-style template; essentially the compiler copies the declaration and pastes it into each object which adheres to the protocol.
swift has protocols as well, here is the relevant documentation: