I'm working with ReactiveCocoa in Swift. I need to use the following method:
rac_signalForSelector(selector: Selector, fromProtocol: Protocol?)
Passing the selector works fine with Selector("method:"), but I cannot find how to pass the delegate protocol to the fromProtocol parameter.
What is the proper way to pass the Protocol type from a delegate to a method signature like this?
EDIT: Adding method documentation and best attempt
The documentation for this method is as follows:
selector - The selector for whose invocations are to be observed. If it doesn’t exist, it will be implemented using information from protocol, and may accept non-object arguments and return a value. This cannot have C arrays or unions as arguments or return type.
protocol - The protocol in which selector is declared. This will be used for type information if the selector is not already implemented on the receiver. This must not be NULL, and selector must exist in this protocol.
I have tried sending in DelegateType.self, and I end up receiving this error:
Could not find an overload for rac_signalForSelector that accepts the supplied arguments
Did you use #objc when you declared your protocol?
I believe SomeProtocol.self is right right way to pass it but since you're passing it into an obj-c API it needs to be prefixed with #objc like this example from the docs:
#objc protocol HasArea {
var area: Double { get }
}
Edit: Turns out the protocol in question is from a library (written in objective-c, so already compatible with objective-c), not defined in Swift.
That being the case, it's probably a compiler bug, so before you do anything else make sure you're using the latest version of Xcode (beta 3 at the time of writing).
If that doesn't work out, I think Tommy's idea to use NSProtocolFromString is the best recourse until the compiler bug is fixed.
Related
In NotificationCenter Class , Why apple has created Observer of type Any?
func addObserver(Any, selector: Selector, name: NSNotification.Name?, object: Any?)
My Reasoning.
If observer is struct then on assigning inside as function parameter, It will be copied then how my observer can receive the notification.
I can't write any function which uses #objc prefix in Struct.
Selector is always be type of #objc.
So What is the use of Any in addObserver.....
It should always be of type AnyObject.
Secondly we already known that NotificationCenter keep the weak copy of observer, And we can't use weak modifier for type Any. Then how apple is managing all this?
Any help in understanding this concept is highly appreciated.
No one chose to make this parameter Any. It's just what they got by default. It's automatically bridged from ObjC:
- (void)addObserver:(id)observer
selector:(SEL)aSelector
name:(nullable NSNotificationName)aName
object:(nullable id)anObject;
The default way that id is bridged is Any. It hasn't been specially refined for Swift. In practice, you can't really use structs meaningfully here. The fact that the compiler won't stop you from calling it in an unhelpful way doesn't imply that it's intended to be used that way.
Why type Any? - because in Objective C it is type id.
Why you can't mark your function as #obj - #obc is the keyword for Swift code which indicates what compiler should add this method to a header file for this Class, yes you can make headers only for Classes.
Selector also is the objective-c term, it just says which function to invoke, similar to msg_send
In NotificationCenter Class , Why apple has created Observer of type Any.
Because all Objective-C id declarations are translated into Swift as Any.
You might object that this really should be AnyObject, because only a class will work here. And indeed, that's the way id used to be translated into Swift. But nowadays you can pass anything where an id is expected, because if it's something Objective-C can't understand, it will be boxed up as a class instance (e.g. as a _SwiftValue) so that it can make the round-trip into Objective-C and back again to Swift. Therefore id is translated as Any.
However, just because you can pass a struct here doesn't mean you should. It won't work, as you've discovered. Objective-C cannot introspect a Swift struct.
There are lots of situations like this, where Cocoa gives you enough room to hang yourself by passing the wrong thing. The contents of a CALayer is typed as Any, but if you pass anything other than a CGImage, nothing will happen. The layerClass if a UIView is typed as AnyClass, but you'd better pass a CALayer subclass. I could go on and on.
I am trying to write a registry in Swift that maps from API's (Protocols) to Implementations (Classes). I would like to be able to provide the registry an API and receive back an instance of the class that implements it. In Objective-C this was fairly trivial - just call NSStringFromProtocol on the protocol and then use that as a key for a dictionary containing the classes that implement them. In Swift, however, we do not have this introspective capability. When I try to do the same I am told that MyAPI.protocol does not have a member "mirrorType". My question to you is how, in Swift, without using #objc protocols, I can map from a protocol itself to the class that implements it. Thanks!
By now it's not possible without using #objc. The solution I've found in this case is using the protocol name (string) as a key for the dictionary for this implementations (In my case I'll always have only one instance per Protocol).
Using #objc will force you to have all your implementations returning AnyObject the equivalent (id) in objective-C (if your function does not return a native objective-C type).
Hope that helps.
What the difference between a method, a selector and a message in Objective-C?
This is a great question.
Selector - a Selector is the name of a method. You're very familiar with these selectors: alloc, init, release, dictionaryWithObjectsAndKeys:, setObject:forKey:, etc. Note that the colon is part of the selector; it's how we identify that this method requires parameters. Also (though it's extremely rare), you can have selectors like this: doFoo:::. This is a method that takes three parameters, and you'd invoke it like [someObject doFoo:arg1 :arg2 :arg3]. There's no requirement that there be letters before each part of the selector components. As I said, this is extremely rare, and you will not find it used in the Cocoa frameworks. You can work with selectors directly in Cocoa. They have the type SEL: SEL aSelector = #selector(doSomething:) or SEL aSelector = NSSelectorFromString(#"doSomething:");
Message - a message is a selector and the arguments you are sending with it. If I say [dictionary setObject:obj forKey:key], then the "message" is the selector setObject:forKey: plus the arguments obj and key. Messages can be encapsulated in an NSInvocation object for later invocation. Messages are sent to a receiver. (ie, the object that "receives" the message).
Method - a method is a combination of a selector and an implementation (and accompanying metadata). The "implementation" is the actual block of code; it's a function pointer (an IMP). An actual method can be retrieved internally using a Method struct (retrievable from the runtime).
Some other related things that you didn't ask for:
Method Signature - a method signature represents the data types returned by and accepted by a method. They can be represented at runtime via an NSMethodSignature and (in some cases) a raw char*.
Implementation - the actual executable code of a method. Its type at runtime is an IMP, and it's really just a function pointer. iOS 4.3 includes a new ability to turn a block into an IMP. This is really cool.
One of the fun things to realize is that the name of a method (the selector) is distinct from the implementation of the method (the IMP). This means that you can swap them around, if you're feeling daring. You can also add and remove methods at runtime, because all you're doing is editing an entry in a hash table: the key is the selector, and the value is the IMP of the method. This allows you to do some really crazy and trippy stuff. It's not for the faint of heart. :)
A method is the implementation which is run when an object or class is asked to perform some action. It is in the scope of its containing class and is therefore different when referenced through some other class. A selector is an identifier which represents the name of a method. It is not related to any specific class or method, and can be used to describe a method of any class, whether it is a class or instance method.
Simply, a selector is like a key in a dictionary. It can tell you what method someone is talking about, but only if you also have the dictionary itself (the class or object). The method is what you get when you ask for the value from the dictionary using the selector as a key.
This site has a good overview of all the terminology in question: http://www.otierney.net/objective-c.html
Check out the link, but I'll give a quick summary:
A method is essentially like a method of function that you are used to in your favourite programming language.
A message (from the article) "A message can be dynamically forwarded to another object. Calling a message on an object in Objective-C doesn't mean that the object implements that message, just that it knows how to respond to it somehow via directly implementing it or forwarding the message to an object that does know how to."
Selectors can mean two things. It can refer to the name of a method, or "refers to the unique identifier that replaces the name when the source code is compiled. Compiled selectors are of type SEL." (from: http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocSelectors.html)
I want to get current method name to use in a format message similar to this one
[NSExeception raise:NSInternalInconsistencyException format:#"You must override %# in a subclass", NSStringFromSelector(_cmd)]
Also, I want to use _cmd to set associated object. I appreciate any idea.
NSStringFromSelector(_cmd); // Objective-C
print(#function) // Swift 4, 5
There is no Swift equivalent of _cmd. There is little reason to use it in Swift.
Consider _cmd in Objective-C. When is it useful? Most of the time, the value of _cmd would be the same as the name of the method that the code is in, so it is already known at compile time, and there is no need for a runtime value. Here are some possible cases when _cmd is useful in Objective-C:
In a macro. The macro is expanded in the code, so if _cmd appears in the macro, then it is inserted into the source where it is used, and so the method name can be used inside the macro. However, such macros do not exist in Swift. Plus macros are compile-time, so a compile-time mechanism like __FUNCTION__ would work similarly.
You can define a C function that takes self and _cmd, and use it (the same function) as the implementation of multiple methods, by adding it using class_addMethod and class_replaceMethod, and the _cmd inside the function will help distinguish between the different method calls. However, class_addMethod and class_replaceMethod are not available in Swift.
Method swizzling is also a process that messes with the implementation of a method. Since in swizzling you swap the implementations of two methods, _cmd will help reveal the actual method name used in the call, which may not match the method that the code is in in the source code, since implementations are swapped. I guess method swizzling may still be possible in Swift, since method_exchangeImplementations is still available in Swift. But in method swizzling, the method you swap in is tailored for the method it is swapping with, so if it is called, there is no ambiguity which method name is being called.
In the case where you manually get the IMP (implementing function) of a method, and manually call it with a different selector. In this case, the inside of the function can see the different selector in _cmd. However, you don't have to worry about this in Swift because the methods that get the IMP are unavailable.
Why are warnings generated when calling methods undeclared in a class interface using conventional means, but not when calling methods using #selector? Is it because selectors can be executed by a different caller than self?
For example:
-(void) doStuff
{
[self doNow]; // Warning: instance method not found
SEL sel = #selector(doNow); // no warnings
}
-(void) doNow {} // this method is not declared in the interface
The "Undeclared Selector" warning is turned off by default. I don't know why. You can turn it back on in the Build Settings.
The documentation for this settings reads:
Warn if a "#selector(...)" expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared before the "#selector(...)" expression, either explicitly in an #interface or #protocol declaration, or implicitly in an #implementation section. This option always performs its checks as soon as a "#selector(...)" expression is found, while -Wselector only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used. [GCC_WARN_UNDECLARED_SELECTOR, -Wundeclared-selector]
A similar question was asked on SO a few weeks ago.
This is basically because selectors are late-bound. They're not looked up until runtime. There are options for forcing verification during compilation. The question I linked to has some more information on how you can do that.
The way #selector works by default is you are telling the compiler; trust me I have this method somewhere in my class. Its the same concept if you do #class yourclassname instead of importing the .h file which contains the class.
This is because the compiler needs to know the signature of the method in order to call it (i.e. [self doNow];), because such a call translates to either a call to objc_msgSend or objc_msgSend_stret depending on whether the method signature has a return type of a struct or not. (Remember the difference between the selector (simply the name with the colons in it, but with no types) and the signature (the types) of a method.) So it needs to warn because it could be calling the wrong function if it doesn't know.
However, just getting a selector (#selector(...)), it doesn't need to know the types. The selector is simply a name, and you've provided the name. It's all about what you use the selector for. If you use it in performSelector:, it also doesn't need to know the types, because that method only works for methods with argument and return types of objects, so there is no ambiguity. Hence there is no need for a warning.