Nsclassfromstring Returning Nil for Nested Class

Instantiating a nested class using NSClassFromString in swift

I find that the following works in a playground (Xcode 8.2 / Swift 3):

// inheriting NSObject is required for `@objc`, at which point `@objc` is optional
class A: NSObject {
    class B: NSObject {
        override var description: String { return "foo" }
    }
}

let str = NSStringFromClass(A.B.self)
guard let anyClass = NSClassFromString(str)
    else { fatalError("no class") }
// cast to a type that defines `init()` so we can instantiate
guard let nsClass = anyClass as? NSObject.Type 
    else { fatalError("class isn't NSObject") }
// call `.init()`, not `nsClass()`; constructor syntax is for static types only
let instance = nsClass.init() 
print(instance) // -> "foo"

The oddball class "name" string is because the ObjC runtime doesn't understand nested classes (or other kinds of types that Swift can define but ObjC can't) -- within Swift itself, such mangled names are how types, functions and such get uniquely defined. (For example, name mangling is also how function overloading works: func foo() and func foo(num: Int) -> Bool have different mangled names internally.)

The bridging machinery can still be made to dynamically resolve a class given its properly mangled Swift name, but Swift name mangling is an implementation detail and subject to change. (At least until Swift 4 locks down the ABI.) So it's safe to pass the result of NSStringFromClass to NSClassFromString within the same program, but not safe to (e.g.) record a mangled name once in testing, ship that mangled name as a string literal or resource in your app, and expect it to work with NSClassFromString later.

Instead, if you want your nested class (or any other Swift-defined class) to have a reliably known name for use in the ObjC runtime, give it an @objc(name) (as described in the docs):

@objc class A: NSObject {
    @objc(A_B) class B: NSObject { /*...*/ }
}
guard let anyClass = NSClassFromString("A_B")
    else { fatalError("no class") }

(Note this snippet won't run by itself -- the class A.B has to be referenced at least once somewhere in your process in order to be registered with the ObjC runtime. This could be as simple as putting let t = A.B.self somewhere in your app's startup code.)

NSClassFromString() returns a class even when class should not be available

To check if CLGeocoder is available and it is really running iOS 5 I used the following:

    if (NSClassFromString(@"CLGeocoder") && 
    [NSClassFromString(@"CLGeocoder") instancesRespondToSelector:
@selector(reverseGeocodeLocation:completionHandler:) ]) {

                        //iOS 5 or later    
       }

NSClassFromString in library

My explanation would be, the library cannot load the class, because it
is not in it's bundle. Is this true?

No, not true. At runtime there are no Libraries, no Projects, just your App, containing all (your) linked code, which is all in your executable.

You could be doing it before Classes have loaded, you could have spelt it wrong, you would have to show some code.

Swift language NSClassFromString

Less hacky solution here: https://stackoverflow.com/a/32265287/308315

Note that Swift classes are namespaced now so instead of "MyViewController" it'd be "AppName.MyViewController"

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Deprecated since XCode6-beta 6/7

Solution developed using XCode6-beta 3

Thanks to the answer of Edwin Vermeer I was able to build something to instantiate Swift classes into an Obj-C class by doing this:

// swift file
// extend the NSObject class
extension NSObject {
    // create a static method to get a swift class for a string name
    class func swiftClassFromString(className: String) -> AnyClass! {
        // get the project name
        if  var appName: String? = NSBundle.mainBundle().objectForInfoDictionaryKey("CFBundleName") as String? {
            // generate the full name of your class (take a look into your "YourProject-swift.h" file)
            let classStringName = "_TtC\(appName!.utf16count)\(appName)\(countElements(className))\(className)"
            // return the class!
            return NSClassFromString(classStringName)
        }
        return nil;
    }
}

// obj-c file
#import "YourProject-Swift.h"

- (void)aMethod {
    Class class = NSClassFromString(key);
    if (!class)
        class = [NSObject swiftClassFromString:(key)];
    // do something with the class
}

EDIT

You can also do it in pure obj-c:

- (Class)swiftClassFromString:(NSString *)className {
    NSString *appName = [[NSBundle mainBundle] objectForInfoDictionaryKey:@"CFBundleName"];
    NSString *classStringName = [NSString stringWithFormat:@"_TtC%d%@%d%@", appName.length, appName, className.length, className];
    return NSClassFromString(classStringName);
}

I hope this will help somebody !

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