Unable to access Swift 4 class from Objective-C: "Property not found on object of type"

Using the latest Xcode 9 beta, I'm seemingly completely unable to access properties on Swift classes. Even odder, I can access the class itself to instantiate it or whatever, but completely unable to access properties on it.

So if I have this Swift class:

import UIKit

class TestViewController: UIViewController {
    var foobar = true
}

And I try to do this:

TestViewController *testViewController = [[TestViewController alloc] init]; // success
testViewController.foobar; // error

What exactly am I doing wrong? New project with Xcode 9.

The rules for exposing Swift code to Objective-C have changed in Swift 4. Try this instead:

@objc var foobar = true

As an optimization, @objc inference have been reduced in Swift 4. For instance, a property within an NSObject-derived class, such as your TestViewController, will no longer infer @objc by default (as it did in Swift 3).

Alternatively, you could also expose all members to Objective-C at once using @objcMembers:

@objcMembers class TestViewController: UIViewController {
    ...
}

This new design is fully detailed in the corresponding Swift Evolution proposal.

Swift 3.2/4.0 / XCode 9.1

You you set swift3.2 in project settings ( //:configuration = Debug SWIFT_VERSION = 3.2 )

you can use your code,(using the correct import file in objc, see below). If You set project to swift 4.0 ( //:configuration = Debug SWIFT_VERSION = 4.0 )

You must prepend @objc for every property.

So:

Swift 3.2:

// MyClass.swift

@objc class MyClass: NSObject{
	
	var s1: String?
	@objc var s2 : String?
}


//

//  ViewController.m

import "MixingObjCAndSwift-Swift.h"
#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
	[super viewDidLoad];    	
	MyClass * mc = [MyClass new];
	NSString * s1 = mc.s1;
	NSString * s2 = mc.s2;
}

works.

Swift 4.0:

// MyClass.swift

@objc class MyClass: NSObject{
	
	var s1: String?
	@objc var s2 : String?
}


.....  
- (void)viewDidLoad {
	[super viewDidLoad];    	
	MyClass * mc = [MyClass new];
	NSString * s1 = mc.s1;
	NSString * s2 = mc.s2;
}

does NOT works: compiler fails:

/Users....ViewController.m:24:21: Property 's1' not found on object of type 'MyClass *'

as s1 is not prepended with @objc.

You must write:

@objc class MyClass: NSObject{
	
	@objc var s1: String?
	@objc var s2 : String?
}

(As a side-note: in C/C++/ObJC file, put always system/general *h files before your "local" class headers.)

Swift 4

just add @objcMembers before class @objcMembers class MyClassObject: NSObject { var s1: String! var s2: String!

} Swift evolutionenter link description here

As for Swift 5.1 I found that "@objc" is not enough, but also make it public "@objc public":

@objc public class SomeClass: NSObject {
    @objc public let amount: NSNumber
...

1. When you add a swift file in your Objective-C project, Xcode will prompt to add Objective-C bridging header file, so allow the header file to be created.
2. In your Objective-C implementation file where you want to access the TestViewController property foobar. Use the following import syntax and replace the ProjectName with your project.

> #import "ProjectName-Swift.h"

Objective-C implementation file:

#import "ViewController.h"
#import "ProjectName-Swift.h"

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    TestViewController *testViewController = [[TestViewController alloc] init]; // success
    BOOL prop = testViewController.foobar;
    NSLog(@"Property: %d", prop);
}

@end

For more details go through the Apple Documents

Xcode 10.2 | Swift 4 Adding @objcMembers before class solved my problem.

@objcMembers class MyClass:NSObject {
   var s1: String!
   var s2: NSMutableArray!
}

I also encountered with this problem in swift 3.0

class declaration was like that

class ClassA {
//statements
}

above class was not accessible in Objective C code and it was also not registered in -Swift.h

once I inherited from NSObject like this:

class ClassA : NSObject {
//statements
} 

the class was accessible in Objective c and got registered with -Swift.h

This solution is useful when you don't want to avoid inheritance from NSobject class.