Inspired by Chris Lattner's concurrency manifesto, this defines a generic thread-safe object in Objective-C, similar to how Chris describes the behavior of actors.

main.m

#import "ThreadSafe.h"

int main(int argc, char **argv) {
    NSNumber *seven = @7;
    
    ThreadSafe<NSNumber *> *threadSafeNumber = [[ThreadSafe alloc] initWithValue:seven];
    NSLog(@"%@", threadSafeNumber.value.stringValue);

    return 0;
}

ThreadSafe.h

@import Foundation;

@interface ThreadSafe<Value>: NSProxy

- (instancetype)initWithValue:(Value)value;

@property (nonatomic, strong, readonly) Value value;

@end

ThreadSafe.m

@interface ThreadSafe ()
    
@property (nonatomic, strong) dispatch_queue_t queue;
@property (nonatomic, strong) id internalValue;

@end


@implementation ThreadSafe

- (instancetype)initWithValue:(id)value {
    _internalValue = value;
    _queue = dispatch_queue_create(NULL, 0);

    return self;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [self.internalValue methodSignatureForSelector:sel];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    dispatch_async(self.queue, ^{
        invocation.target = self.internalValue;
        [invocation invoke];
    });
}

- (id)value {
    return self;
}

@end