GCD accessor

main.m

//
//  main.m
//  test
//
//  Created by Rob Napier on 4/5/19.
//

#import <Foundation/Foundation.h>

@interface MyClass: NSObject
@property (nonatomic, readwrite, copy) NSString *name;
@end

@interface MyClass()
// I often just use one queue for the whole object, but you can also
// split things out so each property has its own queue.
@property (nonatomic, readwrite, strong) dispatch_queue_t queue;
@end

@implementation MyClass {
    NSString *_name;    // Make a backing ivar
}

- (instancetype)init
{
    self = [super init];
    if (self) {
        // Standard concurrent queue
        _queue = dispatch_queue_create("MyClass", DISPATCH_QUEUE_CONCURRENT);
    }
    return self;
}

- (NSString *)name {
    // We have to block here (_sync), but since this is concurrent we will only
    // be blocked if there's a write queued up.
    __block NSString *result;
    dispatch_sync(self.queue, ^{
        result = self->_name;
    });
    return result;
}

- (void)setName:(NSString *)name {
    // Barrier says that this block can't start until all previous blocks have finished
    // and no later blocks may start until this block is done. Make it fast. That's why
    // we do the copy before going into the barrier.
    // All reads that were submitted before this write will see the old value, and all
    // reads that were submitted after this write will see the new value.

    NSString *copy = [name copy];
    dispatch_barrier_async(self.queue, ^{
        self->_name = copy;
    });
}

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MyClass *x = [MyClass new];
        x.name = @"Alice";
        NSLog(@"Hello, %@!", x.name);
    }
    return 0;
}