@synchronized keyword

TTFriendsCollection.h

@class TTUserObject;

@interface TTFriendsCollection : NSObject

- (void)addFriend:(TTUserObject *)user;

- (void)removeFriend:(TTUserObject *)user;

@end

TTFriendsCollection.m

#import "TTFriendsCollection.h"
#import "TTUserObject.h"

@interface TTFriendsCollection ()

@property(nonatomic, strong) NSMutableArray *friendsArray;

@end

@implementation TTFriendsCollection

- (void)addFriend:(TTUserObject *)user {
    @synchronized (self.friendsArray) {
        if (!self.friendsArray) {
            self.friendsArray = [NSMutableArray array];
        }
        
        [self.friendsArray addObject:user];
    }
}

- (void)removeFriend:(TTUserObject *)user {
    @synchronized (self.friendsArray) {
        if (self.friendsArray) {
            [self.friendsArray removeObject:user];
        }
    }
}

@end