集合对象 作为属性(声明不可变)

(参考 高质量iOS 与 OSX 的52 个有效方法)

@interface XXYPerson : NSObject

@property (nonatomic, copy, readonly) NSString *firstName;
@property (nonatomic, copy, readonly) NSString *lastName;
@property (nonatomic, copy, readonly) NSSet *firends;

- (instancetype)initWithFirstName:(NSString *)firstName lastName:(NSString *)lastName;

- (void)addFriends:(XXYPerson *)person;
- (void)removeFridends:(XXYPerson *)person;

@end

#import "XXYPerson.h"

@implementation XXYPerson
{
    NSMutableSet *_internalFriends;
}

- (instancetype)initWithFirstName:(NSString *)firstName lastName:(NSString *)lastName {
    self = [super init];
    if (self) {
        _firstName = firstName;
        _lastName = lastName;
        _internalFriends = [NSMutableSet new];
    }
    return self;
}

- (NSSet *)firends {
    return [_internalFriends copy];
}

- (void)addFriends:(XXYPerson *)person {
    [_internalFriends addObject:person];
}

- (void)removeFridends:(XXYPerson *)person {
    [_internalFriends removeObject:person];
}

@end