假设有一个People类，有一个NSString的属性name
@interface People: NSObject
@property (nonatomic, strong) NSString *name;
@end
然后执行以下代码：
People *p = [[People alloc] init];
NSMutableString *mutableS = [[NSMutableString alloc] init];
[mutableS setString:@"Lucy"];
p.name = mutableS;
NSLog(@"初始值：%@", p.name);
[mutableS setString:@"Lily"];
NSLog(@"变化后：%@", p.name);
执行结果：
2016-07-27 10:17:53.325 Test[1242:44365] 初始值：Lucy
2016-07-27 10:17:53.326 Test[1242:44365] 变化后：Lily
