回归基础：重温ARC

1,ARC 是编译器做的事情（在编译期间插入内存管理相关代码）

ARC evaluates the lifetime requirements of your objects and automatically inserts appropriate memory management calls for you at compile time

2，即使是局部指针变量（属性变量、局部变量  are strong by default），所指对象引用计数也将＋1

ARC ensures that oldLastName is not deallocated before the NSLog statement.

- (void)takeLastNameFrom:(Person *)person {

    NSString *oldLastname = [self lastName];

    [self setLastName:[person lastName]];

    NSLog(@"Lastname changed from %@ to %@", oldLastname, [self lastName]);

}

3，ARC仅仅维护了内存管理，其它的资源还是要手动置空或释放

（记得scrollview滑动时候退出页面 某些系统(因老的sdk中delegate为assign？)会crash）

You do not have to (indeed you cannot) release instance variables, but you may need to invoke [systemClassInstance setDelegate:nil] on system classes and other code that isn’t compiled using ARC.

4，new是可以作为属性字段名的，需要重写get方法

// Won‘t work:

@property NSString *newTitle;

// Works:

@property (getter=theNewTitle) NSString *newTitle;