关键字 nonnull和nullable

iOS 9新增关键字: 用于修饰属性或者方法的参数、方法的返回值

• nonnull不可为空,等同于_Nonnull和__nonnull

  e.g.

    @property (nonatomic, strong, nonnull) UIView *view;
    @property (nonatomic, strong) UIView *_Nonnull view;
    @property (nonatomic, strong) UIView *__nonnull view;
  

• nullable可为空,等同于_Nullable和__nullable

  e.g.

    @property (nonatomic, strong, nullable) UIView *view;
    @property (nonatomic, strong) UIView *_Nullable view;
    @property (nonatomic, strong) UIView *__nullable view;
  

• null_resettable setter方法可传空,getter方法不为空

• _Null_unspecified 不确认是否为空