@public: instance variable can be read and written to directly, using the notation person->age = 32"
@package: instance variable is public, except outside of the framework in which it is specified (64-bit architectures only)
@protected: instance variable is only accessible to its class and derived classes
@private: instance variable is only accessible to its class
（前面加@符号和其它语言进行区分）
@try
{
// attempt to execute the following statements
![self getValue:&value error:&error];
// if an exception is raised, or explicitly thrown...
if (error)
{
@throw exception;
}
}
@catch(NSException *e)
{
// ...handle the exception here
}
@finally
{
// always execute this at the end of either the @try
or @catch block
[self cleanup];
}
@selector(): Returns an SEL pointer to a selector with the specified name. Used in methods like -performSelector:withObject:.
@protocol(): Returns a Protocol pointer to the protocol with the specified name. Used in methods like *-conformsToProtocol:.
@encode(): Returns the type encoding of a type. This type value can be used as the first argument encode in NSCoder -encodeValueOfObjCType:at.
@defs(): Returns the layout of an Objective-C class. For example, to declare a struct with the same fields as an NSObject, you would simply do:
struct
{
@defs(NSObject)
}
@defs is unavailable in the modern Objective-C runtime.
@autoreleasepool{}: If your code contains a tight loop that creates lots of temporary objects, you can use the @autorelease directive to optimize by being more aggressive about how for these short-lived, locally-scoped objects are deallocated. @autoreleasepool replaces and improves upon the old NSAutoreleasePool, which is significantly slower, and unavailable with ARC.
@synchronized(){}: This directive offers a convenient way to guarantee safe execution of a particular code block within a specified context (usually self ). Locking in this way is expensive, however, so for classes aiming for a particular level of thread safety, a dedicated NSLock property or the use of low-level locking functions like OSAtomicCompareAndSwap32(3) are recommended.
@compatibility_alias: Allows existing classes to be aliased by a different name.
@compatibility_alias can be used to significantly improve the experience of working with classes across major OS versions, allowing developers to back-port their own custom implementations of new functionality, without changing how that class is used in the app.