ios-Single case

1 StaticDemoobj *instance;2 3 /**4 1. Rewrite Allocwithzone and instantiate a static variable with dispatch_once5 2. Write a +sharedxxx to facilitate other class calls6  */7 8 //in iOS, the allocation of memory space for all objects will eventually call the Allocwithzone method9 //If you want to do a singleton, you need to override this methodTen //GCD provides a method that is designed to create a singleton One+ (ID) Allocwithzone: (struct_nszone *) Zone A { -     StaticDemoobj *instance; -     the     //dispatch_once is thread-safe, Oncetoken defaults to 0 -     Staticdispatch_once_t Oncetoken; -     //Dispatch_once Macros Ensure that the instructions in the block code are executed only once -Dispatch_once (&oncetoken, ^{ +         //in a multithreaded environment, it will only be executed once and instance will only be instantiated once -Instance =[Super Allocwithzone:zone]; +     }); A     at     returninstance; - } -  -+(instancetype) shareddemoobj - { -     //If two threads are instantiated at the same time, it is possible to create two instances to in //if (!instance) { - //        //Thread 1 0x0000a to //        //Thread 2 0x0000b + //instance = [[Self alloc] init]; - //    } the //    //the pointer returned by the first thread has been modified!  * //return instance; $     return[[Self alloc] init];Panax Notoginseng } -  the  + macro for a single case A  the //. h + #defineSingleton_interface (Class) + (Instancetype) shared# #class; -  $ //. M $ #defineSingleton_implementation (Class) - class*_instance; -  the+ (ID) Allocwithzone: (struct_nszone *) Zone - { Wuyi     Staticdispatch_once_t Oncetoken; theDispatch_once (&oncetoken, ^{  -_instance =[Super Allocwithzone:zone]; Wu     });  -  About     return_instance; $ }  -  -+ (Instancetype) shared##class  - {  A     if(_instance = =Nil) {  +_instance = [[classalloc] init];  the     }  -  $     return_instance; the}

ios-Single case