Two Singleton mode and two Singleton Mode

Two Singleton mode and two Singleton Mode

IOS Singleton has two official methods:

(1) do not use GCD

# Import "ServiceManager. h" static ServiceManager * defaultManager; @ implementation ServiceManager + (ServiceManager *) defaultManager {if (! DefaultManager) defaultManager = [[self allocWithZone: NULL] init]; return defaultManager;} @ end

(2) Use GCD

# Import "ServiceManager. h"

@ Implementation ServiceManager + (ServiceManager *) sharedManager {static metrics predicate; static ServiceManager * sharedManager; dispatch_once (& predicate, ^ {sharedManager = [[ServiceManager alloc] init];}); return sharedManager;} @ end

The dispatch_once function ensures that a code segment in the entire application lifecycle is executed only once!