IOS Study Notes-use dispatch_once to create a singleton

During development, we will use nsicationicationcenter and NSFileManager to obtain their instances through [nsicationicationcenter defacenter center] and [NSFileManager defaultManager]. In fact, this is a singleton. Let's take a look at void dispatch_once (dispatch_once_t * predicate, dispatch_block_t block). The first parameter is predicate, which is the predicate that checks whether the code block represented by the second parameter is called, the second parameter is the code block that will only be called once in the entire application. The code block in the dispach_once function is executed only once, and it is thread-safe. Next we will implement our own singleton. Here is a SchoolManager class, which implements the singleton [cpp] + (SchoolManager *) sharedInstance {www.2cto.com static SchoolManager * sharedManager for this class; static dispatch_once_t onceToken; dispatch_once (& onceToken, ^ {sharedManager = [[SchoolManager alloc] init] ;}); return sharedManager ;}so far, we have implemented a singleton, everything is done, isn't it easy! Use the following method to obtain a unique instance: [cpp] SchoolManager * schoolManager = [SchoolManager sharedInstance]; The preceding describes how to use the dispatch_once function to implement a singleton, you are welcome to add and discuss it!