dispatch_once
Executes a block object once and only once for the lifetime of an application.
void dispatch_once(
dispatch_once_t *predicate,
dispatch_block_t block);
Parameters
predicate
A pointer to a dispatch_once_t structure that
is used to test whether the block has completed or not.
block
The block object to execute once.
Discussion
This function is useful for initialization of global data (singletons) in an application. Always call this function before using or testing any variables that are initialized by the block.
If called simultaneously from multiple threads, this function waits synchronously until the block has completed.
The predicate must point to a variable stored in global or static scope. The result of using a predicate with automatic or dynamic storage is undefined.
dispatch_once不仅意味着代码仅会被运行一次，而且还是线程安全的，这就意味着你不需要使用诸如@synchronized之类的来防止使用多个线程或者队列时不同步的问题。
如果你要共享某个实例
＋(MyInstance*)shareMyInstance（）
｛
static MyInstance *inst=nil;
static dispatch_once_t p;
dispatch_once(&p,^{
inst=[[MyInstance alloc]init];
});
return inst;
｝
你任何时候访问共享实例，需要做的仅是：
MyInstance *myInst = [MyInstance shareMyInstance];
就这样，你在应用中就有一个共享的实例，该实例只会被创建一次。
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