IOS--dispatch_once 创建单例singletons

+(id)shareUserManager
{
    static id userManager;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        userManager = [[self alloc] init];
    });
    return userManager;
}

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 de style="font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102);" >dispatch_once_tde> 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。

1 线程安全

2 很好满足静态分析器要求

3 和自动引用计数（ARC）兼容

4 仅需要少量代码