ARC模式下的单例写法

// 单例
+ (id)sharedInstance
{
    __strong static id sharedObject = nil;

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedObject = [[self alloc] init];
    });
    
    return sharedObject;
}

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 (including Objective-C instance variables) is undefined.

Availability

• Available in iOS 4.0 and later.

Declared In

dispatch/once.h