只执行一次 (多用于单例模式)
dispatch_once(dispatch_once_t *predicate, dispatch_block_t block);
dispatch_once_t *predicate：一个全局的变量 dispatch_block_t block：block函数块
多次执行
dispatch_apply(size_t iterations, dispatch_queue_t queue,void (^block)(size_t));
size_t iterations：执行次数 dispatch_queue_t queue：队列 void (^block)(size_t)：block函数块
具体举例如下：
1、只执行一次时：
//自定义一个block函数块
//定义block
typedef void (^BLOCK)(void);
//将执行代码封装到block中
BLOCK myBlock = ^(){
static int count = 0;
NSLog(@"count=%d",count++);
};
//执行dispatch_once函数
//myblock调用
//myBlock();
//myBlock();
//只会执行一次，GCD once
static dispatch_once_t predicate;
dispatch_once(&predicate, myBlock);
dispatch_once(&predicate, myBlock);
//运行结果：虽然执行两次,只有一个输出
2015-10-07 22:54:34.279 04-GCD-once[3501:278570] count=0
2、多次执行时：
//自定义一个block函数
//定义block
typedef void (^BLOCK)(size_t);
//将函数封装到block
BLOCK myBlock = ^(size_t size){
static int count = 0;
NSLog(@"count=%d",count++);
};
//执行dispatch_apply
//GCD多次执行任务
dispatch_apply(5, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), myBlock);
//运行结果如下：执行了5次,有5个输出
2015-10-07 22:57:59.438 05-GCD-apply[3528:280053] count=0
2015-10-07 22:57:59.438 05-GCD-apply[3528:280093] count=2
2015-10-07 22:57:59.438 05-GCD-apply[3528:280094] count=3
2015-10-07 22:57:59.438 05-GCD-apply[3528:280095] count=1
2015-10-07 22:57:59.439 05-GCD-apply[3528:280053] count=4
