多线程-GCD

1.串行队列：Dispatch Queues或者 Serial Queues 

Dispatch Queues从使用的角度讲更象另一种形式的Operation Queues，

只是 Operation Queuse是用ObjectC的，Dispatch Queues是C的。

dispatch Queues，serial Queues 也被称为私有dispatch Queues。

dispatch_queue_t queue = dispatch_queue_create("myQueue", DISPATCH_QUEUE_SERIAL);

2.并发队列：Concurrent Queues

每个程序系统自动提供了3个Concurrent Queues ，三个优先级，第二个参数没用都设置成0或者NULL就行。

dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
dispatch_queue_t aHQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
  
dispatch_queue_t aLQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);  

3.主队列：main dispatch Queue 

dispatch_queue_t mainQueue = dispatch_get_main_queue();  

4.名词解释：

global 全局

queue  队列

async 异步

sync 同步

5、

//  后台执行：
 dispatch_async(dispatch_get_global_queue(0, 0), ^{
      // something
 });
 // 主线程执行：
 dispatch_async(dispatch_get_main_queue(), ^{
      // something
 });
 // 一次性执行：
 static dispatch_once_t onceToken;
 dispatch_once(&onceToken, ^{
     // code to be executed once
 });
 // 延迟2秒执行：
 double delayInSeconds = 2.0;
 dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
 dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
     // code to be executed on the main queue after delay
 });

6.dispatch_group(组)

dispatch_group_t group = dispatch_group_create();
 dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
      // 并行执行的线程一
 });
 dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
      // 并行执行的线程二
 });
 dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{
      // 汇总结果
 });

dispatch_apply, 作用是把指定次数指定的block添加到queue中
dispatch_apply。这个函数调用单一block多次，并平行运算，然后等待所有运算结束
dispatch_queue_t queue = dispatch_get_global_qeueue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_apply([array count], queue, ^(size_t index){
        [self doSomethingIntensiveWith:[array objectAtIndex:index]];
    });
    [self doSomethingWith:array];