//串行队列 同步执行任务 会在当前线程内执行 //所以打印的会是当前线程的编号 不一定是主线程
-(void)demo
{
//创建串行队列 DISPATCH_QUEUE_SERIAL
// 并行队列 DISPATCH_QUEUE_CONCURRENT
dispatch_queue_t queue=dispatch_queue_create("itcast", DISPATCH_QUEUE_SERIAL);

//创建任务
dispatch_block_t task1=^{
    NSLog(@"%@  同步执行了方法1",[NSThread currentThread]);
};

dispatch_block_t task2=^{
    NSLog(@"%@  同步执行了方法2",[NSThread currentThread]);
};


//加入队列

//同步的方式 执行队列
dispatch_sync(queue, task1);

dispatch_sync(queue, task2);

}

//异步的方式 执行串行队列 会创建一个新的线程来执行任务
-(void)demo2
{
//创建串行队列 DISPATCH_QUEUE_SERIAL
// 并行队列 DISPATCH_QUEUE_CONCURRENT
dispatch_queue_t queue=dispatch_queue_create("itcast", DISPATCH_QUEUE_SERIAL);

//创建任务
dispatch_block_t task1=^{
    NSLog(@"%@  异步执行了方法1",[NSThread currentThread]);
};

dispatch_block_t task2=^{
    NSLog(@"%@  异步执行了方法2",[NSThread currentThread]);
};


//加入队列

//异步的方式 执行队列

//会顺序执行
dispatch_async(queue, task1);

dispatch_async(queue, task2);

}

1.异步执行

dispatch_async(dispatch_get_global_queue(0, 0), ^{
// something
});
2.同步执行

dispatch_sync(dispatch_get_global_queue(0, 0), ^{
// something
});
dispatch_get_global_queue(0, 0) //表示创建一个新的任务（可以理解成线程）执行
dispatch_get_main_queue() //表示使用主线程调用block
3.一次性执行（常用于单例模式）

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
// code to be executed once
});
4.定时执行

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
});
5.自定义任务队列

dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL);
dispatch_async(urls_queue, ^{
　// your code
});
dispatch_release(urls_queue);
6.任务组，汇总

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
});