iOS GCD

dispatch_group、dispatch_group_enter、dispatch_group_leave
只有一层异步任务，多个任务并发执行，完成后通知：

dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
  for (int i = 0; i<10; i++) {
      NSLog(@"任务1 ==============  第%d个任务",i);
    }
})；
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{      
  for (int i = 0; i<10; i++) {
    NSLog(@"任务2 ==============  第%d个任务",i);
  }
});        
dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
  NSLog(@"完成");
});

输出结果：任务1、2执行不确定，但一定是两个都完成之后才会通知

注：如果任务里面又包含异步任务，这个通知会执行时间不定可能是任务1自行一次就通知，也可能2执行一次就通知，要想实现上面的效果，在任务1、2完成之后通知就需要用到
dispatch_group_enter和dispatch_group_leave
示例代码：

dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{       
  dispatch_group_enter(group); 
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),^{
    for (int i = 0; i<10; i++) {
      NSLog(@"任务1 ==============  第%d个任务",i);
    }
    dispatch_group_leave(group);
  });
});
dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ 
  dispatch_group_enter(group);
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    for (int i = 0; i<10; i++) {
      NSLog(@"任务2 ==============  第%d个任务",i);
    }
    dispatch_group_leave(group);
   });
});
dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
  NSLog(@"完成");
});