1.第一种情况：dispatch_group_async直接执行具体任务，内部没有开启别的线程
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{
for (int i=10; i>0; i--) {
NSLog(@"%@ ++%d",[NSThread currentThread],i);
}
});
dispatch_group_async(group, queue, ^{
for (int i=10; i>0; i--) {
NSLog(@"%@ ==%d",[NSThread currentThread],i);
}
});
dispatch_group_notify(group, queue, ^{
NSLog(@"group:over");
});
会等所有任务执行完毕打印group:over：
2021-05-13 17:04:34.606987+0800 test[20638:8113573] group:over
2.第二种情况：dispatch_group_async内嵌dispatch_async或者dispatch_sync操作，需要借助dispatch_group_enter和dispatch_group_enter，否则不会等到所有任务执行完毕就会打印group:over
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{
dispatch_group_enter(group);
dispatch_sync(queue, ^{
for(inti=10; i>0; i--) {
NSLog(@"%@ ++%d",[NSThread currentThread],i);
}
dispatch_group_leave(group);
});
});
dispatch_group_async(group, queue, ^{
dispatch_group_enter(group);
dispatch_sync(queue, ^{
for(inti=10; i>0; i--) {
NSLog(@"%@ ==%d",[NSThread currentThread],i);
}
dispatch_group_leave(group);
});
});
dispatch_group_notify(group, queue, ^{
NSLog(@"group:over");
});
会等所有任务执行完毕打印group:over：
2021-05-13 16:56:55.015755+0800 test[19249:8100876] group:over
3.dispatch_barrier_async和dispatch_barrier_sync不能使用系统的全局并发队列，否则不起作用
区别：
dispatch_barrier_sync在主线程执行，执行完当前任务才能走后面的代码
dispatch_barrier_async在子线程执行，立刻返回，无需的等待当前任务执行完毕即可走后面的代码
