dispatch_apply直到所有任务都执行完毕，才会返回。
例如：
dispatch_apply(5, dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT), ^(size_t n) {
for (int i=0; i<2; i++) {
sleep(1);
NSLog(@"%@ -- %d",[NSThread currentThread], i);
}
});
NSLog(@"all finished");
输出结果
2018-12-01 23:14:59.933527+0800 dispatch_apply-Demo[50299:2325437] <NSThread: 0x600000064fc0>{number = 1, name = main} -- 0
2018-12-01 23:14:59.933550+0800 dispatch_apply-Demo[50299:2325496] <NSThread: 0x6040002603c0>{number = 5, name = (null)} -- 0
2018-12-01 23:14:59.933560+0800 dispatch_apply-Demo[50299:2325497] <NSThread: 0x60400007fc00>{number = 4, name = (null)} -- 0
2018-12-01 23:14:59.933596+0800 dispatch_apply-Demo[50299:2325494] <NSThread: 0x60000027b040>{number = 3, name = (null)} -- 0
2018-12-01 23:15:00.934084+0800 dispatch_apply-Demo[50299:2325437] <NSThread: 0x600000064fc0>{number = 1, name = main} -- 1
2018-12-01 23:15:00.937887+0800 dispatch_apply-Demo[50299:2325497] <NSThread: 0x60400007fc00>{number = 4, name = (null)} -- 1
2018-12-01 23:15:00.937888+0800 dispatch_apply-Demo[50299:2325496] <NSThread: 0x6040002603c0>{number = 5, name = (null)} -- 1
2018-12-01 23:15:00.937980+0800 dispatch_apply-Demo[50299:2325494] <NSThread: 0x60000027b040>{number = 3, name = (null)} -- 1
2018-12-01 23:15:01.935680+0800 dispatch_apply-Demo[50299:2325437] <NSThread: 0x600000064fc0>{number = 1, name = main} -- 0
2018-12-01 23:15:02.937273+0800 dispatch_apply-Demo[50299:2325437] <NSThread: 0x600000064fc0>{number = 1, name = main} -- 1
2018-12-01 23:15:02.937622+0800 dispatch_apply-Demo[50299:2325437] all finished
