iOS如何判断多个异步线程是否执行完毕

一、添加100个操作，怎么知道执行完了

1、dispatch_group_notify

   dispatch_queue_t queue = dispatch_queue_create("jjjjjj", DISPATCH_QUEUE_CONCURRENT);
   dispatch_group_t group = dispatch_group_create();

   for (int i = 0; i<100; i++) {
       dispatch_group_async(group, queue, ^{
           NSLog(@"%@ %d",[NSThread currentThread],i);
       });
   }
   dispatch_group_notify(group, dispatch_get_main_queue(), ^{
       NSLog(@"end");
   });

2、dispatch_barrier_async

    dispatch_queue_t queue = dispatch_queue_create("kkk", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"A");
    });
    dispatch_async(queue, ^{
        NSLog(@"C");
    });
    dispatch_barrier_async(queue, ^{
        NSLog(@"%@",[NSThread currentThread]);
    });

3、waitUntilFinished

    NSOperationQueue *queue = [[NSOperationQueue alloc]init];
    NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op1");
    }];
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op2");
    }];
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"op3");
    }];
    [queue addOperations:@[op1,op2,op3] waitUntilFinished:YES];//YES 阻塞
    NSLog(@"end");

二、5个线程并发，如何实现

1、GCD控制最大并发数

    dispatch_queue_t queue = dispatch_queue_create("hhhhhhh", DISPATCH_QUEUE_CONCURRENT);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(5);
    for (int i = 0; i<10; i++) {
        dispatch_async(queue, ^{
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            NSLog(@"%@ %d",[NSThread currentThread],i);
            dispatch_semaphore_signal(semaphore);
        });
    }

2、 maxConcurrentOperationCount属性设置最大并发数(该属性需要在任务添加到队列中之前进行设置)。maxConcurrentOperationCount默认值是-1；如果值设为0，那么不会执行任何任务；如果值设为1，那么该队列是串行的；如果大于1，那么是并行的。

NSOperationQueue *queue = [[NSOperationQueue alloc]init];
 queue.maxConcurrentOperationCount = 5;
  for (int i =0; i<10; i++) {
      NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
          NSLog(@"%@ %d",[NSThread currentThread],i);
      }];
      [queue addOperation:op1];
  }