ios - 使用 block 会自动创建新线程吗？

Question

刚开始使用块进行工作……非常困惑。我正在使用这样的块:

-(void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
NSDictionary *myDictionary = [[mySingleton arrayPeopleAroundMe] objectAtIndex:indexPath.row];

NSMutableString *myString = [[NSMutableString alloc] initWithString:@"http://www.domain.com/4DACTION/PP_profileDetail/"];
[myString appendString:[myDictionary objectForKey:@"userID"]];
NSURLRequest *urlRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:[myString stringByAddingPercentEscapesUsingEncoding: NSUTF8StringEncoding]]
                                            cachePolicy:NSURLRequestUseProtocolCachePolicy
                                        timeoutInterval:60.0];
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[NSURLConnection
 sendAsynchronousRequest:urlRequest
 queue:queue
 completionHandler: ^( NSURLResponse *response,
                      NSData *data,
                      NSError *error)
 {
     [[mySingleton dictionaryUserDetail] removeAllObjects];
     [[mySingleton arrayUserDetail] removeAllObjects];

     if ([data length] > 0 && error == nil) // no error and received data back
     {
         NSError* error;
         NSDictionary *myDic = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];
         [mySingleton setDictionaryUserDetail:[myDic mutableCopy]];

         NSArray *myArray = [myDic objectForKey:@"searchResults"];
         [mySingleton setArrayUserDetail:[myArray mutableCopy]];

         [self userDetailComplete];
     } else if
         ([data length] == 0 && error == nil) // no error and did not receive data back
     {
         [self serverError];
     } else if
         (error != nil) // error
     {
         [self serverError];
     }
 }];
}

连接完成后，将称为:

-(void)userDetailComplete {
ViewProfile *vpVC = [[ViewProfile alloc] init];
[vpVC setPassedInstructions:@"ViewDetail"];
[[self navigationController] pushViewController:vpVC animated:YES];
}

导致此错误弹出:
“试图从主线程或Web线程以外的线程获取Web锁。这可能是从辅助线程调用UIKit的结果。”
我摆脱错误的唯一方法是将userDetailComplete更改为此:

-(void)userDetailComplete {
dispatch_async(dispatch_get_main_queue(), ^{
ViewProfile *vpVC = [[ViewProfile alloc] init];
[vpVC setPassedInstructions:@"ViewDetail"];
[[self navigationController] pushViewController:vpVC animated:YES];
});
}

我的问题:每次使用块时都会自动启动一个新线程吗？使用积木时，我还应该注意其他陷阱吗？

Answer 1

块不创建线程。它们是闭包；它们只包含可在将来某个时间运行的可运行代码。

这是在后台线程上运行的，因为这是您要求它执行的操作:

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[NSURLConnection
  sendAsynchronousRequest:urlRequest
  queue:queue
  ...

您创建了一个新队列，然后要求 NSURLConnection在该队列上回叫您。如果要在主线程上被调用，请传递 [NSOperationQueue mainQueue]。通常这就是您想要的。