java中的接口是类吗
245
2022-11-05
Core Data浅谈系列之八 : 关于并发
有时候,我们需要有个worker thread来做一些密集型或者长耗时的任务,以避免阻塞住UI,给用户不好的体验。比如从网络上获取一批数据,然后解析它们,并将其输出到存储文件中。这时候,由于数据层发生了变动,我们希望通知到主线程更新UI —— 这就涉及到Core Data的多线程特性。
比如我们一直以来使用的Demo中,添加球员信息的AddPlayerViewController和显示球员列表的PlayerListViewController在进行CURD操作时都是在主ViewController的context中完成的,这通过维持一个属性cdViewController指向主ViewController来实现:
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#pragma mark -#pragma mark - UITableView Delegate- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath{[tableView deselectRowAtIndexPath:indexPath animated:YES];Team *teamObject = [self.teamArray objectAtIndex:indexPath.row];PlayerListViewController *playerListVC = [[[PlayerListViewController alloc] init] autorelease];playerListVC.team = teamObject;playerListVC.cdViewController = self;[self.navigationController pushViewController:playerListVC animated:YES];}
以及:
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#pragma mark -#pragma mark - Player CURD- (void)addPlayer:(id)sender{AddPlayerViewController *addPlayerVC = [[[AddPlayerViewController alloc] init] autorelease];addPlayerVC.cdViewController = self.cdViewController;addPlayerVC.team = self.team;[self presentModalViewController:addPlayerVC animated:YES];}
对于比较小的Demo,这么写代码是可以接受的,虽然也会觉得传递得有点长。
当程序的代码规模比较大,或者说处理的数据比较多时,我们可以通过引入并发特性来做一点优化。
通过创建临时的context来添加球员信息:
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- (IBAction)addBtnDidClick:(id)sender{dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{NSManagedObjectContext *tmpContext = [[NSManagedObjectContext alloc] init];[tmpContext setPersistentStoreCoordinator:sharedPersistentStoreCoordinator];// We don't check the user input."Player" inManagedObjectContext:tmpContext];playerObject.name = self.nameTextField.text;playerObject.age = [NSNumber numberWithInteger:[self.ageTextField.text integerValue]];playerObject.team = self.team;NSError *error = NULL;if (tmpContext && [tmpContext hasChanges] && ![tmpContext save:&error]) {"Error %@, %@", error, [error localizedDescription]);abort();}dispatch_async(dispatch_get_main_queue(), ^{[self dismissModalViewControllerAnimated:YES];});});}
为了响应其它线程的变化,
参考此文档,我们可以先监听消息,然后合并发生了的变化:
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[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mocDidSaveNotification:) name:NSManagedObjectContextDidSaveNotification object:nil];- (void)mocDidSaveNotification:(NSNotification *)notification{NSManagedObjectContext *savedContext = [notification object];if (savedContext == self.managedObjectContext) {return ;}if (savedContext.persistentStoreCoordinator != self.persistentStoreCoordinator) {return ;}dispatch_async(dispatch_get_main_queue(), ^{"Merge changes from other context.\n");[self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification];});}
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2013-01-21 09:56:08.300 cdNBA[573:617] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Illegal attempt to establish a relationship 'team' between objects in different contexts
这是由于我们把主线程context中的team对象传递到临时创建的context中进行操作了。在Core Data的多线程环境中,我们只能传递objectID或者重新fetch:
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addPlayerVC.teamID = [self.team objectID];// ...playerObject.team = (Team *)[tmpContext objectWithID:self.teamID];
这样可以执行过去,控制台输出:
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2013-01-21 10:11:12.834 cdNBA[687:1b03] void _WebThreadLockFromAnyThread(bool), 0x83a91c0: Obtaining the web lock from a thread other than the main thread or the web thread. UIKit should not be called from a secondary thread.2013-01-21 10:11:12.932 cdNBA[687:c07] Merge changes from other context.
第二行日志说明合并变化了,不过第一行告诉我们在非主线程里面访问了一些UI方面的东西。这是由于上面在global_queue里面访问了UITextField,把访问UI的代码提到外面即可。
BTW,在iOS 5以后,苹果提供了更为便捷有效的parent-child context机制,可以参见 这里。
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