Core Data淺談係列之八 ： 關於並發

比如我們一直以來使用的Demo中，添加球員信息的AddPlayerViewController和顯示球員列表的PlayerListViewController在進行CURD操作時都是在主ViewController的context中完成的，這通過維持一個屬性cdViewController指向主ViewController來實現： 

#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：

#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];
}

- (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 *playerObject = [NSEntityDescription insertNewObjectForEntityForName:@"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]) {
            NSLog(@"Error %@, %@", error, [error localizedDescription]);
            abort();
        }

        dispatch_async(dispatch_get_main_queue(), ^{
            [self dismissModalViewControllerAnimated:YES];
        });
    });
}

為了響應其它線程的變化，參考此文檔，我們可以先監聽消息，然後合並發生了的變化：

[[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(), ^{
        NSLog(@"Merge changes from other context.\n");
        [self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
    });
}

這麼做了之後，我們嚐試添加一名球員，會得到如下錯誤信息：

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

這是由於我們把主線程context中的team對象傳遞到臨時創建的context中進行操作了。在Core Data的多線程環境中，我們隻能傳遞objectID或者重新fetch：

addPlayerVC.teamID = [self.team objectID];

// ...

playerObject.team = (Team *)[tmpContext objectWithID:self.teamID];

這樣可以執行過去，控製台輸出：

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.