Question

对于两个核心数据实体A和B，其中A与B具有一对多关系，我试图在后台上下文中执行获取，其父级是主要上下文[与UI相关联]。由于我需要以预取方式获得结果，即核心数据也必须填充关系，如下所示[使用MagicalRecord]：

NSManagedObjectContext* bgContext = [NSManagedObjectContext context];

[bgContext performBlock:^{
    NSpredicate * searchPredicate = <some search criteria in entity A>
    if(!searchPredicate){
        dispatch_async(dispatch_get_main_queue(), ^{
    //completionBlock is a callback block to be executed on main thread.        completionBlock(nil);
            return;
        });
    }
    //Using below request to get relationship data in pre-fetch format.
    NSFetchRequest* aRequest = [A requestAllSortedBy:@"<aColumn>" ascending:YES withPredicate:searchPredicate inContext:bgContext];

    [aRequest setReturnsObjectsAsFaults:YES];
    [aRequest setRelationshipKeyPathsForPrefetching:[NSArray arrayWithObject:@"photos"]];//photos is a relationship between A and B
    [aRequest setIncludesPropertyValues:YES];
    [aRequest setIncludesSubentities:YES];




    NSArray *as = [A executeFetchRequest:aRequest inContext:bgContext];
    if (as == nil) {
        dispatch_async(dispatch_get_main_queue(), ^{
            completionBlock(nil);
            return;
        });
    }
    NSArray *objectIDs =[as valueForKey:@"objectID"];

    NSManagedObjectContext *mainContext = [NSManagedObjectContext defaultContext];
    [mainContext performBlock:^{

        //temporary bad solution
        NSMutableArray *entities = [[NSMutableArray alloc] init];
        for (A *a in as) {
            [entities addObject:[a inContext:mainContext]];
        }
        completionBlock([NSArray arrayWithArray:entities]);
    }];


}];

虽然结果正如预期的那样，但我仍然想知道上面是否接近评论＆＃34; //临时不良解决方案＆＃34;是否有效。即使我将objectID传递给主上下文，那么在这种情况下如何预先获取关系？

Answer 1

最好的选择是获取所有托管对象ID并将其传递给主线程。然后，根据主线程的要求，从对象ID批量加载托管对象并缓存托管对象实例（刷新它们以将它们变成故障但如果内存不足则保留托管对象）。

核心数据：在后台和ObjectIVE中获取

1 个答案: