从核心数据请求分页结果

Question

我有一个相对简单的核心数据sqlite数据库.我试图从DB一次获得结果.

    NSFetchRequest* request = [[[NSFetchRequest alloc] init] autorelease];
    [request setEntity:[...]];

    [request setPredicate:[NSPredicate predicateWithFormat:@"flaggedTime != nil"]];

    NSSortDescriptor* sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"flaggedTime" ascending:NO];
    [request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];

    [request setFetchLimit:pageSize];
    [request setFetchOffset:((pageIndex - 1) * pageSize)];    

    NSArray* results = [self.context executeFetchRequest:request error:NULL];

pageSize为30,测试数据的pageIndex为1,2,3或4(DB中约有80个项目,因此pageIndex = 4应该不返回任何项目).谓词和排序工作正常,结果成功返回.获取限制也可以正常工作.没有错误返回.

问题:我总是从第一页获得结果,就像没有设置fetchOffset一样.我试图删除谓词和排序但无济于事.我可以使fetchOffset工作的唯一情况是当我使用30以下的值时.当然,这对于分页来说毫无意义...

有人知道为什么吗？我会非常感谢每一个答案.

更新:我说的是iOS.测试4.2和5.0.

更新2:简化问题.

    NSFetchRequest* request = [[[NSFetchRequest alloc] init] autorelease];
    [request setEntity:[...];

    NSError* error = nil;

    NSManagedObjectContext* context = [...];

    NSUInteger count = [context countForFetchRequest:request error:&error];
    assert(error == nil);

    NSLog(@"Total count: %u", count);

    request.fetchOffset = 0;    
    request.fetchLimit = 30;

    NSLog(@"Fetch offset: %u, limit: %u", request.fetchOffset, request.fetchLimit);

    NSArray* page1 = [context executeFetchRequest:request error:&error];
    assert(error == nil);

    NSLog(@"Page 1 count: %u", page1.count);

    request.fetchOffset = 30;    
    request.fetchLimit = 30;

    NSLog(@"Fetch offset: %u, limit: %u", request.fetchOffset, request.fetchLimit);

    NSArray* page2 = [context executeFetchRequest:request error:&error];
    assert(error == nil);

    NSLog(@"Page 2 count: %u", page2.count);

得到:

Total count: 34
Fetch offset: 0, limit: 30
Page 1 count: 30
Fetch offset: 30, limit: 30
Page 2 count: 30 (ERROR: should give 4)

Answer 1

我刚刚创建了一个演示项目,尝试以最简单的形式重新创建场景.我创建了一个空白项目,添加了34个对象,然后使用上面列出的完全相同的代码查询它.以下是示例:

CDAppDelegate * delegate = (CDAppDelegate*)[[UIApplication sharedApplication] delegate];
NSManagedObjectContext * context = [delegate managedObjectContext];
for(int i = 0; i < 34;i++){
    CDObject * object = [NSEntityDescription insertNewObjectForEntityForName:@"CDObject"
                                                      inManagedObjectContext:context];
    [object setValue:i];
}
[delegate saveContext];

NSFetchRequest* request = [[[NSFetchRequest alloc] init] autorelease];
[request setEntity:[NSEntityDescription entityForName:@"CDObject"
                               inManagedObjectContext:context]];

 NSError* error = nil;

 NSUInteger count = [context countForFetchRequest:request error:&error];
 assert(error == nil);

 NSLog(@"Total count: %u", count);

 request.fetchOffset = 0;    
 request.fetchLimit = 30;

 NSLog(@"Fetch offset: %u, limit: %u", request.fetchOffset, request.fetchLimit);

 NSArray* page1 = [context executeFetchRequest:request error:&error];
 assert(error == nil);

 NSLog(@"Page 1 count: %u", page1.count);

 request.fetchOffset = 30;    
 request.fetchLimit = 30;

 NSLog(@"Fetch offset: %u, limit: %u", request.fetchOffset, request.fetchLimit);

 NSArray* page2 = [context executeFetchRequest:request error:&error];
 assert(error == nil);

 NSLog(@"Page 2 count: %u", page2.count);

[request release];

日志如下所示:

2011-11-04 14:53:04.530 CDCoreDataTest[77964:207] Total count: 34
2011-11-04 14:53:04.531 CDCoreDataTest[77964:207] Fetch offset: 0, limit: 30
2011-11-04 14:53:04.532 CDCoreDataTest[77964:207] Page 1 count: 30
2011-11-04 14:53:04.533 CDCoreDataTest[77964:207] Fetch offset: 30, limit: 30
2011-11-04 14:53:04.533 CDCoreDataTest[77964:207] Page 2 count: 4

使用您的代码,我能够启动并运行没有问题.这是通过在模拟器上运行的iOS 5.0完成的.您的代码看起来正确,因为您正在尝试完成,因此必须对获取请求或上下文本身进行某些操作...

Answer 2

当您针对未保存的上下文运行获取请求时,将忽略(有时？)的pageOffset属性NSFetchRequest.请注意,OP上下文中的代码从未保存过,而在@kcharwood附加到答案的代码片段中,它实际上已保存.这是一个例子:

- (void) printArrayOfTestEntities:(NSArray *)array{
    NSMutableString * s = [NSMutableString new];
    NSArray * numbers = [array valueForKey:@"someField"];
    for (NSNumber * number in numbers){
        [s appendString:[NSString stringWithFormat:@"%d ", [number intValue]]];
    }

    NSLog(@"fetched objects: %@ \rcount: %d", s, array.count);
}

/* example itself */

NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
[context setPersistentStoreCoordinator:persistentStoreCoordinator];

for(int i = 0; i < 34;i++){
    NSManagedObject * object = [NSEntityDescription insertNewObjectForEntityForName:@"TestEntity"
                                                      inManagedObjectContext:context];
    [object setValue:@(i) forKey:@"someField"];
}

NSFetchRequest * request = [NSFetchRequest fetchRequestWithEntityName:@"TestEntity"];
request.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"someField" ascending:YES]];
request.fetchLimit = 30;

NSArray * result = [context executeFetchRequest:request error:nil];
[self printArrayOfTestEntities:result];

request.fetchOffset = 30;
result = [context executeFetchRequest:request error:nil];
[self printArrayOfTestEntities:result];

[context save:&error];

request.fetchOffset = 0;
result = [context executeFetchRequest:request error:nil];
[self printArrayOfTestEntities:result];

request.fetchOffset = 30;
result = [context executeFetchRequest:request error:nil];
[self printArrayOfTestEntities:result];

日志:

2014-03-01 11:30:06.986 coredatatest[19771:70b] fetched objects: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29  
count: 30
2014-03-01 11:30:06.990 coredatatest[19771:70b] fetched objects: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29  
count: 30
2014-03-01 11:30:06.995 coredatatest[19771:70b] fetched objects: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29  
count: 30
2014-03-01 11:30:06.997 coredatatest[19771:70b] fetched objects: 30 31 32 33   
count: 4