NSFetchedResultsController试图限制显示的记录数 [英] NSFetchedResultsController trying to limit number of records displayed
问题描述
当创建一个 NSFetchRequest 传递给我的 NSFetchedResultsController 我设置fetchLimit属性为3。 p>
现在开始看起来工作正常。我可以修改前三个返回的对象，以任何方式改变它们的顺序，他们都正确地重新洗牌。问题出现时，我改变一个对象，最初落在前三个以外的方式，现在将其带入前三个，或者当简单地添加一个新的对象，因此它将出现在前三个。
我预期会发生什么：插入的对象将其余的按下，一个从底部拖出。
实际上发生了：插入的对象将其余部分按下，并且记录计数增加到4
任何人都可以解释这个问题，这个？
我已经取得了一些进展，基本上通过忽略 numberOfObjects 并返回我想要的表的固定长度的实际长度。这在 controller：didChangeObject：... 中有点麻烦，但似乎工作到目前为止。
- （NSInteger）tableView：（UITableView *）tableView numberOfRowsInSection：（NSInteger）section
{
return kTableSize;
// return [[[self.fetchedResultsController sections] objectAtIndex：section] numberOfObjects]; （NSFetchedResultsChangeType）类型newIndexPath：（NSIndexPath *）newIndexPath（NSIndexPath *）newIndexPath（NSIndexPath *）newIndexPath（NSIndexPath *）newIndexPath（NSFetchedResultsController *）控制器didChangeObject：
{
UITableView * tableView = self.myTableView;
switch（type）{
case NSFetchedResultsChangeInsert：
//只有在insert会影响可见行时才修改表
if（newIndexPath .row< kTableSize）{
//删除最后一行以保持固定长度
[tableView deleteRowsAtIndexPaths：[NSArray arrayWithObject：[NSIndexPath indexPathForRow：（kTableSize - 1）inSection：newIndexPath.section]] withRowAnimation： UITableViewRowAnimationAutomatic];
[tableView insertRowsAtIndexPaths：[NSArray arrayWithObject：newIndexPath] withRowAnimation：UITableViewRowAnimationAutomatic];
}
break;
case NSFetchedResultsChangeDelete：
//只有在删除会影响可见行时才修改表
if（indexPath.row< kTableSize）{
[tableView deleteRowsAtIndexPaths：[NSArray arrayWithObject：indexPath] withRowAnimation：UITableViewRowAnimationAutomatic];
//插入额外的行以保持固定长度
[tableView insertRowsAtIndexPaths：[NSArray arrayWithObject：[NSIndexPath indexPathForRow：（kTableSize - 1）inSection：newIndexPath.section]] withRowAnimation：UITableViewRowAnimationAutomatic];
}
break;
case NSFetchedResultsChangeUpdate：
//如果更新会影响可见行，则只修改表
if（indexPath.row< kTableSize）{
[tableView reloadRowsAtIndexPaths：[NSArray arrayWithObject：indexPath] withRowAnimation：UITableViewRowAnimationAutomatic];
}
break;
case NSFetchedResultsChangeMove：
//只有在移动会影响可见行时才修改表
if（（indexPath.row< kTableSize）||（newIndexPath.row < kTableSize））{
//删除表的旧行或最后一行
if（indexPath.row< kTableSize）{
[tableView deleteRowsAtIndexPaths： [NSArray arrayWithObject：indexPath] withRowAnimation：UITableViewRowAnimationAutomatic];
} else {
[tableView deleteRowsAtIndexPaths：[NSArray arrayWithObject：[NSIndexPath indexPathForRow：（kTableSize - 1）inSection：newIndexPath.section]] withRowAnimation：UITableViewRowAnimationAutomatic];
}
//在表的底部插入新行或行
if（newIndexPath.row
} else {
[tableView insertRowsAtIndexPaths：[NSArray arrayWithObject：[NSIndexPath indexPathForRow：（kTableSize - 1）inSection：newIndexPath.section]] withRowAnimation：UITableViewRowAnimationAutomatic];
}
}
break;
}
}
还需要注意 tableView：cellForRowAtIndexPath：，以确保如果对象数少于表长度，我们不会尝试访问不存在的对象。
When creating a NSFetchRequest to pass over to my NSFetchedResultsController I am setting the fetchLimit property to 3.
Now initially this appears to work fine. I can modify the first three returned objects in any way which changes their order and they all reshuffle correctly. The problem appears when I either alter a object which initially fell outside the first three in a way which now brings it into the first three, or when simply adding a new object so it will appear within the first three.
What I expected to happen: Inserted object pushes the rest down and one drops off the bottom.
What actually happens: Inserted object pushes the rest down and the record count grows to 4?!
Can anyone explain this, or how I should deal with this?
I have made some progress working around this, basically by ignoring numberOfObjects and returning the actual length I want the table fixed at. This takes a bit of trickery in controller:didChangeObject:... but seems to be working so far.
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return kTableSize;
//return [[[self.fetchedResultsController sections] objectAtIndex:section] numberOfObjects];
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
UITableView *tableView = self.myTableView;
switch(type) {
case NSFetchedResultsChangeInsert:
// Only modify table if insert will effect visible rows
if (newIndexPath.row < kTableSize) {
// Delete last row to maintain fixed length
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:(kTableSize - 1) inSection:newIndexPath.section]] withRowAnimation:UITableViewRowAnimationAutomatic];
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
}
break;
case NSFetchedResultsChangeDelete:
// Only modify table if delete will effect visible rows
if (indexPath.row < kTableSize) {
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
// Insert extra row to maintain fixed length
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:(kTableSize - 1) inSection:newIndexPath.section]] withRowAnimation:UITableViewRowAnimationAutomatic];
}
break;
case NSFetchedResultsChangeUpdate:
// Only modify table if update will effect visible rows
if (indexPath.row < kTableSize) {
[tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
}
break;
case NSFetchedResultsChangeMove:
// Only modify table if move will effect visible rows
if ((indexPath.row < kTableSize) || (newIndexPath.row < kTableSize)) {
// Delete old row or last row of table
if (indexPath.row < kTableSize) {
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
} else {
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:(kTableSize - 1) inSection:newIndexPath.section]] withRowAnimation:UITableViewRowAnimationAutomatic];
}
// Insert new row or a row at bottom of table
if (newIndexPath.row < kTableSize) {
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
} else {
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:(kTableSize - 1) inSection:newIndexPath.section]] withRowAnimation:UITableViewRowAnimationAutomatic];
}
}
break;
}
}
Also need to take care in tableView:cellForRowAtIndexPath: to make sure we don't try and access an object which doesn't exist if there are less objects than the table length.
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