hi,
i want you to propose just an idea , a remark, ....on apple documentation is
said
The NSFetchRequest class is used to describe search criteria used to retrieve
data from a persistent store..... implicitely you must record all of them and
update them sometimes....
in your idea there is a lot of different kind of countries ( india, honduras
....) which can have a common property: a selected color that can vary a lot of
times....
why not trying to use an Hash Table in order to use weak reference with
NSHashTable ??....instead of each time ask core data to update all the entire
store....
best regards
Le 10 févr. 2011 à 16:19, WT a écrit :
> Hello list,
>
> a little while ago I posted a question regarding some strange behavior by an
> NSFetchedResultsController (under the same subject line as this message).
> Since then I've narrowed the problem down a bit more and now have a test
> project to show it.
>
> Here's what the goal is, in a nutshell.
>
> I have a list of, say, countries. Some are "selected", others not. I need to
> display all the countries in the same tableview, in the same section, and
> what differentiates the selected ones from the deselected ones is their text
> color. Moreover, the selected ones should be sorted ascending by their
> "index" (an integer that's used to keep track of their position in the list)
> while the deselected ones must be sorted ascending by their "name".
>
> The test project's core data model has a single entity, "CountryCD", with the
> attributes "name", "index", "selected" (representing a boolean), and a
> transformable attribute "indexOrName" which returns the entity's index if
> it's selected or its name if it's deselected.
>
> - (id) indexOrName;
> {
> if ([self.selected isEqualToNumber: [NSNumber numberWithBool: YES]])
> {
> return self.index;
> }
> else
> {
> return self.name;
> }
> }
>
> The fetched results controller is defined as usual but contains two sort
> descriptors:
>
> NSSortDescriptor* sortBySelected = [[NSSortDescriptor alloc]
> initWithKey: @"selected" ascending: NO];
>
> NSSortDescriptor* sortByIndexOrName = [[NSSortDescriptor alloc]
> initWithKey: @"indexOrName" ascending: YES];
>
> the idea being that entities get sorted first by their "selected" status and
> then by their "indexOrName" attribute. Since that attribute returns the
> entity's "index" or its "name", depending on the entity's "selected" status,
> the fetched results controller *should* sort the entities as desired.
>
> Should, but doesn't. And that's the problem I can't seem to solve.
>
> In the test project, I create country entities in the following order
>
> [self createCountryOfName: @"India"];
> [self createCountryOfName: @"Honduras"];
> [self createCountryOfName: @"Germany"];
> [self createCountryOfName: @"Denmark"];
> [self createCountryOfName: @"Brazil"];
> [self createCountryOfName: @"Egypt"];
> [self createCountryOfName: @"Australia"];
> [self createCountryOfName: @"China"];
> [self createCountryOfName: @"Finland"];
>
> and make every second one deselected, which *should* result in them being
> displayed as follows:
>
> // (ordered by index)
> // India
> // Germany
> // Brazil
> // Australia
> // Finland
>
> // (ordered by name)
> // China
> // Denmark
> // Egypt
> // Honduras
>
> Alas, they're displayed like this (or some other seemingly random order):
>
> (selected)
> Brazil
> Germany
> Australia
> India
> Finland
>
> (deselected)
> Honduras
> Denmark
> Egypt
> China
>
> Neither the selected ones are sorted by their "index" attribute nor the
> deselected ones are sorted by their "name" attribute. In fact, the test
> project shows that "indexOrName" is not accessed at all when the fetched
> results controller does its fetch.
>
> First I thought that there might be something wrong with the way I
> implemented the fetched results controller, but if you replace "indexOrName"
> in the sort descriptor with either "index" or "name", it works as would be
> expected.
>
> So then I thought that using a transformable attribute is not the way to go
> in order to achieve the result I need to achieve, but I can't see any other
> way to do it.
>
> If using a transformable attribute isn't the way to go, can anyone suggest an
> alternative? If using a transformable attribute should work, then why doesn't
> it as I've implemented it? It appears to me that NSFetchedResultsController
> ignores a sort descriptor based on a transformable attribute.
>
> The test project can be found here:
> http://www.restlessbrain.com/FRC_Test.zip
>
> I'd greatly appreciate any help. I've already spent several days trying to
> figure this out but got nowhere and ran out of ideas.
>
> Thanks in advance.
> WT
>
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