我想使用NSFetchedResultsController和NSSortDescriptor提供自定义排序。
here
),我尝试使用NSSortDescriptor派生类来重写compareObject:toObject:message。
here
).
我的问题是:如何强制NSFetchedResultscontroller使用compareObject:toObject:message对数据进行排序?(它是否能与大数据集一起工作)
另一个解决方案是:
-调用performFetch通过SQL对数据排序(未调用compareObject)
-对数据进行修改并将其反转。
它在我的情况下确实有效,但它是一个黑客,我不确定它是否总是有效(尤其是在大数据集(大于批处理大小)的情况下)。
更新:您可以使用CoreDataBooks示例进行复制。
在RootViewController.m中,添加这个丑陋的hack:
- (void)viewWillAppear:(BOOL)animated {
Book* book = (Book *)[NSEntityDescription insertNewObjectForEntityForName:@"Book"
inManagedObjectContext:[self fetchedResultsController].managedObjectContext];
[[self fetchedResultsController] performFetch:nil];
[[self fetchedResultsController].managedObjectContext deleteObject:book];
[self.tableView reloadData];
}
MySortDescriptor *myDescriptor = [[MySortDescriptor alloc] init];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:myDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
添加MySortDescriptor类:
@implementation MySortDescriptor
-(id)init
{
if (self = [super initWithKey:@"title" ascending:YES selector:@selector(compare:)])
{
}
return self;
}
- (NSComparisonResult)compareObject:(id)object1 toObject:(id)object2
{
//set a breakpoint here
return [[object1 valueForKey:@"author" ] localizedCaseInsensitiveCompare:[object2 valueForKey:@"author" ] ];
}
//various overrides inspired by [this blog post][3]
- (id)copy
{
return [self copyWithZone:nil ];
}
- (id)mutableCopy
{
return [self copyWithZone:nil ];
}
- (id)mutableCopyWithZone:(NSZone *)zone
{
return [self copyWithZone:zone ];
}
- (id)copyWithZone:(NSZone*)zone
{
return [[MySortDescriptor alloc] initWithKey:[self key] ascending:[self ascending] selector:[self selector]];
}
- (id)reversedSortDescriptor
{
return [[[MySortDescriptor alloc] initWithKey:[self key] ascending:![self ascending] selector:[self selector]] autorelease];
}
@end
