Invalidating section identifier - ios

Question

I am using a transient attribute to sort the core data objects into separate table view sections. It is called 'sectionIdentifier'. The getter for this attribute is inside a NSManagedObject subclass, called ToDoItem.m . The problem is that during app execution, the new added objects are always shown under the TODAY section. After a new launch of the app, all objects are shown under the expected row. I was told by an expert user, that when setting a new object, the sectionIdentifier must be invalidated, but I don't know how to invalidate it. Here is my NSManagedObject subclass code:
#import "ToDoItem.h"
#import "ToDoGroup.h"
#import "ToDoSubItem.h"
#implementation ToDoItem
#dynamic todoDescription;
#dynamic todoName;
#dynamic todoDueDate;
#dynamic sectionIdentifier;
#dynamic todogroup;
#dynamic todosubitems;
-(NSString *)sectionIdentifier{
[self willAccessValueForKey:#"sectionIdentifier"];
NSString *tmp = [self primitiveValueForKey:#"sectionIdentifier"];
[self didAccessValueForKey:#"sectionIdentifier"];
if (!tmp){
NSDate *date = self.todoDueDate;
NSDate *todayDate = [NSDate date];
NSLog(#"date= %#",date);
NSLog(#"todayDate = %#",todayDate);
NSCalendar *calendar = [NSCalendar currentCalendar];
NSInteger comps = (NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit);
NSDateComponents *date1Components = [calendar components:comps fromDate:date];
NSDateComponents *date2Components = [calendar components:comps fromDate:todayDate];
date = [calendar dateFromComponents:date1Components];
todayDate = [calendar dateFromComponents:date2Components];
if([date
compare:todayDate] == NSOrderedSame) {
tmp = #"1";//TODAY
}
else if([date
compare:todayDate] == NSOrderedDescending){
tmp = #"2";//OVERDUE
}
else if ([date
compare:todayDate] == NSOrderedAscending){
tmp =#"0";//UPCOMING
}
NSLog(#"Tmp= %#",tmp);
[self setPrimitiveValue:tmp forKey:#"sectionIdentifier"];
}
return tmp;
}
#end
Any help is welcome...

Answer 1

The problem is that the section identifier is calculated and cached from the todoDueDate, but not updated automatically when the todoDueDate changes.
The DateSectionTitles/APLEvent.m
sample code from Apple shows how such an automatic update can be achieved.
In your case, you should add the following methods to the managed object subclass
ToDoItem:
- (void)setTodoDueDate:(NSDate *)newDate
{
// If the todoDueDate changes, the section identifier become invalid.
[self willChangeValueForKey:#"todoDueDate"];
[self setPrimitiveValue:newDate forKey:#"todoDueDate"];
[self didChangeValueForKey:#"todoDueDate"];
// Set the section identifier to nil, so that it will be recalculated
// when the sectionIdentifier method is called the next time:
[self setPrimitiveValue:nil forKey:#"sectionIdentifier"];
}
+ (NSSet *)keyPathsForValuesAffectingSectionIdentifier
{
// If the value of todoDueDate changes, the section identifier may change as well.
return [NSSet setWithObject:#"todoDueDate"];
}