Add additional argument to an existing NSPredicate

Question

Is it possible to take an existing NSPredicate and add an additional argument to it?

In one of my tableviews I am passing in a NSPredicate to use in for my NSFetchedResultsController like so:

[fetchedResults setPredicate:self.predicate];

This is working just fine and will show the content based on the existing NSPredicate. But I want to take this one step further by adding an UISegmentedControl to the tableView.

- (IBAction)segmentChange:(id)sender {
     switch (selectedSegment) {
            case kDisplayDVD:
                // Add argument to existing NSPredicate
                break;
            case kDisplayVHS:
                // Add argument to existing NSPredicate
                break;
            default:
                break;
        }

Based on which segment the user has chosen I would like to add an argument to the existing NSPredicate. Is this at all possible?

Answer 1

Sure!

Let's say that your objects have a property called display that's an int (or rather, an enum that corresponds to your kDisplayDVD, kDisplayVHS, etc). Then you can do:

- (IBAction) segmentChange:(id)sender {
  NSPredicate * newCondition = [NSPredicate predicateWithFormat:@"display = %d", selectedSegment];
  NSPredicate * newPredicate = [NSCompoundPredicate andPredicateWithSubpredicates:[NSArray arrayWithObjects:[self predicate], newCondition, nil]];
  [self setPredicate:newPredicate];
}

So if [self predicate] is (foo = @"bar" OR baz > 42), your new predicate will be display = 1 AND (foo = @"bar" OR baz > 42)