Question

I have a UISegmentControl with my UITableView that sorts the data. I’d like to be able to do things:

(1) default sort (so when the user turns on the app for the first time, it would select the first segment, and sort by that action)

(2) remember where the user was between table loads. What I mean by this is, similar to Apple’s coverflow, when I go to a different cover, the UITableView repopulates. So if the last time the user was there, the sort was on the 3rd segment, then it would remember that.

I’m a bit new to object-oriented design, and this was my best guess to not have the same redundant code everywhere: (MarkersList is a NSMutableArray)

- (NSArray *)sortByName:(NSArray *)sortDescriptors {
    return [self.MarkersList sortedArrayUsingDescriptors:sortDescriptors];
}

- (NSArray *)sortByRSID:(NSArray *)sortDescriptors {
    return [self.MarkersList sortedArrayUsingDescriptors:sortDescriptors];
}

- (void)setSortedMarkersList:(NSArray *)sortedArray {
    if (self.MarkersList != nil) {
        [self.MarkersList removeAllObjects];
    }
    [self.MarkersList addObjectsFromArray:sortedArray];
}

- (IBAction)sortButtonPressed:(UISegmentedControl *)segmentControl {

    // Create sort descriptors
    NSSortDescriptor *nameDescriptor = [[[NSSortDescriptor alloc] initWithKey:@"Name" ascending:YES selector:@selector(localizedCaseInsensitiveCompare:)] autorelease];
    NSSortDescriptor *rsID = [[NSSortDescriptor alloc] initWithKey:@"ID" ascending:YES];

    if ([segmentControl selectedSegmentIndex] == NAME) { // this is #define 0 
        NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:nameDescriptor, rsIDDescriptor, nil];
        NSArray *sortedArray = [self sortByGene:sortDescriptors];
        [self setSortedMarkersList:sortedArray];
        [sortDescriptors release];

    }

    else if ([segmentControl selectedSegmentIndex] == RS_ID) {
        NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:rsIDDescriptor, resultDescriptor, nameDescriptor, nil];
        NSArray *sortedArray = [self sortByRSID:sortDescriptors];
        [sortDescriptors release];
        [self setSortedMarkersList:sortedArray];
    }

    [self.MarkersTableView reloadData];
}

I haven’t implemented the third sort yet since it’s not just an NSString or NSNumber like the other two yet. So far, I think it works correctly. However, the problem I have is to implement (1), I would need to call sortByName when my table is loaded. I could just create the NSSortDescriptors again, but that seems redundant. Is there a more OOD way to achieve this?

For (2), I’m guessing I could save that index for the table in a dictionary, and retrieve it when that table is loaded. Or something along those lines, not really sure.

Any help is greatly appreciated. Thanks!

Answer 1

This isn’t so much an OO question, as iOS doesn’t always let us use best practices to solve a problem. The best bet for your situation, I think, is to store the information on the selected sort in the NSUserDefaults. In your -viewDidLoad method, check if the selected sort object exists in NSUserDefaults, use it if it does, and if not choose a reasonable default value.

Don’t worry about creating NSSortDescriptors with each load of the application, unless you have done profiling and determined that a large amount of time is spent building it. Serializing and deserializing the NSSortDescriptors would be far more inefficient than just recreating it when needed. Apple spends a lot of time optimizing frequently used classes like NSSortDescriptor.

Regarding #2, you can use indexPathsOfVisibleRows on the UITableView to get an array of visible indices, store the first one in UserDefaults, then on load (or pop from the next view controller if that occurs) call –scrollToRowAtIndexPath:atScrollPosition:animated:.

Once you’re code is functioning, I recommend you watch the various videos on iTunes U regarding profiling and Instruments. It’s a wonderful tool that is often overlooked, and really helps concentrate effort where it’s needed.