iPhone UITableView. How do turn on the single letter alphabetical list like the Music App?

Question

In the iPhone music app, selecting Artist, Songs, or Albums presents a tableView with a verticl list of single letters at the righthand side of the UI that enables rapid scr

Answer 1

If you're using a NSFetchedResultsController, you can just do:

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView {
    return [frc sectionIndexTitles];
}

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index {
    return [frc sectionForSectionIndexTitle:title atIndex:index];
}

Answer 2

I came up with an alternative approach to handling a single letter alphabet list without using sections. It's similar to Zaph's answer but instead of getting any value from returning a new index (since we'll always have 1 section), we calculate the index for the location of the first item in the array that begins with a certain character, then scroll to it.

The downside is this requires searching the array every time (is this absolutely terrible?), however I didn't notice any lag or slow behavior in the iOS simulator or on my iPhone 4S.

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView {
  return[NSArray arrayWithObjects:@"A", @"B", @"C", @"D", @"E", @"F", @"G", @"H", @"I", @"J", @"K", @"L", @"M", @"N", @"O", @"P", @"Q", @"R", @"S", @"T", @"U", @"V", @"W", @"X", @"Y", @"Z", nil];
}

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index {

  NSInteger newRow = [self indexForFirstChar:title inArray:self.yourStringArray];
  NSIndexPath *newIndexPath = [NSIndexPath indexPathForRow:newRow inSection:0];
  [tableView scrollToRowAtIndexPath:newIndexPath atScrollPosition:UITableViewScrollPositionTop animated:NO];

  return index;
}

// Return the index for the location of the first item in an array that begins with a certain character
- (NSInteger)indexForFirstChar:(NSString *)character inArray:(NSArray *)array
{
  NSUInteger count = 0;
  for (NSString *str in array) {
    if ([str hasPrefix:character]) {
      return count;
    }
    count++;
  }
  return 0;
}

---

adding property to store last selected index like

 @property (assign, nonatomic) NSInteger previousSearchIndex;

and storing this property every time like:

- (NSInteger)indexForFirstChar:(NSString *)character inArray:(NSArray *)array
{
    NSUInteger count = 0;
    for (NSString *str in array) {
        if ([str hasPrefix:character]) {
            self.previousSearchIndex = count;
            return count;
        }
        count++;
    }
    return self.previousSearchIndex;
}

and updating scrollToRow code like:

 [tableView scrollToRowAtIndexPath:newIndexPath atScrollPosition:UITableViewScrollPositionTop animated:YES];

Do this method even better and with nice animation.

Answer 3

If you're using MonoTouch, override the SectionIndexTitles(UITableView) method in the UITableViewDataSource class. Just return an array of strings and the subclass takes care of the rest.

class TableViewDataSource : UITableViewDataSource
{
  public override string[] SectionIndexTitles(UITableView tableView) 
  { 
    return new string[] { /*your string values */};
  }
}

*just a hint for those of us using C# and Mono (.NET) to write iPhone apps. :)

Answer 4

Here's a simple solution in Swift, assuming you have your title headers in an array. If the title couldn't be found, it will return the previous index in the array.

func sectionIndexTitlesForTableView(tableView: UITableView) -> [String]? {
    return "ABCDEFGHIJKLMNOPQRSTUVWXYZ".characters.flatMap{String($0)}
}

func tableView(tableView: UITableView, sectionForSectionIndexTitle title: String, atIndex index: Int) -> Int {
    return self.headerTitles.filter{$0 <= title}.count - 1
}

Answer 5

Here is a modified version of Kyle's function that handles the case of clicking an index for which you do not have a string:

- (NSInteger)indexForFirstChar:(NSString *)character inArray:(NSArray *)array
{
    char testChar = [character characterAtIndex:0];
    __block int retIdx = 0;
    __block int lastIdx = 0;

    [array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        char firstChar = [obj characterAtIndex:0];

        if (testChar == firstChar) {
            retIdx = idx;
            *stop = YES;
        }

        //if we overshot the target, just use whatever previous one was
        if (testChar < firstChar) {
            retIdx = lastIdx;
            *stop = YES;
        }

        lastIdx = idx;
    }];
    return retIdx;
}

Answer 6

Implement the delegate methods -sectionIndexTitlesForTableView: and -tableView:sectionForSectionIndexTitle:atIndex:

See the UITableViewDataSource documentation for more info.