For selecting mutually exclusive categories in a dependent hierarchy (i.e. tracing a taxonomy). See -> http://www.cocoanetics.com/2011/03/expandingcollapsing-tableview-sections/

HPGroupSelectViewController.h

#import <UIKit/UIKit.h>

@interface HPGroupSelectViewController : UIViewController <UITableViewDataSource, UITableViewDelegate>

@end

HPGroupSelectViewController.m

#import "HPGroupSelectViewController.h"
#import "DTCustomColoredAccessory.h"
#import "HPActionBar.h"
#import "HPApiClient.h"
#import "MBProgressHUD.h"

#import "HPUI.h"
#import "HPTitleLabel.h"
#import "HPUser.h"

@interface HPGroupSelectViewController ()
@property (strong, nonatomic) NSArray *sectionTitles;
@property (strong, nonatomic) NSMutableDictionary *selections;
@property (nonatomic, strong) NSMutableIndexSet *expandedSections;
@property (nonatomic, strong) UITableView *tableView;
@property (strong, nonatomic) HPTitleLabel *titleLabel;

@property (nonatomic, strong) NSDictionary *buildings;
@property (nonatomic, strong) NSArray *locations;
@property (nonatomic, strong) NSArray *departments;
@property (nonatomic, strong) NSDictionary *organizations;

@end

@implementation HPGroupSelectViewController

#pragma mark - UITableViewDataSource

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (indexPath.row == 0) {
        return ([self currentSelectionFromSection:indexPath.section]) ? 60 : 44;
    } else {
        return 38;
    }
}

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 4;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return (![self.expandedSections containsIndex:section]) ? 1 : [self itemsForSection:section].count + 1;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell;
    NSString *currentSelection = [self currentSelectionFromSection:indexPath.section];
    if (indexPath.row == 0) {
        cell = [tableView dequeueReusableCellWithIdentifier:@"SectionCell"];
        cell.accessoryView=nil;
        if (!cell)  {
            cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:@"SectionCell"];
            cell.selectionStyle = UITableViewCellSelectionStyleNone;
            cell.textLabel.font = [HPUI fontWithName:@"Ronnia-SemiBold" size:20.0f];
            cell.detailTextLabel.font = [HPUI fontWithName:@"Ronnia-Light" size:16.0f];
        }
        cell.textLabel.text = self.sectionTitles[indexPath.section];
        cell.detailTextLabel.text = currentSelection;
        DTCustomColoredAccessoryType accessoryType = ([self.expandedSections containsIndex:indexPath.section]) ? DTCustomColoredAccessoryTypeUp : DTCustomColoredAccessoryTypeDown;
        cell.accessoryView = [DTCustomColoredAccessory accessoryWithColor:[UIColor grayColor] type:accessoryType];
        cell.accessoryView.userInteractionEnabled = NO;
        cell.contentView.alpha = 1;
        if (indexPath.section == 1 || indexPath.section == 3) {
            if (![self currentSelectionFromSection:indexPath.section - 1]) cell.contentView.alpha = 0.4;
        }
    } else {
        cell = [tableView dequeueReusableCellWithIdentifier:@"ItemCell"];
        cell.accessoryView=nil;
        if (!cell) {
            cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"ItemCell"];
            cell.selectionStyle = UITableViewCellSelectionStyleGray;
            cell.textLabel.font = [HPUI fontWithName:@"Ronnia-Regular" size:16.0f];
            cell.textLabel.xOrigin += 20;
        }
        NSArray *sectionItems = [self itemsForSection:indexPath.section];
        NSString *item = [sectionItems objectAtIndex:indexPath.row - 1];
        cell.textLabel.text = [NSString stringWithFormat:@"     %@", item];
        cell.accessoryType = ([item isEqual:currentSelection]) ? UITableViewCellAccessoryCheckmark : UITableViewCellAccessoryNone;
    }
    return cell;
}

#pragma mark - UITableView Delegate

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (!indexPath.row) {
        [self toggleSection:indexPath tableView:tableView];
    } else {
        NSArray *sectionItems = [self itemsForSection:indexPath.section];
        NSString *currentSelection = self.selections[self.sectionTitles[indexPath.section]];
        if (![currentSelection isEqual:[sectionItems objectAtIndex:indexPath.row - 1]]) {
            self.selections[self.sectionTitles[indexPath.section]] = [sectionItems objectAtIndex:indexPath.row - 1];
        } else {
            [self.selections removeObjectForKey:self.sectionTitles[indexPath.section]];
        }
        
        if (indexPath.section == 0 || indexPath.section == 2)
            [self.selections removeObjectForKey:self.sectionTitles[indexPath.section + 1]];
        
        NSInteger countChild = (indexPath.section == 0 || indexPath.section == 2) ? 1 : 0;
        [self.expandedSections removeIndex:indexPath.section];
        [self.tableView reloadSections:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(indexPath.section, 1 + countChild)] withRowAnimation:NO];
    }
    [tableView deselectRowAtIndexPath:indexPath animated:YES];
}

#pragma mark - Private

- (NSArray *)itemsForSection:(NSInteger)index
{
    switch (index) {
        case 0:
            return self.departments;
        case 1:
            return [self.organizations objectForKey:[self currentSelectionFromSection:0]];
        case 2:
            return self.locations;
        case 3:
            return [self.buildings objectForKey:[self currentSelectionFromSection:2]];
        default:
            return nil;
    }
}

- (NSString *)currentSelectionFromSection:(NSInteger)index
{
    return self.selections[self.sectionTitles[index]];
}

-(void)toggleSection:(NSIndexPath*)indexPath tableView:(UITableView *)tableView
{
    if ((indexPath.section == 1 || indexPath.section == 3) && ![self currentSelectionFromSection:indexPath.section - 1])
        return;
    
    BOOL currentlyExpanded = [self.expandedSections containsIndex:indexPath.section];
    if (currentlyExpanded)
        [self.expandedSections removeIndex:indexPath.section];
    else    
        [self.expandedSections addIndex:indexPath.section];
    
    UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
    DTCustomColoredAccessoryType accessoryType = (!currentlyExpanded) ? DTCustomColoredAccessoryTypeUp : DTCustomColoredAccessoryTypeDown;
    cell.accessoryView = [DTCustomColoredAccessory accessoryWithColor:[UIColor grayColor] type:accessoryType];
    cell.accessoryView.userInteractionEnabled = NO;
    [self.tableView reloadSections:[NSIndexSet indexSetWithIndex:indexPath.section] withRowAnimation:NO];
}

-(void)didPressCancelButtonOnGroupEdit:(id)sender
{
    [self.navigationController dismissModalViewControllerAnimated:YES];
}

- (void)failWithMessage:(NSString *)status
{
    [MBProgressHUD hideHUDForView:self.view animated:YES];
    MBProgressHUD *hud = [MBProgressHUD showHUDAddedTo:self.view animated:NO];
    [hud setLabelText:status];
    [self performBlockOnMainThread:^{
        [MBProgressHUD hideHUDForView:self.view animated:YES];
        [self performBlockOnMainThread:^{hud.labelText = @"";} afterDelay:0.33f];
    } afterDelay:1.5f];
}

-(void)finishSelectingGroup
{
    [MBProgressHUD showHUDAddedTo:self.view animated:YES];
    NSMutableArray *savedGroups = [NSMutableArray arrayWithCapacity:4];    
    for (int i = 0; i < 4; i++) {
        NSString *selection = [self currentSelectionFromSection:i];
        if (selection) [savedGroups addObject:selection];
    }
    
    [HPApiClient saveGroupList:savedGroups success:^(NSDictionary *results) {
        [MBProgressHUD hideAllHUDsForView: self.view animated:YES];
        [self dismissModalViewControllerAnimated:YES];
        [HPSessionManager activeUser].groups = [results objectForKey:@"groups"];
    } failure:^(NSString *status, id json) {
        [self failWithMessage:status];
    }];
}

#pragma mark - VC Lifecycle

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.expandedSections = [[NSMutableIndexSet alloc] init];
    self.sectionTitles = @[@"Departments", @"Organizations", @"Locations", @"Buildings"];
    self.selections = [[NSMutableDictionary alloc] initWithCapacity:4];
    self.titleLabel = [[HPTitleLabel alloc] init];
    self.navigationItem.titleView = self.titleLabel;
    [self.titleLabel setTitleLabelText:@"Edit Groups"];

    self.tableView = [[UITableView alloc] initWithFrame:self.view.frame style:UITableViewStyleGrouped];
    self.tableView.delegate = self;
    self.tableView.dataSource = self;
    self.tableView.separatorStyle = UITableViewCellSeparatorStyleNone;
    self.tableView.yOrigin = 0;
    self.tableView.height = self.view.height - self.navigationController.navigationBar.height - 44;
    self.tableView.backgroundView = nil;
    self.view.backgroundColor = [[UIColor alloc] initWithPatternImage:[UIImage imageFromDiskNamed:@"gray_bg.png"]];
    [self.view addSubview:self.tableView];
    
    self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"Cancel" style:UIBarButtonItemStyleDone target:self action:@selector(didPressCancelButtonOnGroupEdit:)];
    
    HPActionBar *actionBar = [[HPActionBar alloc] initWithButtonType: HPTabbarActionButtonCheckMark];
    [actionBar.actionButton setTarget:self action:@selector(finishSelectingGroup) forControlEvents:UIControlEventTouchUpInside];
    actionBar.yOrigin = self.view.height - 44 - actionBar.height;
    [self.view addSubview: actionBar];
    
    [MBProgressHUD showHUDAddedTo:self.view animated:YES];
    [HPApiClient getGroupList:^(NSDictionary *results) {
        [MBProgressHUD hideAllHUDsForView:self.view animated:YES];
        NSDictionary *groups = results[@"groups"];
        self.departments = groups[@"Departments"];
        self.organizations = groups[@"Organizations"];
        self.locations = groups[@"Locations"];
        self.buildings = groups[@"Buildings"];
        self.expandedSections = [[NSMutableIndexSet alloc] init];
        [self.tableView reloadData];
    } failure:^(NSString *status, id json) {
        [self failWithMessage:status];
    }];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
}


@end