why is my variable setting itself to nil?

Question

In a UITableViewController, under didSelectRowAtIndexPath, I set a property in an NSObject file to a value. I can use NSLog and see that the variable has indeed been set. However, when I use the table view and click on a cell that activates didSelectRowAtIndexPath, I noticed that the variable sets itself to nil when the view changes to another UITableViewController. I even did a NSLog under viewDidLoad to check this property value, and it returned nil. Why is this? How can I make it so it doesn't change itself to nil?

First UITableView.m:

#import "enchantmentViewController.h"
#import "levelViewController.h"
#import "dataBrain.h"

@interface enchantmentViewController ()

@property (nonatomic, strong) dataBrain *brain;
@property (nonatomic, strong) levelViewController *level;

@end

@implementation enchantmentViewController

@synthesize brain = _brain;
@synthesize level = _level;

- (dataBrain *)brain
{
    if (!_brain){
        _brain = [[dataBrain alloc] init];
    }
    return _brain;
}

- (levelViewController *)level
{
    if (!_level){
        _level = [[levelViewController alloc] init];
    }
    return _level;
}

- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Uncomment the following line to preserve selection between presentations.
    // self.clearsSelectionOnViewWillAppear = NO;

    // Uncomment the following line to display an Edit button in the navigation bar for this view controller.
    // self.navigationItem.rightBarButtonItem = self.editButtonItem;
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return (interfaceOrientation == UIInterfaceOrientationPortrait);
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    //warning Potentially incomplete method implementation.
    // Return the number of sections.
    return [self.brain enchantmentNumberOfSections];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    //#warning Incomplete method implementation.
    // Return the number of rows in the section.
    return [self.brain enchantmentNumberOfCells:section];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

    static NSString *CellIdentifier = @"enchantment";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    if (!cell) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
        cell.selectionStyle = UITableViewCellSelectionStyleNone;
    }

    cell.textLabel.text = [self.brain enchantmentCellText:indexPath];

    return cell;

}

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
    return [self.brain enchantmentSectionData:section];
}


/*
// Override to support conditional editing of the table view.
- (BOOL)tableView:(UITableView *)tableView canEditRowAtIndexPath:(NSIndexPath *)indexPath
{
    // Return NO if you do not want the specified item to be editable.
    return YES;
}
*/

/*
// Override to support editing the table view.
- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (editingStyle == UITableViewCellEditingStyleDelete) {
        // Delete the row from the data source
        [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
    }   
    else if (editingStyle == UITableViewCellEditingStyleInsert) {
        // Create a new instance of the appropriate class, insert it into the array, and add a new row to the table view
    }   
}
*/

/*
// Override to support rearranging the table view.
- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath
{
}
*/

/*
// Override to support conditional rearranging of the table view.
- (BOOL)tableView:(UITableView *)tableView canMoveRowAtIndexPath:(NSIndexPath *)indexPath
{
    // Return NO if you do not want the item to be re-orderable.
    return YES;
}
*/

#pragma mark - Table view delegate

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    [tableView deselectRowAtIndexPath:indexPath animated:YES];
    [self.brain convertSelectedTableCellToNumberOfPossibleEnchantments:indexPath];
    [self.brain test];
}

@end

Second UITableViewController code:

#import "levelViewController.h"
#import "dataBrain.h"

@interface levelViewController ()

@property (nonatomic, strong) dataBrain *brain;

@end

@implementation levelViewController

@synthesize brain = _brain;


- (dataBrain *)brain
{
    if (!_brain){
        _brain = [[dataBrain alloc] init];
    }
    return _brain;
}

- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Uncomment the following line to preserve selection between presentations.
    // self.clearsSelectionOnViewWillAppear = NO;

    // Uncomment the following line to display an Edit button in the navigation bar for this view controller.
    // self.navigationItem.rightBarButtonItem = self.editButtonItem;
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return (interfaceOrientation == UIInterfaceOrientationPortrait);
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    // Return the number of sections.
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    // Return the number of rows in the section.
    //NSLog(@"needing sections");

    NSLog(@"value: %@", [self.brain levelNumberOfCells]);
    return [self.brain.numberOfLevelsForCurrentEnchantment integerValue];
    //[self.brain levelNumberOfCells];

}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"level";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    if (!cell) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
        cell.selectionStyle = UITableViewCellSelectionStyleNone;
    }

    cell.textLabel.text = @"jo";//[self.brain levelCellText:indexPath];

    return cell;
}

@end

[self.brain test] returns the value of the variable that is in question.

Answer 1

So each viewController has its own brain, which gets automatically allocated when need be. You set a variable in one VC's brain, then switch VC's, so now when you ask for something about its brain, you're asking about a different brain -- one you haven't made any changes to.