Possible memory leak in UIViewController with UITableView

Question

I have a UIViewController thats presented modally. When I watch the memory allocations Instrument, the memory usage increases when the view is presented, but when it's exited the memory isn't released. If I keep opening and closing the view, the memory just keeps getting higher. Instruments doesn't report a memory leak! What could be causing this? The View Controller code is below (I've skipped the didSelectRow code). Dealloc is always called.

EDIT - I am using ARC

.h

#import <UIKit/UIKit.h>
@class OutlineTextUILabel;

@interface StoreViewController : UIViewController <UITableViewDelegate, UITableViewDataSource> {

    int starCount;
    NSMutableArray *_singleUseArray;
    NSMutableArray *_fullUseArray;

}

@property (weak, nonatomic) IBOutlet UITableView *tableView;
@property (weak, nonatomic) IBOutlet OutlineTextUILabel *starCountLbl;
- (IBAction)exitBtnPressed:(id)sender;

.m

#import "StoreViewController.h"
#import "NSUserDefaults+MPSecureUserDefaults.h"
#import "PowerUpCell.h"
#import "OutlineTextUILabel.h"
#import "PowerUpSingleton.h"
#import "PowerUp.h"

#define kPrefsNumberOfStars             @"numberOfStars"

@interface StoreViewController ()

@end

@implementation StoreViewController
@synthesize tableView = _tableView;
@synthesize starCountLbl;

#pragma mark View Methods

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Display star count
    NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
    BOOL valid = NO;
    starCount = [prefs secureIntegerForKey:kPrefsNumberOfStars valid:&valid];
    if (!valid) {
        NSLog(@"Stars Tampered With!");
        self.starCountLbl.text = @"Err";
    } else {
        self.starCountLbl.text = [NSString stringWithFormat:@"%d",starCount];
    }

    // Tableview setup
    CGRect frame2 = CGRectMake(0, 0, 320, 40);
    UIView *footer = [[UIView alloc] initWithFrame:frame2];
    footer.backgroundColor = [UIColor clearColor];
    self.tableView.tableFooterView = footer;
    self.tableView.opaque = NO;
    self.tableView.backgroundView = nil;
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:YES];

    if (![[PowerUpSingleton sharedList] refreshArray]) {
        NSLog(@"Error, %s",__FUNCTION__);
    } else {
        [self performSelectorOnMainThread:@selector(workOutSingleUseToDisplay) withObject:nil waitUntilDone:YES];
        [self performSelectorOnMainThread:@selector(workOutFullUseToDisplay) withObject:nil waitUntilDone:YES];
        [self.tableView reloadData];
    }
}

- (void)workOutSingleUseToDisplay
{
    _singleUseArray = [[NSMutableArray alloc] init];
    for (PowerUp *pu in [[PowerUpSingleton sharedList] sharedArray]) {
        if (!pu.fullUnlock) {
            [_singleUseArray addObject:pu];
        }
    }
}

- (void)workOutFullUseToDisplay
{
    _fullUseArray = [[NSMutableArray alloc] init];
    for (PowerUp *pu in [[PowerUpSingleton sharedList] sharedArray]) {
        if (pu.prefFullName != nil) {
            [_fullUseArray addObject:pu];
        }
    }

}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

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

- (void)viewDidUnload {
    [self setTableView:nil];
    [self setStarCountLbl:nil];
    [super viewDidUnload];
}

#pragma mark TableView Setup Methods

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 2;
}

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
    if (section == 0) {
        return @"Single Use";
    } else if (section == 1) {
        return @"Use forever";
    }

    return nil;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    if (section == 0) {
        return [_singleUseArray count];
    } else if (section == 1) {
        return [_fullUseArray count];
    }

    return 0;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSString *cellIdentifier;
    if (indexPath.section == 0) {
        cellIdentifier = @"powerUpCellSingleUse";
    } else if (indexPath.section == 1) {
        cellIdentifier = @"powerUpCell";
    }

    PowerUpCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
    if (cell == nil) {
        cell = [[PowerUpCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
    }

    if (indexPath.section == 0) {
        PowerUp *tmpPU = [_singleUseArray objectAtIndex:indexPath.row];
        cell.descriptionLbl.text = tmpPU.displayName;
        int cost = tmpPU.costSingle;
        cell.costLbl.text = [NSString stringWithFormat:@"%d",cost];
        if (cost > starCount) {
            cell.costLbl.textColor = [UIColor redColor];
        } else {
            cell.costLbl.textColor = [UIColor blueColor];
        }
        int howMany = tmpPU.numberOwned;
        cell.howManyLbl.text = [NSString stringWithFormat:@"%d",howMany];

    } else if (indexPath.section == 1) {
        PowerUp *tmpPU = [_fullUseArray objectAtIndex:indexPath.row];
        cell.descriptionLbl.text = tmpPU.displayName;
        int cost = tmpPU.costFull;
        cell.costLbl.text = [NSString stringWithFormat:@"%d",cost];
        if (cost > starCount) {
            cell.costLbl.textColor = [UIColor redColor];
        } else {
            cell.costLbl.textColor = [UIColor blueColor];
        }
        if (tmpPU.fullUnlock) {
            cell.costLbl.textColor = [UIColor greenColor];
            cell.costLbl.text = @"---";
        }
    }

    return cell;
}

#pragma mark -

- (IBAction)exitBtnPressed:(id)sender
{
    [self dismissModalViewControllerAnimated:YES];
}

- (void)dealloc
{
    NSLog(@"%s",__FUNCTION__);
    self.tableView = nil;
    self.starCountLbl = nil;
}

@end

EDIT ------------- Something seems not to be right. I've added an NSLog to the cell alloc, and it's never called, even though the cells are created!

PowerUpCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
    if (cell == nil) {
        NSLog(@"new cell");
        cell = [[PowerUpCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
    }

EDIT July 1st ------ I've added a Navigation controller and now use push instead of modal and this problem is still here. I've taken heap shots with Instruments by moving back and forward between views a few times and it seems maybe the cells are still hanging around, as this screenshot shows gesture recogniser still around from a previous load of the view.

Answer 1

It is because you used your IBOutlets as weak, instead of using strong.

I actually believe this is a flaw in the XCode environment, as it should warn you of this kind of behavior.

As a best practice, I would suggest letting XCode generate the IBOutlets by dragging the views to the code in the Interface Builder, to avoid such annoying pitfalls.

Answer 2

Looks like you've already found some ways around this, but just in case this helps:

1) Make sure you haven't got Zombies turned on while you're debugging, as this causes objects to hang around after you think they should be dealloc-ed (Edit scheme -> Run -> Diagnostics).

2) You're using ARC and so I assume storyboards or at least prototype UITableView cells in your storyboard/NIB? If so, then the reason your NSLog() below never gets called is because the dequeueReusableCellWithIdentifier call knows to create cells from these prototype cells via the defined cellIdentifier. Pretty handy.

PowerUpCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
    if (cell == nil) {
        NSLog(@"new cell");
        cell = [[PowerUpCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
    }

You have to rely on the UITableView to manage this cache of UITableViewCells, and release them appropriately. So it's possible they are just hanging around because your UITableView isn't being released (though I think you're saying it is).