UIScrollView Memory-Allocation and Release

Question

I am working on a pretty simple and straightforward app that uses a couple of pages with scroll views. I have an array of images in ViewDidLoad and then a mutable array that shows current page plus and minus 1. There is a purge page method which is supposed to remove the extra images from the superview. When running this on an actual device with a large quantity of images, a memory warning comes up after scrolling through 50 or so images and then the app crashes. I ran this through instruments and see that the memory in increasing substantially with each swipe. When creating the array I used imageNamed as well as imageWithContentsOfFile and either way gives close to the same result. I realize there have been multiple scrollview questions here, but somehow I cannot seem to get past this one. Frustrating to say the least. I hope someone can look at this with a fresh set of eyes and shed some light on my problem. Thanks very much in advance.

EDIT: Forgot to mention ARC use. Yes, I use ARC.

Code: .h file

#import <UIKit/UIKit.h>

@interface FirstViewController : UIViewController <UIScrollViewDelegate>

@property (nonatomic, strong) IBOutlet UIScrollView *scrollView;
@property (nonatomic, strong) IBOutlet UIPageControl *pageControl;

@end

and the .m file

#import "FirstViewController.h"

@interface FirstViewController ()

@property (nonatomic, strong) NSArray *pageImages;
@property (nonatomic, strong) NSMutableArray *pageViews;

- (void)loadVisiblePages;
- (void)loadPage:(NSInteger)page;
- (void)purgePage:(NSInteger)page;
@end

@implementation FirstViewController


@synthesize scrollView = _scrollView;
@synthesize pageControl = _pageControl;

@synthesize pageImages = _pageImages;
@synthesize pageViews = _pageViews;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)didReceiveMemoryWarning
{
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];

    // Release any cached data, images, etc that aren't in use.
}

- (void)loadVisiblePages {
    // First, determine which page is currently visible
    CGFloat pageWidth = self.scrollView.frame.size.width;
    NSInteger page = (NSInteger)floor((self.scrollView.contentOffset.x * 2.0f + pageWidth)     / (pageWidth * 2.0f));

// Update the page control
self.pageControl.currentPage = page;

// Work out which pages you want to load
NSInteger firstPage = page - 1;
NSInteger lastPage = page + 1;

// Purge anything before the first page
for (NSInteger i=0; i<firstPage; i++) {
    [self purgePage:i];
}

// Load pages in our range
for (NSInteger i=firstPage; i<=lastPage; i++) {
    [self loadPage:i];
}

// Purge anything after the last page
for (NSInteger i=lastPage+1; i<self.pageImages.count; i++) {
    [self purgePage:i];
}

}

- (void)purgePage:(NSInteger)page {
    if (page < 0 || page >= self.pageImages.count) {
    // If it's outside the range of what you have to display, then do nothing
    return;
    }

    // Remove a page from the scroll view and reset the container array
    UIView *pageView = [self.pageViews objectAtIndex:page];
    if ((NSNull*)pageView != [NSNull null]) {
        [pageView removeFromSuperview];
        [self.pageViews replaceObjectAtIndex:page withObject:[NSNull null]];


    }

}


- (void)loadPage:(NSInteger)page {
    if (page < 0 || page >= self.pageImages.count) {
        // If it's outside the range of what you have to display, then do nothing
        return;
    }

    // 1
    UIView *pageView = [self.pageViews objectAtIndex:page];
    if ((NSNull*)pageView == [NSNull null]) {
        // 2
        CGRect frame = self.scrollView.bounds;
        frame.origin.x = frame.size.width * page;
        frame.origin.y = 0.0f;

        // 3
        UIImageView *newPageView = [[UIImageView alloc] initWithImage:[self.pageImages objectAtIndex:page]];
        newPageView.contentMode = UIViewContentModeScaleAspectFit;
        newPageView.frame = frame;
        [self.scrollView addSubview:newPageView];
        // 4
       [self.pageViews replaceObjectAtIndex:page withObject:newPageView];

    }
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    // Load the pages that are now on screen
    [self loadVisiblePages];
}

- (void)viewDidLoad {
    [super viewDidLoad];

    // 1
    self.pageImages = [NSArray arrayWithObjects:
                       [UIImage imageNamed:@"BBD1.jpg"],
                       [UIImage imageNamed:@"BBD2.jpg"],
                       [UIImage imageNamed:@"BBD3.jpg"],
                       [UIImage imageNamed:@"BBD4.jpg"],
                       [UIImage imageNamed:@"BBD5.jpg"],
                       [UIImage imageNamed:@"BBD6.jpg"],
                       [UIImage imageNamed:@"BBD7.jpg"],
                  .....
                       [UIImage imageNamed:@"BBD292.jpg"],
                       [UIImage imageNamed:@"BBD293.jpg"],
                       [UIImage imageNamed:@"BBD294.jpg"],
                       [UIImage imageNamed:@"BBD295.jpg"],
                       [UIImage imageNamed:@"BBD296.jpg"],
                       [UIImage imageNamed:@"BBD297.jpg"],
                       [UIImage imageNamed:@"BBD298.jpg"],
                       [UIImage imageNamed:@"BBD299.jpg"],
                       [UIImage imageNamed:@"BBD300.jpg"],                       


                       nil];

    NSInteger pageCount = self.pageImages.count;

    // 2
    self.pageControl.currentPage = 0;
    self.pageControl.numberOfPages = pageCount;

    // 3
    self.pageViews = [[NSMutableArray alloc] init];
    for (NSInteger i = 0; i < pageCount; ++i) {
        [self.pageViews addObject:[NSNull null]];
    } 
}

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

    // 4
    CGSize pagesScrollViewSize = self.scrollView.frame.size;
    self.scrollView.contentSize = CGSizeMake(pagesScrollViewSize.width *     self.pageImages.count, pagesScrollViewSize.height);

    // 5
    [self loadVisiblePages];
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
}

- (BOOL)shouldAutorotateToInterfaceOrientation:    (UIInterfaceOrientation)interfaceOrientation
{
    if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone) {
        return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
    } else {
        return YES;
    }
}

@end

I really appreciate you taking the time to look at this and hope there is someone out there who can point me in the right direction.

THANKS!

Answer 1

Loading the actual images into the array incurs a much larger overhead than storing just the names in the array and loading the image like

UIImage *image = [[UIImage alloc] initWithContentsOfFile:[self.pageImages objectAtIndex:page]];
UIImageView *newPageView = [[UIImageView alloc] initWithImage:image];
[image release];

At this point, only the newPageView will hold a reference to the image in memory, and when that newPageView is removed from it's superview, the image in memory will be released.

Answer 2

You don't say if you are using ARC or not, but without ARC there are obvious problems in memory management. In loadPage: at comment "// 3" you create newPageView with a retain count of 1, then add it as a subview of self.scrollView and to the array self.pageViews. Thus it has an overall retain count of +3. In purgePage: you remove it from self.scrollView and self.pageViews but are still left with an overall retain count of +1. Thus the UIImageView for each page is forgotten but not deallocated.

You can fix this by using ARC in your project or modifying the line where you create newPageView to be instead:

UIImageView *newPageView = [[[UIImageView alloc] initWithImage:[self.pageImages objectAtIndex:page]] autorelease];