Possible optimization to unload the views+controllers which are no longer visible in UIScrollView and PageControl

Question

I have this page control code same from the apple sample. Here i have a subview (controller.view) which contains a ImageView. Now problem is with memory management. All works fine. But when i scroll 5-10 pages. RAM gets filled. I tried to release the view+controller but did not find any proper place/way that work. I want to release the views which are not currently visible. (except current,previous & next view)

- (void)applicationDidFinishLaunching:(UIApplication *)application {
    NSMutableArray *controllers = [[NSMutableArray alloc] init];
    for (unsigned i = 0; i < kNumberOfPages; i++) {
        [controllers addObject:[NSNull null]];
    }
    self.viewControllers = controllers;
    [controllers release];

    scrollView.pagingEnabled = YES;
    scrollView.contentSize = CGSizeMake(scrollView.frame.size.width * kNumberOfPages, scrollView.frame.size.height);
    scrollView.showsHorizontalScrollIndicator = NO;
    scrollView.showsVerticalScrollIndicator = NO;
    scrollView.scrollsToTop = NO;
    scrollView.delegate = self;

    pageControl.numberOfPages = kNumberOfPages;
    pageControl.currentPage = 0;

    [self loadScrollViewWithPage:0];
    [self loadScrollViewWithPage:1];
}
- (void)loadScrollViewWithPage:(int)page {
    if (page < 0) return;
    if (page >= kNumberOfPages) return;

    PageControlExampleViewControl *controller = [viewControllers objectAtIndex:page];
    if ((NSNull *)controller == [NSNull null]) {
        controller = [[PageControlExampleViewControl alloc] initWithPageNumber:page];
        [viewControllers replaceObjectAtIndex:page withObject:controller];
        [controller release];
    }

    if (nil == controller.view.superview) {
        CGRect frame = scrollView.frame;
        frame.origin.x = frame.size.width * page;
        frame.origin.y = 0;
        controller.view.frame = frame;
        [scrollView addSubview:controller.view];
    }
}

- (void)scrollViewDidScroll:(UIScrollView *)sender {
  if (pageControlUsed) {
        return;
    }
    CGFloat pageWidth = scrollView.frame.size.width;
    int page = floor((scrollView.contentOffset.x - pageWidth / 2) / pageWidth) + 1;
    pageControl.currentPage = page;

    [self loadScrollViewWithPage:page - 1];
    [self loadScrollViewWithPage:page];
    [self loadScrollViewWithPage:page + 1];

}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    pageControlUsed = NO;
}
- (IBAction)changePage:(id)sender {
    int page = pageControl.currentPage;
    [self loadScrollViewWithPage:page - 1];
    [self loadScrollViewWithPage:page];
    [self loadScrollViewWithPage:page + 1];
    CGRect frame = scrollView.frame;
    frame.origin.x = frame.size.width * page;
    frame.origin.y = 0;
    [scrollView scrollRectToVisible:frame animated:YES];
    pageControlUsed = YES;
}

Answer 1

I tried for several hours the code of Deepmist. It did the job but I received memory warnings and my app crashed every 25-30 pages scrolled (I am using big images in the pages). In Instruments I noticed a very large use of memory: even if the unnecessary views were removed time by time from superview and the relative viewControllers were replaced with NSNulls, Instruments showed that real memory increased on every pagescroll of 4-5MB!

Searching on the web I found that this is a common problem. If you also have this problem, you should try the following checks:

1) in each view, be sure to use imageWithContentsOfFile instead of imageNamed. As documented imageNamed cache images and increase memory size.

2) in the Deepmist code, after:

[controller.view removeFromSuperview];

you also have to set to nil the view:

controller.view=nil;

This trick solved the memory consumption which now is stable for the only three views loaded (current, current-1 and current+1 to avoid flashing in the pagescroll).

Hope this helps!

Answer 2

I didn't test this but I imagine you could write a method that does the opposite of loading like so:

- (void)unloadScrollViewWithPage:(int)page {
    if (page < 0) return;
    if (page >= kNumberOfPages) return;

    PageControlExampleViewControl *controller = [viewControllers objectAtIndex:page];

    if ((NSNull *)controller != [NSNull null]) {
        if (nil != controller.view.superview)
            [controller.view removeFromSuperview];

        [viewControllers replaceObjectAtIndex:page withObject:[NSNull null]];            
    }
}

Then add some code to your didScroll method like so:

[self unloadScrollViewWithPage:page - 2];
[self loadScrollViewWithPage:page - 1];
[self loadScrollViewWithPage:page];
[self loadScrollViewWithPage:page + 1];
[self unloadScrollViewWithPage:page + 2];

Answer 3

in loadScrollViewWithPage method, just before if (nil == controller.view.superview), cycle trough all views and remove them all except current -1, current and current + 1 but only if method was called with current index view. Also, don't forget to replace those view controllers in viewControllers array with NSNulls.

Answer 4

try this VSScrollview , it reuses its views like UITableview reuses its cell.