Question

My app runs in Instruments taking up approximately 700 KB of Live Bytes on average while running. However, every time it loads a new full-screen image, the memory allocations jump about 10 MB for a second, and then recover to the normal 700 KB level.

This is okay at the beginning, but once it has happened a few times I receive memory warnings and the app quits, even though the total Live Bytes stabilises well under the 1 MB mark.

I have created a test project to see why this is happening. It is a Single View Application with only the following code in the View Controller:

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSString *imgFile = [[NSBundle mainBundle] pathForResource:@"00-bg" ofType:@"png"];
    UIImage *img = [[UIImage alloc] initWithContentsOfFile:imgFile];

    UIImageView *backgroundImageView = [[UIImageView alloc] initWithImage:img];
    [img release];

    [self.view addSubview:backgroundImageView];
    [backgroundImageView release];
}

The output from Instruments (Leaks) looks like this:

I have tried both ARC and non-ARC, and the only difference is the length of the spike (ARC seems to hold onto the memory for longer).

I have also tried both UIImage imageNamed: and initWithContentsOfFile: but the results are the same.

Why is this spike happening? And is there anything I can do to avoid it?

Answer 1

To display them on screen, iOS has to uncompress your images and that’s where your spike comes from.

2048 * 1536 = 3145728 pixels. At 4 bytes per pixel that is 12 MB.