IOS / iPhone loading images into UIImage based on slider value memory issue

Question

I am loading images into a UIImage with the values of a slider (these are pages or slides, if you like). This UIImage switches them very fast with the use of a slider. Problem is, at some point, the app crashes on the device with an error of:

2011-04-02 17:39:01.836 Book1[2123:307] Received memory warning. Level=1

Here's the code:

- (IBAction)slidePages:(id)sender{
    int sliderValue = pageSlider.value;

    NSString *slideToPage = [NSString stringWithFormat:@"P%i.jpg", sliderValue];
    imagePlaceholder.image = [UIImage imageNamed:slideToPage];

    pageDisplay.text = [NSString stringWithFormat:@"Page %i", sliderValue];

    currentPage = sliderValue;  
}

Is there anything I could do to make it more efficient? Maybe the error is somewhere else but I'm guessing it has to do with the fast loading of images.

Still, I don't know how iOS deals with this. Every time I load a new image into the UIImage what happens with the "unloaded" one?

Thanks in advance.

score 3 · Accepted Answer · edited May 23 '17 at 11:47

3

One imortant aspect of [UIImage imageNamed] is that it caches all images loaded in that way and they never get unloaded, even if you dealloc the UIImage that was created! This is good in some circumstances (e.g. smallish images in UITableView cells), but bad in others (e.g. large images).

The solution is to use [UIImage imageWithData] which does not do this caching and which unloads the data when the UIImage is dealloc'd.

More info and discussion here:

Difference between [UIImage imageNamed...] and [UIImage imageWithData...]?

Update

This question has some good info on the question of [UIImage imageNamed:] not emptying its cache when a memory warning occurs.

edited May 23 '17 at 11:47

Community

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answered Apr 03 '11 at 00:24

occulus

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Oh, thanks! I'll try this and let you know. Seems like this is the answer. – HotFudgeSunday Apr 03 '11 at 11:13
Or you can close the [imageView setImage[UIImage imageNamed:@"xxx.png"]]; into NSAutoreleasePool ... that does the trick for me :) – Ondrej Rafaj May 02 '11 at 16:48
Are you sure about that, Ondrej? Have you confirmed that the data cached by imageNamed is actually released by using the allocations monitor or a similar instruments tool? – occulus May 04 '11 at 14:48

Marcelo Cantos · Answer 2 · 2011-04-02T23:20:28.403

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[UIImage imageNamed:] caches images, so every image thus loaded effectively leaks. I don't know what the canonical solution is, but one option might be to load the image with CGImageCreateWithJPEGDataProvider() and initialise it with [UIImage imageWithCGImage:].

The original (wrong) answer:

~~You may need to release the previous image before loading the current one.~~

edited Apr 02 '11 at 23:20

answered Apr 02 '11 at 22:24

Marcelo Cantos

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score 0 · Answer 3 · answered Apr 02 '11 at 22:31

Loading a bunch of large images in iOS has always been a memory issue. As Marcelo mentioned, you should only keep around images are you currently viewing. All other images should be released so they can be garbage collected and the memory freed up. In my experience, even loading 2 fairly large images (500k-2mb each) will cause memory issues, especially on older devices with less RAM.

IOS / iPhone loading images into UIImage based on slider value memory issue

3 Answers3