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.

Answer 1

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.