Handling out of memory errors in iPhone

Question

I would like to handle out of memory errors in iPhone to execute logic with lesser memory requirements in case I run of of memory. In particular, I would like to do something very similar to the following pseudo-code:

UIImage* image;
try  {
    image = [UIImage imageNamed:@"high_quality_image.png"];
} catch (OutOfMemoryException e) {
    image = [UIImage imageNamed:@"low_quality_image.jpg"];
}

First I attempt to load a high-quality image, and if I run out of memory while doing it, then I use a lower quality image.

Would this be possible? Is there some kind of exception or notification than can be handled when an out of memory error occurs?

The out of memory warning is not what I'm looking for, as it's received before the phone runs out of memory. I would like to know if the logic I'm executing failed because an out of memory error, and deal with this accordingly.

Alternatively, something like this could also help:

UIImage* image;
if (enoughMemory) {
    image = [UIImage imageNamed:@"high_quality_image.png"];
} else {
    image = [UIImage imageNamed:@"low_quality_image.jpg"];
}

Answer 1

The iPhone has a low memory warning that is triggered when a certain memory threshold is reached. This callback is issued to the application and must be responded to appropriately or your application runs the risk of being preempted by the OS. At that time, then, it would be good for your application to convert the high-memory images to more low memory ones.

Be careful here, though, as the size of the file on disk is not necessarily representative of how much memory it will consume once it has been loaded and prepped for use on the device.

See here for more information on handling low memory warnings.

Answer 2

Edit: it looks like this wasn't quite the answer to your question.

---

What are you trying to do with your images? Some actions, like storing a full-sized image returned from the image picker controller are particularly heavy memory users.

What I would recommend doing is always assuming you need the smallest sized images at all times and:

1. If you will need a high-resolution image later, cache it to disk.
2. All images for display should be the minimum resolution you can get away with.
3. When you need the high-resolution image, pull it back out of the cache.
4. Use the low memory warnings as a sign to release any images that you have in memory, but aren't visible.

Now, it's possible that this situation won't work for you. Reading and writing large images to disk on the iPhone is slow. If that's the case what you'll have to do is follow @fbrereto's suggestion: load the large image, watch out for memory warnings and if you get one, drop it and load the small image.

You definitely shouldn't ignore memory warnings, but not responding to one won't necessarily crash your app. My iPhone 3G will get memory warnings almost every time I get an image back from the UIImagePickerController, and there's nothing I can do about it.

Answer 3

here is an answer that will get you the current memory used by your app and you can decide what to do based on that info.

http://stackoverflow.com/questions/2786539/iphone-memory-usuage