Any way to get a Cached UIImage from my 'Documents' directory?

Question

I know that the -imageNamed: method returns a Cached UIImage, but the problem is that my image file is stored in 'Documents', and the -imageNamed: method seems to only search the Bundle... I am currently (reluctantly) using -imageWithContentsOfFile: to get my image from 'Documents' but it is not the same...Scaling up/down a UIImageView containing the resulting image is choppy and awkward. Scaling the same UIImageView containing an image created with -imageNamed: however appears very smooth. So, again: How can I get a cached UIImage from my 'Documents' if I cannot use -imageNamed:?

Answer 1

What about writing your own image cache? You have all the pieces in place, now you just need to encapsulate it and keep a record of images you've already loaded.

Answer 2

You can cache UIImages yourself just as -imageNamed: does. It just loads them, and then holds onto them. You can hold onto them, too, using an NSDictionary and implement your own -imageNamed:

But I'm more concerned about the trouble you're having with scaling. How are your images getting into Documents, how are you scaling them, and have you tested the same image file stored in the bundle? I doubt that -imageNamed: has anything to do with this. I would more suspect things like the fact that the bundle has some compression applied to it (though I don't yet have a theory on why this would matter in practice), differences in the file, or differences in how the rest of the program is behaving during scaling (causing contention on the disk or CPU). Caching is unlikely related to this issue.

I'd do some profiling w/ Instruments to try to find out where the choppiness is coming from. Are you maxing out the disk, CPU, memory? What's the bottleneck?

Answer 3

The simplest way would be an NSMutableDictionary storing the cached images and a clear cache method:

@interface UIImage (CacheExtensions)
+ (id)cachedImageWithContentsOfFile:(NSString *)path;
+ (void)clearCache;
@end

static NSMutableDictionary *UIImageCache;

@implementation UIImage (CacheExtensions)
+ (id)cachedImageWithContentsOfFile:(NSString *)path
{
    id result;
    if (!UIImageCache)
        UIImageCache = [[NSMutableDictionary alloc] init];
    else {
        result = [UIImageCache objectForKey:path];
        if (result)
            return result;
    }
    result = [UIImage imageWithContentsOfFile:path];
    [UIImageCache setObject:result forKey:path];
    return result;
}
+ (void)clearCache
{
    [UIImageCache removeAllObjects];
}
@end

Note: you should call +[UIImage clearCache] from your didReceiveMemoryWarning method. Also, clearCache will invalidate all objects in the cache, not just unused items; a UIImage subclass and more complicated caching mechanism would be required to remedy this.