Image caching for images in the Documents directory

Image.h

#import <Foundation/Foundation.h>

@interface Image : NSObject

+(void) clearCache;

+(UIImage *) imageInDocuments:(NSString *)imageName ;

+(void)addToDictionary:(NSString *)imageName image:(UIImage *)image;

@end

Image.m

#import "Image.h"

@implementation Image

static NSDictionary * cache;
static NSDictionary * fifo;
static NSNumber * indexFifo;
static NSInteger maxFifo = 25;

+(void)initialize {
    [self clearCache];
}

+(void) clearCache {
    cache = [[NSDictionary alloc] init];
    fifo = [[NSDictionary alloc] init];
    indexFifo = [NSNumber numberWithInt:0];
}

+(UIImage *) imageInDocuments:(NSString *)imageName {
    UIImage * imageFromCache = [cache objectForKey:imageName];
    if(imageFromCache != nil) return imageFromCache;
    
    NSString * path = [NSHomeDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:@"/Documents/%@", imageName, nil]];
    UIImage * result = [UIImage imageWithContentsOfFile:path];
    [self addToDictionary:imageName image:result];
    return result;
}

+(void)addToDictionary:(NSString *)imageName image:(UIImage *)image {
    
    NSMutableDictionary *mFifo = [fifo mutableCopy];
    NSString * imageToRemoveFromCache = [mFifo objectForKey:indexFifo];
    [mFifo setObject:imageName forKey:indexFifo];
    fifo = [NSDictionary dictionaryWithDictionary:mFifo];
    // indexFifo is like a cursor which loop in the range [0..maxFifo];
    indexFifo = [NSNumber numberWithInt:([indexFifo intValue] + 1) % maxFifo];
    
    NSMutableDictionary * mcache = [cache mutableCopy];
    [mcache setObject:image forKey:imageName];
    if(imageToRemoveFromCache != nil) [mcache removeObjectForKey:imageToRemoveFromCache];
    cache = [NSDictionary dictionaryWithDictionary:mcache];
}

@end