Persistently stores and recovers NSCoding compliant objects with a expiry time

KDPersistantCache.h

//
//  KDPersistantCache.h
//  KDPrototype
//    Stores NSCoding compliant objects persistantly in the NSCachesDirectory 
//    for a specified period of time.
//
//  Created by  on 18/04/2012.
//  Copyright (c) 2012 __MyCompanyName__. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface KDPersistantCache : NSObject

// Cache length is |NSTimeInterval| which is measured in seconds.
- (void) addObject:(id<NSCoding>)objectToCache 
       forCacheKey:(NSString*)cacheKey
       cahceLength:(NSTimeInterval)timeToLive;

// Will reutrn nill if the object donst exist or is expired.
- (id) objectForCacheKey:(NSString*)cacheKey;

// Goes though each object, 
// if the date is passed the time to live
// removes the object from the persistant store
- (void) purgeExpiredCahceItems;

@end

KDPersistantCache.m

//
//  KDPersistantCache.m
//  KDPrototype
//
//  Created by  on 18/04/2012.
//  Copyright (c) 2012 __MyCompanyName__. All rights reserved.
//

#import "KDPersistantCache.h"


@interface KDPersistantCache()
@property (copy, nonatomic) NSString* cachePath;
@end


@implementation KDPersistantCache
@synthesize cachePath = m_cachePath;

- (NSString*) cachePath 
{
    if (!m_cachePath) {
        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
        m_cachePath = [paths objectAtIndex:0];
        BOOL isDir = NO;
        NSError *error;
        if (! [[NSFileManager defaultManager] fileExistsAtPath:m_cachePath isDirectory:&isDir]
            && isDir == NO) 
        {
            [[NSFileManager defaultManager] createDirectoryAtPath:m_cachePath
                                      withIntermediateDirectories:NO
                                                       attributes:nil
                                                            error:&error];
        }
    }
    return m_cachePath;
}



- (NSString*) pathForCahceKey:(NSString*)cacheKey 
{
    NSString* path = [[self cachePath] stringByAppendingPathComponent:cacheKey];
    path = [path stringByAppendingPathExtension:@"kdioscache"];
    
    return path;
}

- (void) addObject:(id<NSCoding>)objectToCache 
       forCacheKey:(NSString*)cacheKey
       cahceLength:(NSTimeInterval)timeToLive 
{
    NSString* path = [self pathForCahceKey:cacheKey];
    NSDate* expireTime = [NSDate dateWithTimeIntervalSinceNow:timeToLive];
    
    NSData* objectData = [NSKeyedArchiver archivedDataWithRootObject:objectToCache];
    
    NSDictionary* objectAndExpireTime = [NSDictionary dictionaryWithObjectsAndKeys:
                                         expireTime, @"expireTime",
                                         objectData, @"object" , nil];
    
    [objectAndExpireTime writeToFile:path atomically:YES];
}

- (id) objectForCacheKey:(NSString*)cacheKey
{
    NSString* path = [self pathForCahceKey:cacheKey];
    
    NSDictionary* objectAndExpireTime = [NSDictionary dictionaryWithContentsOfFile:path];
    
    if (objectAndExpireTime) 
    {
        NSDate* expireTime = [objectAndExpireTime objectForKey:@"expireTime"];
        
        // Ensure the cache has not expired
        if ([expireTime laterDate:[NSDate date]] == expireTime) 
        {
            NSData* objectData = [objectAndExpireTime objectForKey:@"object"];
            id objectRecoveredFromCache = [NSKeyedUnarchiver unarchiveObjectWithData:objectData];
            
            return objectRecoveredFromCache;
        } 
    }
    
    return nil;
}



@end