Cache Expiration Implementation using NSCache

caching, cocoa-touch

Solution

Should the cache automatically invalidate itself by using a fixed interval timer?

This would be a bad solution, because you might add something seconds before the timer fires. The expiry should be based on the specific item's age. (It would, of course, be possible to conditionally invalidate items using a timer; see the comments on this answer.)

Here's an example. I thought about subclassing `NSCache`, but decided it was simpler to use composition.

Interface

//
//  ExpiringCache.h
//
//  Created by Aaron Brager on 10/23/13.

#import <Foundation/Foundation.h>

@protocol ExpiringCacheItem <NSObject>

@property (nonatomic, strong) NSDate *expiringCacheItemDate;

@end

@interface ExpiringCache : NSObject

@property (nonatomic, strong) NSCache *cache;
@property (nonatomic, assign) NSTimeInterval expiryTimeInterval;

- (id)objectForKey:(id)key;
- (void)setObject:(NSObject <ExpiringCacheItem> *)obj forKey:(id)key;

@end

Implementation

//
//  ExpiringCache.m
//
//  Created by Aaron Brager on 10/23/13.

#import "ExpiringCache.h"

@implementation ExpiringCache

- (instancetype) init {
    self = [super init];

    if (self) {
        self.cache = [[NSCache alloc] init];
        self.expiryTimeInterval = 3600;  // default 1 hour
    }

    return self;
}

- (id)objectForKey:(id)key {
    @try {
        NSObject <ExpiringCacheItem> *object = [self.cache objectForKey:key];

        if (object) {
            NSTimeInterval timeSinceCache = fabs([object.expiringCacheItemDate timeIntervalSinceNow]);
            if (timeSinceCache > self.expiryTimeInterval) {
                [self.cache removeObjectForKey:key];
                return nil;
            }
        }

        return object;
    }

    @catch (NSException *exception) {
        return nil;
    }
}

- (void)setObject:(NSObject <ExpiringCacheItem> *)obj forKey:(id)key {
    obj.expiringCacheItemDate = [NSDate date];
    [self.cache setObject:obj forKey:key];
}

@end

Notes

- Assumes you're using ARC.

- I didn't implement `setObject:forKey:cost:` since the NSCache documentation all but tells you not to use it.

- I use a @try/@catch block, since technically you could add an object to the cache that doesn't respond to `expiringCacheItemDate`. I thought about using `respondsToSelector:` for this, but you could add an object that doesn't respond to that too, since NSCache takes `id` and not `NSObject`.

Sample code

#import "ExpiringCache.h"

@property (nonatomic, strong) ExpiringCache *accountsCache;

- (void) doSomething {
    if (!self.accountsCache) {
        self.accountsCache = [[ExpiringCache alloc] init];
        self.accountsCache.expiryTimeInterval = 7200; // 2 hours
    }

    // add an object to the cache
    [self.accountsCache setObject:newObj forKey:@"some key"];

    // get an object
    NSObject *cachedObj = [self.accountsCache objectForKey:@"some key"];
    if (!cachedObj) {
        // create a new one, this one is expired or we've never gotten it
    }
}

Problem

Original source