TTURLCache to cache my image and data - iphone

Question

I used three20's photo gallery in my app and it worked really well and looked neat and pretty. And basically I don't need to worry about all the fetch-photos-from-the-server thing since three20 has fetched and managed and cached the data for me, all of which are done behind the scene.
That's when I started to notice this magic caching mechanism implemented in three20, called TTURLCache. In its header file I find the following interface methods:
/**
* Stores a data on disk.
*/
- (void)storeData:(NSData*)data forURL:(NSString*)URL; //method 1
- (void)storeData:(NSData*)data forKey:(NSString*)key;
/**
* Stores an image in the memory cache.
*/
- (void)storeImage:(UIImage*)image forURL:(NSString*)URL; //method 2
/**
* Stores an etag value in the etag cache.
*/
- (void)storeEtag:(NSString*)etag forKey:(NSString*)key; //method 3
My questions:
Method 1 and method 2 in the above code: are they only capable of storing data, or are they capable of both fetching and storing?
What is an etag in method 3?
Method 1 says it will store the data on disk, while method 2 says it will store the image in memory. If I use method 2 to store an image, will it get lost once I kill and restart my app?
Thanks a lot in advance.

Answer 1

Those are the storage methods. The retrieval methods are separate. (See below.)
Etags are a way to optimize network traffic. See Wikipedia for a good intro.
I think that Three20 both stores the image file on disk, and the decoded image itself in memory. Certainly, any in-memory caches would be cleared if the app re-started, but remember that under iOS 4, that probably happens less often that you think.
To get data out of the cache, try one of:
/**
* Gets the data for a URL from the cache if it exists and is newer than a minimum timestamp.
*
* #return nil if hthe URL is not cached or if the cache entry is older than the minimum.
*/
- (NSData*)dataForURL:(NSString*)URL expires:(NSTimeInterval)expirationAge
timestamp:(NSDate**)timestamp;
- (NSData*)dataForKey:(NSString*)key expires:(NSTimeInterval)expirationAge
timestamp:(NSDate**)timestamp;
/**
* Gets an image from the in-memory image cache.
*
* #return nil if the URL is not cached.
*/
- (id)imageForURL:(NSString*)URL;
- (id)imageForURL:(NSString*)URL fromDisk:(BOOL)fromDisk;
I would advise reading through the source, though. If memory serves, TTURLCache isn't all that big a class.