Does NSCache persist across ViewControllers? - ios

Question

Its my first time using NSCache and I know that it doesn't persist on app lunch.
But does it persist across ViewControllers?... Meaning... If I set the cache object on ViewController A and then I move to ViewController B can I still access it?
My question is related to an issue that I am having in my code. I am on ViewController B and I set the cache object. Then move to ViewController B and try to retrieve that object but is never found.
Is that normal or there is a problem in my code??? My Views are quite inexpensive so I see no reason why it would be dropping the cache object
ViewController A (Using Cache)
- (void) searchDone:(NSDictionary*)response {
NSString * str = input.text;
str = [str stringByReplacingOccurrencesOfString:#" " withString:#""];
NSString* cachedKey = [str stringByReplacingOccurrencesOfString:#"#" withString:#""];
cachedKey = [cachedKey lowercaseString];
//
// Cache
//
NSCache * cache = [[NSCache alloc]init];
NSDictionary* chachedData = [cache objectForKey:cachedKey];
// Check for a cached version of this
if ( chachedData ) {
NSLog(#"There is a cache");
NSTimeInterval timeDifferenceBetweenDates = [chachedData[#"time"] timeIntervalSinceNow];
CGFloat time = fabsf(timeDifferenceBetweenDates);
if ( time < sysTraditionalSearchCacheTime ) {
NSLog(#"using cache");
NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:input.text,#"input",chachedData,#"response",nil];
[[NSNotificationCenter defaultCenter]
postNotificationName:#"closeSearch"
object:nil
userInfo:dictionary];
return;
}
[cache removeObjectForKey:cachedKey];
}
ViewController B (Cache Setter)
- (void) notificationCloseSearch:(NSNotification*) notification {
input.text = [[notification userInfo] valueForKey:#"input"];
NSDictionary* response = [[notification userInfo] valueForKey:#"response"];
NSString * str = input.text;
str = [str stringByReplacingOccurrencesOfString:#" " withString:#""];
NSString* cachedKey = [str stringByReplacingOccurrencesOfString:#"#" withString:#""];
cachedKey = [cachedKey lowercaseString];
//
// Cache
//
NSCache * cache = [[NSCache alloc]init];
NSDictionary* chachedData = [cache objectForKey:cachedKey];
// Check for a cached version of this
if ( chachedData ) {
NSTimeInterval timeDifferenceBetweenDates = [chachedData[#"time"] timeIntervalSinceNow];
CGFloat time = fabsf(timeDifferenceBetweenDates);
if ( time >= sysTraditionalSearchCacheTime ) {
[cache removeObjectForKey:cachedKey];
}
} else { // if there is no cache then set one
NSLog(#"setting cache key %#",cachedKey);
NSDate* now = [NSDate date];
NSMutableDictionary* newResopnse = [[NSMutableDictionary alloc]initWithDictionary:response];
[newResopnse setObject:now forKey:#"time"];
response = [[NSDictionary alloc] initWithDictionary:newResopnse];
[cache setObject:response forKey:cachedKey];
NSLog(#"cached %#",[cache objectForKey:cachedKey]);
}
}

Answer 1

NSCache is a some kind of NSMutableDictionary.
The difference is that when NSCache detects excessive memory pressure it will release some of it key-value pairs.
In ViewControllerA you create NSCache object
NSCache * cache = [[NSCache alloc]init];
And in ViewControllerB you create one more NSCache object again
NSCache * cache = [[NSCache alloc]init];
So it will be two different objects with two different set of key-value pairs.
If you need some kind of storage you can write singleton class which will contain one NSCache object.