How to check NSCache instance value set or not - ios

Question

I have an instance of NSCache, like: NSCache *imageCache; It is basically used for holding some remote "image" values with different "#keys". I alloc & init NSCache in a Global class at the beginning and set a if else condition, like this:
if (self.imageCache == nil)
{
self.imageCache = [[NSCache alloc] init];
}
else
{
NSLog(#"cache set");
}
I #import that "Global Class" in all of ViewControllers, so that I don't have to parse images every time. But the problem is when I go to other ViewControllers It seems like, NSCache alloc & init every time. Because It takes same time to load the images as 1stVC. I think the if else condition didn't working perfectly or it's not the appropriate way to check either NSCache set or not.
Can anyone tell me whats wrong with it? One thing more, The imageCache is used from a global variable.
Thanks in advance.
Have a good day.
ADDITION:
This is the method where I load the UIButtons in UIScrollView as subView. This is a UIViewClass which I add in my "EveryViewController" as a subView Just take have a look on the if (cachedImage) line. It works fine. But when I want to check either the NSCache (iADImageCache) set or not, it shows me it's not set. But which should be set. In this situation how can I check all several iADImageCache with their different #"Key" name?
Thanks again.
-(void) loadUIButton
{
[self loadScrollView];
for (int i = 0; i < [iADDisplayArray count]; i++)
{
adButtonOutLet = [[UIButton alloc] initWithFrame:CGRectMake(i*320, 0, ButtonWidth, ButtonHight)];
currentAd = [iADDisplayArray objectAtIndex:i];
NSString *path = currentAd.bannerIconURL;
NSURL *url = [NSURL URLWithString:path];
NSMutableURLRequest *requestWithBodyParams = [NSMutableURLRequest requestWithURL:url];
NSData *imageData = [NSURLConnection sendSynchronousRequest:requestWithBodyParams returningResponse:nil error:nil];
UIImage *originalImage = [UIImage imageWithData:imageData];
UIImage *cachedImage = [self.iADImageCache objectForKey:currentAd.bannerIconURL];
if (cachedImage)
{
[adButtonOutLet setImage:cachedImage forState:UIControlStateNormal];
//NSLog(#"OnecachedImage %#", cachedImage);
}
else
{
[self.iADImageCache setObject:originalImage forKey:currentAd.bannerIconURL];
[adButtonOutLet setImage:originalImage forState:UIControlStateNormal];
NSLog(#"OneimageCache %#", self.iADImageCache);
}
adButtonOutLet.userInteractionEnabled= YES;
[adButtonOutLet setTag:i];
[adButtonOutLet addTarget:self action:#selector(goToURL:) forControlEvents:UIControlEventTouchUpInside];
[self.iADScrollView addSubview:adButtonOutLet];
}
}

Answer 1

You could make a singleton class, instance at application did finish launching and use it wherever you are, through all the view controllers. In the singleton class put a property
#property(nonatomic,strong) NSCache* imageCache;
then instance it just once in the singleton class init method. In this way you don't have to care about it, and you can just add images to that cache. Of course you have to check if the image is cached or not based on the existance of a key inside that cache.
NSCache* globalCache = [SingletonClass sharedInstanceMethod].imageCache;
UIImage *imageX = [globalCache objectForKey: #"keyX"];
if (!imageX) {
// download your image
imageX = <Download method>;
[globalCache setObject: imageX forKey: #"keyX"];
}
// Do your stuff (like showing the image)
....
...
..
.
Hope it helps

Answer 2

Unless actually needed across the whole program, I would suggest another approach that would restrict the caching to the class that actually needs to use it by adding the class method:
+ (NSCache *)staticCacheForClass
{
static NSCache *staticCache;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^
staticCache = [NSCache new];
staticCache.name = #"myCache";
});
return staticCache;
}
Using this - you restrict the changes to the cache to be internal to the class that uses it and avoid creating another singleton. The cache is also preserved across creating and destroying instances of this class.
That said, a singleton could be a viable solution if you need to access the cache from different classes, since a cache singleton is not really a global state.
Cheers.