How to cache file downloads with AFNetworking? - ios

Question

I am trying to download some files.
For each cell, I check if the file exists and is already stored in a directory, if not I download it.
But I also download files for the next couple of cells.
What happens, if I am trying to download a file for an invisible cell ahead of time, but then the user scrolls to the cell and the file is not fully downloaded yet.
Is there anyway to prevent it from downloading twice?
Is there anyway to know that the file is already getting downloaded?
I am using AFNetworking.

Answer 1

What you're trying to do has a name and it's caching.
AFNetworking already implements it as per the official FAQs
Does AFNetworking have any caching mechanisms built-in?
AFNetworking takes advantage of the caching functionality already provided by NSURLCache and any of its subclasses. So long as your NSURLRequest objects have the correct cache policy, and your server response contains a valid Cache-Control header, responses will be automatically cached for subsequent requests.
Set up the cache as follows and you'll be ok
- (void)setupCache {
NSURLCache *urlCache = [[NSURLCache alloc] initWithMemoryCapacity:1024*1024*4 // 1MB mem cache
diskCapacity:1024*1024*5 // 5MB disk cache
diskPath:nil];
[NSURLCache setSharedURLCache:urlCache];
}
Then just perform requests normally and if the resource is in cache you'll get a cache hit and you won't be downloading it twice.
Note that if you have to support anything below iOS5, you have to use an alternative URL cache like SDURLCache and set it up like follows
- (void)setupCache {
SDURLCache *urlCache = [[SDURLCache alloc] initWithMemoryCapacity:1024*1024 // 1MB mem cache
diskCapacity:1024*1024*5 // 5MB disk cache
diskPath:[SDURLCache defaultCachePath]];
[NSURLCache setSharedURLCache:urlCache];
}
The reason is well explained in this article, but it can be summarized by the following quote:
Before iOS5, NSURLCache just saved requests to memory, even if the documentation said otherwise - the diskCapacity property was silently ignored