Eric,
I think one of the more effective ways to cache images is to store
your image to the file system, and then store meta-data for that image
in a local SQL database. That meta-data should include a "last
updated" date for the image. Then, instead of requesting the image
directly from the server, first request its meta-data from the server,
which will return meta-data indicating if the image has been updated
on the server or not. If it has, download the new image, write it to
your local file system, and update the meta-data in your local SQL
database. If not, just continue to use the existing local image.
I don't know what sizes your images are, or how many you are
processing at once, but if you are going to try to construct an in-
memory cache for the images mentioned, make SURE you pay close
attention to the
- (void)didReceiveMemoryWarning
method and manage your cache accordingly should that method get
called. Otherwise, what might appear as a wonderful optimization may
end up being an effective means to crash your app.
I hope that helps.
Brad
On May 12, 2009, at 2:22 PM, Luke the Hiesterman wrote:
On May 12, 2009, at 2:13 PM, Eric E. Dolecki wrote:
Okay, so I know I can somehow cache them... but I have some noob
questions.
- how do I cache the images themselves? Some binary data object
written to
the disk? How?
If you're caching to memory, all you really need to do is retain the
UIImage and keep a reference around so you can get to it later. To
cache to the disk, you could encode your UIImage to NSData using
NSKeyedArchiver, and then write to a file.
- how do I check if the image has already been loaded (check an
NSMutableArray populated with dictionaries with a url string and
url key?)
I would have the following keys in my dictionary:
1. URLKey
2. MemoryCacheReferenceKey - this contains a reference to a UIImage
in memory
3. FileReferenceKey - this contains a file path where the image can
be cached on disk.
- if I find that the image has been cached (somehow), how do I use
that
image instead?
Using the above dictionary scheme, I would first check if my
MemoryCacheReferenceKey is valid. If so, I use that UIImage and be
done. Otherwise, I would check for a file at the location in
FileReferenceKey and use that if it exists. If I don't find anything
in either of those caches, I would download the image using the
URLKey.
Hope that helps.
Luke
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