Best image caching strategy in iOS

Question

In my app, I have a UITableView which displays fairly large images and loads a moderately designed Xib file to display it in. Each image is around 700KB to 1MB in size. The flow is virtually never ending, it loads more and more as we scroll down. So you can imagine that I am running into memory issues.

I have tried using SDImageCache and NSCache. The former used disk memory for caching images. In both cases, the caches somehow didn't clear images automatically. I had to manually clear them when I got a Received memory warning prompt. And each time I clear these caches, the memory freed seems to be lesser each subsequent time.

Now I confused as to which cache strategy I must use for such a long list of images. Might I be having some leaks somewhere? They certainly didn't show up when I profiled the app.

P.S.: I am loading the images from the web. Just to be clear.

Answer 1

From the docs:

UIImage 

+(UIImage *)imageNamed:(NSString *)name

Discussion This method looks in the system caches for an image object with the specified name and returns that object if it exists. If a matching image object is not already in the cache, this method loads the image data from the specified file, caches it, and then returns the resulting object.

So I guess leaving this to the UIImage class is a good approach.

Hope this helps!

Answer 2

As we implemented it in both Android and iOS: once you can show on the screen only 2-3 images. Load in memory 2 more for the downwards scroll and 2 more for the upwards one. So you have in memory 7 images. Display them. The other images must be stored in files (when you download them). If the user scrolls too fast, do not show all the sequence of the images, instead show some "loading" icons in place of the images. When the scrolling stops, show the appropriate image + the previous one + the next one + prepare 2 more (for upwards scroll) and 2 more for downwards scroll.