I've got an app I'm working on where we handle a LOT of images at once in a scrollview. (Here's how it looks, each blue block being in image on a scrollview expanding to the right: http://i.imgur.com/o7lFx.png) So to be able to handle the large strain doing this puts on memory. So I've implemented a bunch of techniques such as reusing imageviews etc which have all worked quite successfully in keeping my memory usage down. Another thing I do is instead of keeping the actual image in memory (which I of course couldn't do for all of them because that would run out of memory very quickly) I only keep the image's filepath in memory and then read the image when the user scrolls to an area of the scroll view near that image. However, although this is memory efficient, it's causing a LOT of lag in the scrollview because of the fact that it has to constantly read images from the disk. I can't think of a good solution on how to fix this. Basically right now the app draws to the screen only the visible uiimageviews and while the user scrolls the app will look to see if it can dequeue another imageview so it doesn't have to allocate another one and at that point it reads the image into memory, but as I said it's causing the scrolling action to be very slow. Any ideas on a strategy to use to fix this? Does anyone know what the native photos app does to handle this kind of thing? Thanks so much!
I can suggest you a simple solution to balance both the memory and the computer processing. You only keep small images like thumbnails in memory and only keep about 20 of them. One project that I am doing, I keep 20 thumbnail images (100 x 100) recently accessed, which doesn't cost a lot of memory. I believe that it costs about 200 kb all the time but comparing to a general available memory. I think it is good enough.
It also depends on your use case : if user scroll really fast and you don't know when will they go. You can have even smaller images than the thumnail and when you show it on the UIImageView, you resize it to fit. When user stops scrolling for a while. You can start loading bigger images and then you have a nicer images. User may not even notice about the process
I don't think there is a solution that can be fast and using as less memory as possible. Because we have memory, maybe not big but have enough if we use it smartly.