iPhone: Reading many images quickly

Question

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: https://i.sstatic.net/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!

Answer 1

I had a similar issue with a PDF viewer, The recommended way to do this is to have as low a res image as you can get away with and if you are allowing the user to blow the image up/zoom, then have two versions or three versions of that image increasing the res as you go.

Put as much code as you can get away with in the didDecelerate method (like loading in higher res images like vodkhang talks about), rather than processing loads in didScroll. Recycle Views out of scope as you have said. and beware of autoreleased Context based Image Creation functions.

Load images in on background threads intelligently (based on the scrollView Offset position and zoom level), and think about using CALayer/Tiled Layer drawing for larger images.

Three20 (an open source iOs lib) has a great Photo Viewer that can be subclassed, it has thumbnail navigation, large image paging, caching and gestures right out of the box.

Answer 2

Slow scrolling performance might mean that you're blocking the main thread while loading images. In that case, the scrolling animation won't continue until the images are loaded, which would indeed cause pretty choppy scrolling performance.

It would be better to lazily load requested images in the background, while the main thread continues to handle the scrolling animation. A library that provides this functionality (among other things) is the 'three20' library. See the Tidbits document, and scroll down to the bottom where the 'TTImageView' class is described.

Answer 3

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.