Question

TL:DR

What technique does Apple use to make Photo.app so fast, even with large images?

Long Version

I watched Apple’s WWDC 2010 video about scroll views to learn how to replicate Photo.app pagination behavior and low memory utilization (PhotoScroller Demo). It works well, but since images are loaded only when they are needed, when I try to paginate to another image, the app locks while the JPEG is being decompressed.

The same video shows a tiling technique to get better performance, but since I’m using photos taken from the camera and stored in the app, that doesn’t seem feasible (having multiple copies of each photo, in different resolutions, would consume too much space – 4MB vs 27MB). Also, using iExplorer I noticed Photo.apps has only a copy of each photo (it doesn’t even have a small thumbnail copy for the gallery).

What technique did Apple use to make Photos.app so fast? How can I get that same performance in my app?

I’m a bit confused if this should be here or on Programmers,
since there’s no code in the question, but F.A.Q. says that algorithm
questions are part of Stackoverflow, and the tags here match it
better.

Answer 1

So if you just show one image fullscreen you can do this:

In the WWDC11 Session 104 – Advanced Scroll View Techniques they talk about infinite scrolling and how to do it. The basic idea is to scroll the view and after scrolling reposition the (UIImage)view inside the scroll view so it appears centered or whatever you layout constraints are.You could then load the new UIImage into the UIImageView. Since you only have one UIImageView it should be pretty low memory consuming. I am not sure about how the loading times of the images will behave though.

Maybe preload the next UIImage to the left and right to the current image and then load it into the UIImageView after reposition the scrollView can help here.