What is an acceptable FPS for scrolling, and what are tips for improving performance?

Question

I see in many WWDC video's that says you want to achieve 60.0 FPS as close as possible to get a better smooth scrolling experience. I have a UIScrolLView which loads up image and a couple of table view's at once. Currently I am getting 30 FPS. This is half of what the recommended FPS. Just wondering what FPS do you guys typically get for a table view/scroll view that loads up images and other heavy stuff/rendering stuff.

Any other tips for optiziming FPS? I've spend the past week till now firing up Instruments using the time profiler, allocations, and core animation tool to optimize as much as I can.

Just to clarify a bit on what I have. I have a masonry/waterfall/pinterest style layout on the iPad. So it's not just a regular UITableView. It's a UIScrollView that fills out the whole screen, and is filled with a couple of UIView's. Each of this view has a 150x150 UIImageView and a UITableView and also it has some attributed label, drawn using Core Text. So at one glance when you see the screen, you can see 5-8 table view at one shot, each cell again has a UIImageView and then each cell renders attributed label drawn using core text.

So you can just image how deep and complicated this is. This is not just a regular table view with a UIImageView. I know how to get 60 FPS with just one UITableView in an iPhone with a UIImage. The concept is to load images asynchrounously and not to block the main thread as much as possible.

EDIT:

It seems that the problem here is the UITableView that I have inside my view.. when I remove that from the UIView I get really smooth scrolling..

I uploaded a sample project which is a simpler version of what I have, but it clearly shows the problem. The link is here

Answer 1

Many things affect render performance, here are some items you can check:

• Profile - You said you already did this, so great job! Unfortunately profiling is often overlooked, even though it can reveal unexpected problems. In one app I was working on a calendar with different cells representing dates. At first scrolling between cells slow, which was unexpected. I thought maybe it was drawing a few cells too many. After profiling I found that [NSCalender currentCalender] was using 85% of my CPU time! After fixing that everything scrolled great!

• Images - Large images put a lot of load in CoreGraphics. Scrolling especially requires a lot of draw operations to move them around. One tip is to scale images on the device as little as you can, that makes CoreGraphics' job a lot easier. If an image is twice as large as the view displaying it, resize the UIImage before displaying it in the view. iOS devices handle PNGs best. They are compressed by a tool (pngcrush) at compile time and iOS has special hardware for rendering them.

Edit: JPGs are probably a better option for photos. iOS devices have dedicated JPG decoders as well.

• Custom Drawing - If possible, cutback on the amount of custom CGContext drawing you do. Lots of custom drawing has negative effects on animation speed. I would considering using an image over complex custom drawing, if possible.

• Cull - Only draw things you need to. UITableView automatically unloads and loads cells as they appear, so this is done for you, but any custom CGContext drawing should only be done when that part is visible. Also automatic view shadows can be very slow in my experience.

• Reuse - Use the reuse identifier on UITableView, this will allow UITableView to reuse cell objects rather than reallocating as it scrolls - look at the answer to this question. Also reuse UIImages rather than allocating multiple for the same file. imageNamed caches images automatically but imageFromContents of file does not.

• Create your own - You could create your own grid view class that culls it's subviews views hidden off screen, and scrolls with lazy content loading. By writing a custom solution you can fully control the process and create a design optimized for the usage context. For most use cases you will have a hard time building something better than the Apple standard, but I have seen it done in specific cases.

• Last resort - Reduce the size of the offending view (improves filtrate), break content into multiple pages, downsize images, cut out older devices that don't perform as well. I would settle for 30 FPS before sacrificing most of that stuff. Devices will continue to get faster, older devices will be eliminated, and your app will gradually get faster.

Answer 2

I get close to 60 fps with my UITableViewController where the table contains about 2000 cells and each cell pulls an image from the web. The trick is to lazy load the images as you need them. This sample code from Apple is pretty helpful.

The general idea is to keep the UI responsive by not blocking the main thread. Perform downloads and other time-consuming tasks on another thread.