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

By | December 16, 2017
Questions:

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 150×150 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

Answers:

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.

  • 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.

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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.

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I would do something called Lazy Loading, which doesn’t load the images until they are actually seen.

Here’s a great example on how to do so: http://www.cocoacontrols.com/platforms/ios/controls/mhlazytableimages

Good Luck!

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What I’ve done is to use NSCache. I’ve created a small class with properties that conforms to the NSCache data protocol (its really easy to do). So what I do is create a relationship between each cell in the main table and various things worth caching: NSAttributed strings, images etc – really anything that takes work to create. I don’t preload it but you could.

When you are asked to provide a cell by the tableview, look in your cache for your primary object. If there, pull all all the objects you need. If the cache does not have the object, then get the data the old fashion way, but before you finish, save it in the cache too.

This really helped me reduce “stutter” when scrolling the cell. Also, do NOT animate anything in the cell – that kills performance. Everything should be fully rendered.

Another thing to remember – make sure ever view which can be set to opaque has its property set to YES. That for sure helps the system render the cell (including the backgound view if you use one.)

EDIT:

So you provided information that included UITableViews may the root problem. So two suggestions:

1) Can you step back and figure out how to make the scrollView a single UITableView? With table headers and footers, and section headers and footers, and even the ability to essentially make a cell a floating view, can’t you figure out how to rearchitect what you have?

2) So you decide no to suggestion 1. Then, do this. Think of the space used by the tableview as being a container view. Whenever the tableview is edited, take an image snapshot of it and keep this image around. As soon as the user starts to scroll, swap the tableViews out for the images. When the scrollView stops swap the UITableView back in. This of course will take some fine tuning. In fact, you could probably overlay an opaque image snapshot over the table (which will hide it and prevent it from being asked to draw itself) during scrolling.

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There are a few things you can do in general to get better table view performance:

1) Switch to Loren Brichter’s method of drawing UITableViewCell’s (for lack of a better link: http://www.therefinedgeek.com.au/index.php/2010/12/21/fast-scrolling-uitableview-updates-for-ios-4-2/)

Basically, all his code does is render all your cell content as one opaque UIView, which UITableView (and CoreGraphics) can very quickly blast onto a UITableViewCell

If you don’t want to do all your cell design in drawRect:, you can still use nibs, but:

  • Make sure every subview is marked opaque
  • Don’t have any transparent/semi-transparent subviews
  • Don’t have any images with an alpha channel != 1.0f.

2) Don’t let UIImageView do any scaling to display your image, give it a correctly-sized UIImage

3) If you’re using iOS 5 and above, you can register a nib for a particular cell identifier. That way, when you call [tableView dequeueReusableCellWithIdentifier:], you are guaranteed to get a cell. Cell allocation is faster (according to Apple), and you get to write less code:

- (void)viewDidLoad {
    UINib *nib = [UINib nibWithNibName:@"MyCell" bundle:nil];
    [self.tableView registerNib:nib forCellReuseIdentifier:@"MyCellIdentifier"];
}

// ...

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    static NSString *cellIdentifier = @"MyCellIdentifier";
    MyCell *cell = (MyCell *)[tableView dequeueReusableCellWithIdentifier:cellIdentifier];

    // Commented out code is no longer needed
    //if (cell == nil) {
    //    cell = [[MyCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
    //}

    // setup cell

    return cell;
}

4) Displaying images downloaded from the web

  • Display a default image (something to look at while the real image is downloading)
  • Start the download on a separate thread (hint: use GCD’s dispatch_async())
  • When the image comes in, cache it (hint: NSCache), and display it on the cell
    • Do all image downloading/caching off the main thread; the only thing you should be doing on the main thread is setting the image (remember UI code HAS to be on the main thread!)

You’ll probably want to write an async-capable UIImageView (or use an existing library).

Stay away from EGOImageView, even though it has async downloading, it does cache lookup (which happens to be on disk, so that means costly disk IO) on the main thread before dispatching to a background thread for the download. I used to use it, but I ended up writing my own set of classes to handle this, and it’s significantly faster.

Just follow these, and stuff other ppl have written here, and you’ll have table views that scroll like glass in no time 🙂

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the human eye sees at about 60 FPS, so that’s why it’s recommended, but 30 FPS will also appear very smooth, especially when a regular user is viewing it, as opposed to you trying to find as much to fix as possible. This is obviously dependent on how fast the scrolling goes, if the difference from frame to frame is a movement of a few pixels, 30 FPS will do just fine, but faster movement will require a higher FPS to appear smooth

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You want 60fps, but 30 fps doesn’t look too terrible in actuality. But I would try to achieve 60fps for a smoother look while scrolling.

There are many Performance Improvement possibilities, that are also shown by various tutorials

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While 60 FPS is the ideal, games like Halo run very prettily in 30 FPS. The battlefield chaos in Halo probably involve more surprising, rapid motion than most lists, even complex ones like yours!

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