Using CGLayer to Cache Cells in a UITableView

Question

I am trying to improve the performance of scrolling in our app. I have followed all of the generally accepted advice (draw it yourself with CG, the cell is opaque, no subviews, etc.) but it still stutters sometimes when we have background CPU and network activity.

One solution stated here:

http://www.fieryrobot.com/blog/2008/10/08/more-glassy-scrolling-with-uitableview/

is to cache bitmap snapshots of the cells, which we tried. But the bitmaps were fuzzy and took up a ton of memory (~a few hundred kb each).

One suggestion in the comments of that link is to cache the cells using CGLayer or CALayer(?) because they go to the graphic card's memory. So a few questions,

1) Has any tried this? Sample code?

2) How much memory does the iphone/ipod touch graphics card have? Does this make any sense?

3) Any other suggestions for speeding things up?

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More information

I used the CPU sampler (on the phone) and systematically eliminated things from the cell to figure out the problem. A few things:

1) It isn't the setup of the cell. If I remove just the drawing calls (drawinrect etc), but leave the setup, it is glassy.

2) It isn't the drawing of the smallest image (25x25 png), if I put that in it is fine.

3) If I add the second or third image (the first is a big background 320x1004kB, the other is a button image 61x35 4kB) it stutters. I am grabbing both UIImages in a class method, so it is cached.

4) The text is also a problem. It looks like by drawRect spends 75% of its time in the three NSString drawInRect methods I am using. Like:

[mytext drawInRect:drawrect withFont:myFont lineBreakMode:UILineBreakModeTailTruncation];

Those calls seem to go through webcore, perhaps that causes some of the stutters? I need to be able to format two lines of text, and one small paragraph of text. I need the ability to truncate the text with ellipses. Can I perform these out of the critical path and cache them? May I can do that part with a layer?

Answer 1

Before you go too deep into the optimisation, are you using the cell reuse mechanism ([tableView dequeueReusableCellWithIdentifier:]) for creating UITableViewCells in your delegate's tableView:cellForRowAtIndexPath: ?

Also, have you run the code in the Simulator using Instruments' Activity Monitor? Set the interval to 1 ms (I found the default - 10 ms - too big for this) and see where most of your time is spent when doing the scrolling.

Answer 2

CGLayers are not cached on the GPU, they are used during the process of drawing Core Graphics elements to a context. CALayers do have their visual contents cached on the GPU. This can "hide" some of your memory usage, but you're still going to run into memory problems if you hold on to a lot of CALayers.

Honestly, if you've followed the table view best practices, as described by Loren Brichter and others, of drawing all of your content via Core Graphics in one layer, making your cell opaque, and obeying the cell reuse mechanism of the table view, there isn't much more you can do. Overloading the CPU on the iPhone will cause stuttering of your scrolling, no matter how optimized you can make it. The inertial scrolling animation does require some CPU power to run.

One last thing to make sure of is that the background CPU and network processes you refer to really are running on a background thread. Anything running in the main thread will cause your interaction methods to pause while that task is processing, potentially adding to the choppiness of your scrolling.