When does a view (or layer) require offscreen rendering?

Question

Hello
this weekend I started to watch the 2011 WWDC videos. I\'ve found really interesting topics about iOS. My favorites were about performance and graphics, but I\'ve

Answer 1

Offscreen-rendering is one of the worst defined topics in iOS rendering, today. When Apple's UIKit engineers refer to offscreen-rendering, it has a very specific meaning, and a ton of third-party iOS dev blogs are getting it wrong.

When you override "drawRect:", you're drawing via the CPU, and spitting out a bitmap. The bitmap is packaged up and sent to separate process that lives in iOS, the render server. Ideally, the render server just displays the data on screen.

If you fiddle with properties on CALayer, like turning on drop shadows, the GPU will perform additional drawing. This additional work is what UIKit engineers mean when they say "off-screen rendering." This is always performed with hardware.

The issue with off-screen drawing isn't necessarily the drawing. The off-screen pass requires a context switch, as the GPU switches its drawing destination. During this switch, the GPU is idle.

While I don't know a full list of properties that trigger an off-screen pass, you can diagnose this with the Core Animation Instrument's "Color Offscreen-rendered layer" toggle. I assume any property other than alpha is performed via an offscreen pass.

With early iOS hardware, it was reasonable to say "do everything in drawRect." Nowadays GPUs are better, and UIKit has features like shouldRasterize. Today, it's a balancing act between the time spent in drawRect, the number of off-screen passes, and the amount of blending. For the full details, watch the 2014 WWDC session 419, "Advanced Graphics and Animation for iOS Apps."

That all said, it's good to understand what's going on behind-the-scenes, and keep it in the back of your head so you don't do anything insane, but you should start from the simplest solution. Then test it on the slowest hardware you support. If you aren't hitting 60FPS, use Instruments to measure things and figure it out. There are a few possible bottlenecks, and if you aren't using data to diagnose things, you're just guessing.