Predict the output
On a 120 Hz ProMotion display, how much time is there per frame?
// 1 second / 120 frames = ?Divide one second by the refresh rate.
Choose an answer, then open Show answer below to check it.
Answer: B About 8.3 ms
Correct. 1000 ms ÷ 120 ≈ 8.3 ms, and the app gets only part of that.
A frame is dropped (a hitch) when the app or the render server misses the display's deadline: about 16.7 ms at 60 Hz and 8.3 ms at 120 Hz on ProMotion devices. Causes are main-thread work during scrolling or animation (decoding, layout, synchronous I/O) and expensive rendering such as offscreen passes. Move work off the main thread, cache layout, and simplify layers.
Explain hitches and how you fix them.
Say your answer out loud, then check which of these points you covered:
Commit hitches come from your app's work; render hitches from the render server.
The Animation Hitches instrument separates the two.
CADisplayLink.preferredFrameRateRange sets the rate for custom animations.
Decode images with preparingForDisplay() off the main thread.
Seniors name both hitch types and the 120 Hz budget.
Think of your answer first, then open the hint.
What is a commit hitch?
The app took too long preparing a frame's layer changes.
How do you find hitches in the field?
MetricKit hitch rate and Organizer.
Suggest a fix if something here is wrong or out of date.