Is it guaranteed that every frame takes 16 ms in Android?

# compose

[Mehdi Haghgoo]

Is it guaranteed that every frame takes 16 ms in Android?

[romainguy]

The refresh rate depends on the display. Newer displays have high refresh rates and even variable refresh rates.

[romainguy]

Of course it also depends on what you mean exactly, on a 60Hz display a new vsync fires every 16.6ms, but the all may take more time and miss a frame

[romainguy]

Also things are made a bit more complicated by the fact android gives apps a longer window by firing vsync ahead of time

[romainguy]

Tl;dr don’t assume 16ms and if you need to sync with the display, use the Choreographer API

[Mehdi Haghgoo]

There is test snippet on Android CS, Compose section:

// Forward the clock by 2 frames: 1 for toggle and then 1 for count
composeTestRule.mainClock.advanceTimeBy(32)

It apparently assumes each frame takes 16ms. So, I assume this test could fail on some devices.

Sample: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/[…]java/androidx/compose/ui/test/samples/MainTestClockSamples.kt

[romainguy]

No it's not an issue in this case

[romainguy]

Tests are setup with a special monotic test clock that guarantees a frame delay of 16ms