Animation

What happens when we put animations, such as AnimatedBuilder or CircularProgressIndicator, into the PreemptBuilder's builder argument (let's name it "smooth region")? This section will tell you. No worries - it will be quite simple :)

Background

The animations are indeed driven by Tickers under the hood. An animation may look to be driven by a AnimationController, but under the hood that controller utilizes ticker. Luckily, Ticker has a minimalist API: It triggers onTick callback in each and every frame (this is the story without flutter_smooth).

If we only implement the infra layer using ideas before this section, animations inside smooth region will not animate at all. This is because, even though the smooth region has build/layout/paint/etc called per 16.67ms via preempt, the AnimationController does not see any onTick and thus never change the animating value, so the output UI is unchanged, thus user faces jank even though it is literally 60FPS.

Solution

Just make extra calls to onTick. More specifically, one extra onTick per extra render.

Notice that we need to provide the correct time stamp to onTick's argument. For example, if the first preempt render provides 1000ms, the second preempt should provide 1016.67ms, etc. By doing so, AnimationController will change its animation value according to the time stamp, and we see animations on screen at full smooth.