Add smooth/interpolated scrolling to the scroll container by dweymouth · Pull Request #5971 · fyne-io/fyne

dweymouth · 2025-10-08T21:57:28Z

Description:

Use an animation to smooth out the scroll, similar to what is done in web browsers and some other UI frameworks

Checklist:

Tests included. // <- kind of hard to unit test an animation
Lint and formatter run with no errors.
Tests all pass.

Jacalz · 2025-10-09T16:33:02Z

+
+	smoothScrollingFactor = 0.5
+
+	disableSmoothScrollingEnvKey = "FYNE_DISABLE_SMOOTH_SCROLLING"


Would this perhaps be better off in fyne_settings instead?

Maybe? On Slack yesterday Andy didn't even think it was needed at all but I ended up adding it as a relatively easy way to turn it off in unit test code. We do have a precedent of environment variables being used to control things that aren't exposed in fyne_settings, like FYNE_CACHE and FYNE_DISABLE_DPI_DETECTION.

It may be an unpopular opinion but I'm just kind of against environment variables like this and many of the ones we already have. It feels to me like they never are documented anywhere and just kind of rot away. As such, I get a feeling that very few actually use them so the code becomes more complicated for little gain.

I could just remove it and test if the fyne.CurrentApp() is the test app?

That sounds like a good plan

Jacalz · 2025-10-09T20:12:52Z

+
+	checkSmoothScrollOnce          = sync.Once{}
+	checkEnvDisableSmoothScrolling = func() {
+		checkSmoothScrollOnce.Do(func() {


FYI: We actually can get away with just an if statement and a Boolean now that the code is single threaded.

Don't we still need to worry about people who haven't completed the migration?

not really, race condition prevention on the old model is on a "reasonable effort" level now rather than an absolute promise

This is also a place where a race condition would be of little consequence if it happens. Worst case, it is loaded twice

andydotxyz

I'm a little unsure about the algorithm here.

It looks like it will move a half pixel after each frame following a scroll event - is this assuming a 2x scale factor?
What about differences in how fast the user was scrolling? How does it relate to the inertial scroll on mobile?

andydotxyz · 2025-10-11T09:44:23Z


+func (s *Scroll) animateScroll(_ float32) {
+	diff := s.Offset.Subtract(s.targetOffset)
+	if math.Abs(float64(diff.X)) < 0.5 && math.Abs(float64(diff.Y)) < 0.5 {


This 0.5 check seems like it should be comparing to the smooth scroll factor instead?

They're different values which just both happen to be 0.5. This is just an absolute threshold in dp where a scroll will just snap to the requested position instead of animating (or continuing to animate). The smoothScrollingFactor is an exponential decay factor.

OK thanks - then in this case I have a different question - why is 0.5 the right value and should it too be a named constant?

andydotxyz · 2025-10-11T09:45:21Z

 	// what fraction of the page to scroll when tapping on the scroll bar area
 	pageScrollFraction = float32(0.95)
+
+	smoothScrollingFactor = 0.5


Why 0.5? does this assume HiDPI? What about UHDPI like smart phones?

It's somewhat arbitrary but it's the exponential decay factor of the animation curve. It's pixel-independent. Since we're holding this off until 2.7.1 I may try to implement a more sophisticated ease-in/ease-out curve instead of just this decaying exponential.

I see - it seems I misread the "*" as "+", apologies.

The curves are already coded into the animation code so maybe using the value parameter in the callback could use that instead of coding it in?

Yeah I was thinking about that, the issue with that for now is that there's no way to extend a running animation properly (ie when the user continues to scroll so the endpoint target for the animation needs to be adjusted on-the-fly) which is what led to me opening #5970. It doesn't look too hard to fix though so I can look into it for 2.7.1

dweymouth added 2 commits October 8, 2025 14:04

smooth / interpolate scrolling in scroll container

9a811cf

disable smooth scrolling for unit tests

32b4c87

dweymouth marked this pull request as ready for review October 9, 2025 15:37

use sync.Once instead of sync.OnceFunc (introduced in Go 1.21)

618b63a

dweymouth mentioned this pull request Oct 9, 2025

[Feature request] Smooth scrolling in lists dweymouth/supersonic#576

Open

Jacalz reviewed Oct 9, 2025

View reviewed changes

dweymouth added 2 commits October 9, 2025 13:30

switch to bool flag from sync.Once

0950f17

fix missed syntax error

7d034ed

andydotxyz reviewed Oct 11, 2025

View reviewed changes


		smoothScrollingFactor = 0.5

		disableSmoothScrollingEnvKey = "FYNE_DISABLE_SMOOTH_SCROLLING"

Uh oh!

Uh oh!

Conversation

dweymouth commented Oct 8, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Description:

Checklist:

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

andydotxyz left a comment

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

dweymouth commented Oct 8, 2025 •

edited

Loading