Scroll optimize

[singularity-s0]

Still need to:

• fix scroll controller

[w568w]

Until we figure out a fix to all broken features (i.e. ScrollBar cannot scroll, double tap on title cannot jump to the top), this PR should not be merged.

[w568w]

Also PTAL at issue #95. If we use a paging method like TieBa, there should be pretty less problems and bugs.

(In fact, this PR works at cross purposes against that.)

[w568w]

There are some problems preventing this merge:

• On iOS, after using scroll-to-bottom in hole_detail, double tapping on the notification bar (which triggers a scroll in PrimaryScrollController) weirdly makes the list scroll to the bottom again, rather than to the top.
• If entering hole_detail with a floor located (e.g. from search results or other's references), the ScrollBar on the page looks good but cannot be dragged, throwing an exception indicating that there is no ScrollPosition attached to the PrimaryScrollController controlled by the scroll bar.
• scrollable_positioned_list's bug: if item's size changes during scrolling by index, list will scroll to an incorrect position. See [scrollable_positioned_list] If an image loads during scroll, list scrolls to incorrect position google/flutter.widgets#330. It seems that the repository is out of maintenance, so we won't expect an official fix for the issue in the short term. (It is NOT a big problem if we drop usage of keframe, because our item widgets' height will not change without it.)

However, removing keframe will bring pros and cons. It does have some issues, like slowing down the normal loading speed even under low render pressure, i.e. user is scrolling the list slowly, but still seeing the following items' transformation from blank to a normal widget. But it really optimizes the frame lag caused by rapid scrolling, giving users a better experience when scrolling.

What do you think towards these problems? @singularity-s0

p.s. I think that Flutter is really doing a bad job with scrolling things. We have struggled with ListView, ScrollBar, PrimaryScrollController for many many times.

[singularity-s0]

Is it possible to use Scrollable.ensureVisible() for locating items instead?

[w568w]

Is it possible to use Scrollable.ensureVisible() for locating items instead?

Looks hopeful. You can start a new branch and do some tests on it.

[singularity-s0]

Is it possible to use Scrollable.ensureVisible() for locating items instead?

Looks hopeful. You can start a new branch and do some tests on it.

Tests complete. You may revert to the previous implementation and merge these branches.

[w568w]

The PR and the branch is verified as unworkable. Close it.

[w568w]

Shall we have a try on https://github.com/superlistapp/super_sliver_list? It looks promising.

Edit: also see https://github.com/fzyzcjy/flutter_smooth.