Summary:
Builds off of cae7179c94
- Make the prop a dictionary for more configuration options
- Rename `maintainPositionAtOrBeyondIndex` -> `maintainVisibleContentPosition` + `minIndexForVisible`
- Add autoscroll threshold feature
Given the async native of RN JS and background layout, there is no way to trigger the scrollTo from JS without risking a delay, so we add the feature in native code.
== Test Plan ==
ScrollViewExample:
https://youtu.be/pmY8pxC9PRs
Reviewed By: shergin
Differential Revision: D6729160
fbshipit-source-id: 70f9bae460ce84567857a4f696da78ce9b3b834c
Summary:
== Problem / Background ==
Most lists paginate in a single direction (standard infinite list), but some paginate in both directions. Most common example is a chat thread where new messages show up on the bottom, and old content can be loaded by scrolling up. Comment threads are another example.
Right now, adding content to the bottom of a scroll view is smooth - the content doesn't jump. But when adding to the top of the scrollview, the content gets pushed down, which is jarring (note this may appear reversed because of inverting the list which is common for chat applications).
== Approach ==
The basic idea is simple - we set a flag in JS, then for every uimanager transaction, we record which is the first eligible and visible view in the ScrollView, and compare it's new origin to the old one. If it has changed, we update the contentOffset of the ScrollView to compensate.
This is done by observing `willPerformMounting` directly (only from scrollviews that have this new property set), and then observing the prev state with prependUIBlock and making the update synchronously in addUIBlock to avoid any flicker.
There is also a way to skip views that we don't care about, like a spinner at the top of the view that we don't want to stay in place - we actually want it to get pushed up by the new content, replaced visually in the viewport.
== Notes ==
Most chat applications will probably want to do a scrollToTop when new content comes in and the user is already scrolled at or near the bottom.
This is glitchy if visible children are re-ordered, which could be fixed with additional logic, but it doesn't come up in the type of applications we're targetting here so punting on that.
== Test Plan ==
https://youtu.be/4GcqDGz9eOE
Reviewed By: shergin
Differential Revision: D6696921
fbshipit-source-id: 822e7dfcb207006cd1ba098356324ea81f619428
Semen Zhydenko
Spencer Ahrens
Stepan Hruda
Valentin Shergin