Support scrollable UI nodes (Overflow::Scroll?)
#8074
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nicoburns
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nicoburns
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Overflow::ScrollOverflow::Scroll)
nicoburns
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Overflow::Scroll)Overflow::Scroll?)
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Sep 23, 2024
# Objective - Fixes #8074 - Adopts / Supersedes #8104 ## Solution Adapted from #8104 and affords the same benefits. **Additions** - [x] Update scrolling on relayout (height of node or contents may have changed) - [x] Make ScrollPosition component optional for ui nodes to avoid checking every node on scroll - [x] Nested scrollviews **Omissions** - Removed input handling for scrolling from `bevy_ui`. Users should update `ScrollPosition` directly. ### Implementation Adds a new `ScrollPosition` component. Updating this component on a `Node` with an overflow axis set to `OverflowAxis::Scroll` will reposition its children by that amount when calculating node transforms. As before, no impact on the underlying Taffy layout. Calculating this correctly is trickier than it was in #8104 due to `"Update scrolling on relayout"`. **Background** When `ScrollPosition` is updated directly by the user, it can be trivially handled in-engine by adding the parent's scroll position to the final location of each child node. However, _other layout actions_ may result in a situation where `ScrollPosition` needs to be updated. Consider a 1000 pixel tall vertically scrolling list of 100 elements, each 100 pixels tall. Scrolled to the bottom, the `ScrollPosition.offset_y` is 9000, just enough to display the last element in the list. When removing an element from that list, the new desired `ScrollPosition.offset_y` is 8900, but, critically, that is not known until after the sizes and positions of the children of the scrollable node are resolved. All user scrolling code today handles this by delaying the resolution by one frame. One notable disadvantage of this is the inability to support `WinitSettings::desktop_app()`, since there would need to be an input AFTER the layout change that caused the scroll position to update for the results of the scroll position update to render visually. I propose the alternative in this PR, which allows for same-frame resolution of scrolling layout. **Resolution** _Edit: Below resolution is outdated, and replaced with the simpler usage of taffy's `Layout::content_size`._ When recursively iterating the children of a node, each child now returns a `Vec2` representing the location of their own bottom right corner. Then, `[[0,0, [x,y]]` represents a bounding box containing the scrollable area filled by that child. Scrollable parents aggregate those areas into the bounding box of _all_ children, then consider that result against `ScrollPosition` to ensure its validity. In the event that resolution of the layout of the children invalidates the `ScrollPosition` (e.g. scrolled further than there were children to scroll to), _all_ children of that node must be recursively repositioned. The position of each child must change as a result of the change in scroll position. Therefore, this implementation takes care to only spend the cost of the "second layout pass" when a specific node actually had a `ScrollPosition` forcibly updated by the layout of its children. ## Testing Examples in `ui/scroll.rs`. There may be more complex node/style interactions that were unconsidered. --- ## Showcase  ## Alternatives - `bevy_ui` doesn't support scrolling. - `bevy_ui` implements scrolling with a one-frame delay on reactions to layout changes.
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What problem does this solve or what need does it fill?
The ability to create a scrollable region is a common requirement when building a user interface.
What solution would you like?
Build in support for scrolling enabled by
Overflow::Scrollor some similar marker on a node. It should be possible to control whether a node scrolls independently in each axis.Additional context
Scrolling is currently implemented as an example in the
ui/ui.rsexample. But this:The text was updated successfully, but these errors were encountered: