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bevy/examples/ui/scroll.rs

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UI Scrolling (#15291) # 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 **Resolution** _Edit: Below resolution is outdated, and replaced with the simpler usage of taffy's `Layout::content_size`._ When recursively iterating the children of a node, each child now returns a `Vec2`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 ![scroll](https://github.com/user-attachments/assets/1331138f-93aa-4a8f-959c-6be18a04ff03) ## Alternatives - `bevy_ui` doesn't support scrolling. - `bevy_ui` implements scrolling with a one-frame delay on reactions to layout changes.
2024-09-23 17:17:58 +00:00
//! This example illustrates scrolling in Bevy UI.
use bevy::{
a11y::{
accesskit::{NodeBuilder, Role},
AccessibilityNode,
},
input::mouse::{MouseScrollUnit, MouseWheel},
picking::focus::HoverMap,
prelude::*,
winit::WinitSettings,
};
fn main() {
let mut app = App::new();
app.add_plugins(DefaultPlugins)
.insert_resource(WinitSettings::desktop_app())
.add_systems(Startup, setup)
.add_systems(Update, update_scroll_position);
app.run();
}
const FONT_SIZE: f32 = 20.;
const LINE_HEIGHT: f32 = 21.;
fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
// Camera
commands.spawn((Camera2dBundle::default(), IsDefaultUiCamera));
//root node
commands
.spawn(NodeBundle {
style: Style {
width: Val::Percent(100.0),
height: Val::Percent(100.0),
justify_content: JustifyContent::SpaceBetween,
flex_direction: FlexDirection::Column,
..default()
},
..default()
})
.insert(Pickable::IGNORE)
.with_children(|parent| {
// horizontal scroll example
parent
.spawn(NodeBundle {
style: Style {
width: Val::Percent(100.),
flex_direction: FlexDirection::Column,
..default()
},
..default()
})
.with_children(|parent| {
// header
parent.spawn((
TextBundle::from_section(
"Horizontally Scrolling list (Shift + Mousewheel)",
TextStyle {
font: asset_server.load("fonts/FiraSans-Bold.ttf"),
font_size: FONT_SIZE,
..default()
},
),
Label,
));
// horizontal scroll container
parent
.spawn(NodeBundle {
style: Style {
width: Val::Percent(80.),
margin: UiRect::all(Val::Px(10.)),
flex_direction: FlexDirection::Row,
overflow: Overflow::scroll_x(), // n.b.
..default()
},
background_color: Color::srgb(0.10, 0.10, 0.10).into(),
..default()
})
.with_children(|parent| {
for i in 0..100 {
parent.spawn((
TextBundle::from_section(
format!("Item {i}"),
TextStyle {
font: asset_server
.load("fonts/FiraSans-Bold.ttf"),
..default()
},
),
Label,
AccessibilityNode(NodeBuilder::new(Role::ListItem)),
))
.insert(Style {
min_width: Val::Px(200.),
align_content: AlignContent::Center,
..default()
})
.insert(Pickable {
should_block_lower: false,
..default()
})
.observe(|
trigger: Trigger<Pointer<Down>>,
mut commands: Commands
| {
if trigger.event().button == PointerButton::Primary {
commands.entity(trigger.entity()).despawn_recursive();
}
});
}
});
});
// container for all other examples
parent
.spawn(NodeBundle {
style: Style {
width: Val::Percent(100.),
height: Val::Percent(100.),
flex_direction: FlexDirection::Row,
justify_content: JustifyContent::SpaceBetween,
..default()
},
..default()
})
.with_children(|parent| {
// vertical scroll example
parent
.spawn(NodeBundle {
style: Style {
flex_direction: FlexDirection::Column,
justify_content: JustifyContent::Center,
align_items: AlignItems::Center,
width: Val::Px(200.),
..default()
},
..default()
})
.with_children(|parent| {
// Title
parent.spawn((
TextBundle::from_section(
"Vertically Scrolling List",
TextStyle {
font: asset_server.load("fonts/FiraSans-Bold.ttf"),
font_size: FONT_SIZE,
..default()
},
),
Label,
));
// Scrolling list
parent
.spawn(NodeBundle {
style: Style {
flex_direction: FlexDirection::Column,
align_self: AlignSelf::Stretch,
height: Val::Percent(50.),
overflow: Overflow::scroll_y(), // n.b.
..default()
},
background_color: Color::srgb(0.10, 0.10, 0.10).into(),
..default()
})
.with_children(|parent| {
// List items
for i in 0..25 {
parent
.spawn(NodeBundle {
style: Style {
min_height: Val::Px(LINE_HEIGHT),
max_height: Val::Px(LINE_HEIGHT),
..default()
},
..default()
})
.insert(Pickable {
should_block_lower: false,
..default()
})
.with_children(|parent| {
parent
.spawn((
TextBundle::from_section(
format!("Item {i}"),
TextStyle {
font: asset_server.load(
"fonts/FiraSans-Bold.ttf",
),
..default()
},
),
Label,
AccessibilityNode(NodeBuilder::new(
Role::ListItem,
)),
))
.insert(Pickable {
should_block_lower: false,
..default()
});
});
}
});
});
// Bidirectional scroll example
parent
.spawn(NodeBundle {
style: Style {
flex_direction: FlexDirection::Column,
justify_content: JustifyContent::Center,
align_items: AlignItems::Center,
width: Val::Px(200.),
..default()
},
..default()
})
.with_children(|parent| {
// Title
parent.spawn((
TextBundle::from_section(
"Bidirectionally Scrolling List",
TextStyle {
font: asset_server.load("fonts/FiraSans-Bold.ttf"),
font_size: FONT_SIZE,
..default()
},
),
Label,
));
// Scrolling list
parent
.spawn(NodeBundle {
style: Style {
flex_direction: FlexDirection::Column,
align_self: AlignSelf::Stretch,
height: Val::Percent(50.),
overflow: Overflow::scroll(), // n.b.
..default()
},
background_color: Color::srgb(0.10, 0.10, 0.10).into(),
..default()
})
.with_children(|parent| {
// Rows in each column
for oi in 0..10 {
parent
.spawn(NodeBundle {
style: Style {
flex_direction: FlexDirection::Row,
..default()
},
..default()
})
.insert(Pickable::IGNORE)
.with_children(|parent| {
// Elements in each row
for i in 0..25 {
parent
.spawn((
TextBundle::from_section(
format!("Item {}", (oi * 25) + i),
TextStyle {
font: asset_server.load(
"fonts/FiraSans-Bold.ttf",
),
..default()
},
),
Label,
AccessibilityNode(NodeBuilder::new(
Role::ListItem,
)),
))
.insert(Pickable {
should_block_lower: false,
..default()
});
}
});
}
});
});
//Nested scrolls example
parent
.spawn(NodeBundle {
style: Style {
flex_direction: FlexDirection::Column,
justify_content: JustifyContent::Center,
align_items: AlignItems::Center,
width: Val::Px(200.),
..default()
},
..default()
})
.with_children(|parent| {
// Title
parent.spawn((
TextBundle::from_section(
"Nested Scrolling Lists",
TextStyle {
font: asset_server.load("fonts/FiraSans-Bold.ttf"),
font_size: FONT_SIZE,
..default()
},
),
Label,
));
// Outer, horizontal scrolling container
parent
.spawn(NodeBundle {
style: Style {
column_gap: Val::Px(20.),
flex_direction: FlexDirection::Row,
align_self: AlignSelf::Stretch,
height: Val::Percent(50.),
overflow: Overflow::scroll_x(), // n.b.
..default()
},
background_color: Color::srgb(0.10, 0.10, 0.10).into(),
..default()
})
.with_children(|parent| {
// Inner, scrolling columns
for oi in 0..30 {
parent
.spawn(NodeBundle {
style: Style {
flex_direction: FlexDirection::Column,
align_self: AlignSelf::Stretch,
overflow: Overflow::scroll_y(),
..default()
},
background_color: Color::srgb(0.05, 0.05, 0.05)
.into(),
..default()
})
.insert(Pickable {
should_block_lower: false,
..default()
})
.with_children(|parent| {
for i in 0..25 {
parent
.spawn((
TextBundle::from_section(
format!("Item {}", (oi * 25) + i),
TextStyle {
font: asset_server.load(
"fonts/FiraSans-Bold.ttf",
),
..default()
},
),
Label,
AccessibilityNode(NodeBuilder::new(
Role::ListItem,
)),
))
.insert(Pickable {
should_block_lower: false,
..default()
});
}
});
}
});
});
});
});
}
/// Updates the scroll position of scrollable nodes in response to mouse input
pub fn update_scroll_position(
mut mouse_wheel_events: EventReader<MouseWheel>,
hover_map: Res<HoverMap>,
mut scrolled_node_query: Query<&mut ScrollPosition>,
keyboard_input: Res<ButtonInput<KeyCode>>,
) {
for mouse_wheel_event in mouse_wheel_events.read() {
let (mut dx, mut dy) = match mouse_wheel_event.unit {
MouseScrollUnit::Line => (
mouse_wheel_event.x * LINE_HEIGHT,
mouse_wheel_event.y * LINE_HEIGHT,
),
MouseScrollUnit::Pixel => (mouse_wheel_event.x, mouse_wheel_event.y),
};
if keyboard_input.pressed(KeyCode::ShiftLeft) || keyboard_input.pressed(KeyCode::ShiftRight)
{
std::mem::swap(&mut dx, &mut dy);
}
for (_pointer, pointer_map) in hover_map.iter() {
for (entity, _hit) in pointer_map.iter() {
if let Ok(mut scroll_position) = scrolled_node_query.get_mut(*entity) {
scroll_position.offset_x -= dx;
scroll_position.offset_y -= dy;
}
}
}
}
}
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