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use crate::lowering::{LoweringContext, NodeBuilder};
use crate::ui::traits::Lower;
use crate::ui::Node;
use fission_ir::{op::{FlexDirection, LayoutOp, Op}, NodeId};
use serde::{Deserialize, Serialize};
use std::sync::Arc;
/// A virtualized vertical list that only builds visible items.
///
/// For large data sets, `LazyColumn` dramatically reduces the node count by
/// rendering only the items within the scroll viewport plus a small buffer.
/// Each item is assumed to have a uniform `item_height`.
///
/// When `item_height` is 0 or negative, virtualisation is disabled and all
/// children are rendered (equivalent to wrapping a `Column` in a `Scroll`).
///
/// # Example
///
/// ```rust,ignore
/// LazyColumn {
/// children: Arc::new(
/// items.iter()
/// .map(|item| Text::new(item.name.clone()).into_node())
/// .collect()
/// ),
/// item_height: 48.0,
/// ..Default::default()
/// }
/// ```
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct LazyColumn {
/// Explicit node identity (used for scroll-offset tracking).
pub id: Option<NodeId>,
/// All items in the list (only the visible slice is lowered each frame).
pub children: Arc<Vec<Node>>,
/// Uniform height of each item in layout points. Set to 0 to disable
/// virtualisation.
pub item_height: f32,
}
impl LazyColumn {
pub fn into_node(self) -> crate::ui::Node {
crate::ui::Node::LazyColumn(self)
}
}
impl Lower for LazyColumn {
fn lower(&self, cx: &mut LoweringContext) -> NodeId {
let scroll_id = self.id.unwrap_or_else(|| cx.next_node_id());
if self.item_height <= 0.0 {
let col_id = cx.next_node_id();
let mut column_children = Vec::new();
for (i, child) in self.children.iter().enumerate() {
let child_scope = NodeId::derived(col_id.as_u128(), &[i as u32]);
cx.push_scope(child_scope);
column_children.push(child.lower(cx));
cx.pop_scope();
}
let mut col = NodeBuilder::new(
col_id,
Op::Layout(LayoutOp::Flex {
direction: FlexDirection::Column,
wrap: fission_ir::op::FlexWrap::NoWrap,
flex_grow: 0.0,
flex_shrink: 0.0,
padding: [0.0; 4],
gap: None,
align_items: fission_ir::op::AlignItems::Stretch,
justify_content: fission_ir::op::JustifyContent::Start,
}),
);
col.add_children(column_children);
let col_id = col.build(cx);
let content_id = cx.next_node_id();
let mut content_box = NodeBuilder::new(
content_id,
Op::Layout(LayoutOp::Box {
width: None,
height: None,
min_width: None,
max_width: None,
min_height: None,
max_height: None,
padding: [0.0; 4],
flex_grow: 0.0,
flex_shrink: 0.0,
aspect_ratio: None,
}),
);
content_box.add_child(col_id);
let content_id = content_box.build(cx);
let mut scroll = NodeBuilder::new(
scroll_id,
Op::Layout(LayoutOp::Scroll {
direction: FlexDirection::Column,
show_scrollbar: true,
width: None,
height: None,
min_width: None,
max_width: None,
min_height: None,
max_height: None,
padding: [0.0; 4],
flex_grow: 1.0,
flex_shrink: 1.0,
}),
);
scroll.add_child(content_id);
return scroll.build(cx);
}
// Get layout info from previous frame
let (viewport_width, viewport_height) = if let Some(layout) = cx.layout {
let viewport = layout.viewport_size;
if let Some(geom) = layout.get_node_geometry(scroll_id) {
let w = geom.rect.width();
let h = geom.rect.height();
let w = if w > 0.0 {
if viewport.width > 0.0 { w.min(viewport.width) } else { w }
} else {
viewport.width
};
let h = if h > 0.0 {
if viewport.height > 0.0 { h.min(viewport.height) } else { h }
} else {
viewport.height
};
(if w > 0.0 { Some(w) } else { None }, if h > 0.0 { h } else { 600.0 })
} else {
(if viewport.width > 0.0 { Some(viewport.width) } else { None }, if viewport.height > 0.0 { viewport.height } else { 600.0 })
}
} else {
(None, 600.0)
};
let scroll_offset = cx.runtime_state.scroll.get_offset(scroll_id);
let item_h = self.item_height.max(1.0);
let total_count = self.children.len();
let start_index = (scroll_offset / item_h as f32).floor() as usize;
let visible_count = (viewport_height as f32 / item_h as f32).ceil() as usize + 1; // +1 buffer
let end_index = (start_index + visible_count).min(total_count);
let start_index = start_index.min(total_count); // Clamp
// Column
let col_id = cx.next_node_id(); // Reserve ID for column wrapper
// Build children
let mut column_children = Vec::new();
// Top Spacer
if start_index > 0 {
let spacer_height = item_h * start_index as f32;
if spacer_height > 0.0 {
let spacer_id = NodeId::derived(col_id.as_u128(), &[u32::MAX - 1]);
let spacer = NodeBuilder::new(
spacer_id,
Op::Layout(LayoutOp::Box {
width: None,
height: Some(spacer_height),
min_width: None,
max_width: None,
min_height: None,
max_height: None,
padding: [0.0; 4],
flex_grow: 0.0,
flex_shrink: 0.0,
aspect_ratio: None,
}),
);
column_children.push(spacer.build(cx));
}
}
// Visible Items
// We do NOT push `col_id` as a shared scope because children need stable IDs regardless of skip.
// Instead, we derive a unique scope for each child index.
if start_index < end_index {
let visible = &self.children[start_index..end_index];
for (offset, child) in visible.iter().enumerate() {
let i = start_index + offset;
let child_scope = NodeId::derived(col_id.as_u128(), &[i as u32]);
cx.push_scope(child_scope);
column_children.push(child.lower(cx));
cx.pop_scope();
}
}
// No pop_scope needed for col_id since we didn't push it.
// Bottom Spacer
let remaining = total_count.saturating_sub(end_index);
if remaining > 0 {
let spacer_height = item_h * remaining as f32;
if spacer_height > 0.0 {
let spacer_id = NodeId::derived(col_id.as_u128(), &[u32::MAX]);
let spacer = NodeBuilder::new(
spacer_id,
Op::Layout(LayoutOp::Box {
width: None,
height: Some(spacer_height),
min_width: None,
max_width: None,
min_height: None,
max_height: None,
padding: [0.0; 4],
flex_grow: 0.0,
flex_shrink: 0.0,
aspect_ratio: None,
}),
);
column_children.push(spacer.build(cx));
}
}
let mut col = NodeBuilder::new(
col_id,
Op::Layout(LayoutOp::Flex {
direction: FlexDirection::Column,
wrap: fission_ir::op::FlexWrap::NoWrap,
flex_grow: 0.0,
flex_shrink: 0.0,
padding: [0.0; 4],
gap: None,
align_items: fission_ir::op::AlignItems::Stretch,
justify_content: fission_ir::op::JustifyContent::Start,
}),
);
col.add_children(column_children);
let col_id = col.build(cx);
let content_id = cx.next_node_id();
let mut content_box = NodeBuilder::new(
content_id,
Op::Layout(LayoutOp::Box {
width: None,
height: None,
min_width: None,
max_width: None,
min_height: None,
max_height: None,
padding: [0.0; 4],
flex_grow: 0.0,
flex_shrink: 0.0,
aspect_ratio: None,
}),
);
content_box.add_child(col_id);
let content_id = content_box.build(cx);
// Scroll
let mut scroll = NodeBuilder::new(
scroll_id,
Op::Layout(LayoutOp::Scroll {
direction: FlexDirection::Column,
show_scrollbar: true,
width: viewport_width,
height: None,
min_width: None,
max_width: None,
min_height: None,
max_height: None,
padding: [0.0; 4],
flex_grow: 1.0,
flex_shrink: 1.0,
})
);
scroll.add_child(content_id);
scroll.build(cx)
}
}