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use super::*;
impl<H: UiHost> UiTree<H> {
pub(super) fn prepaint_virtual_list_window_from_interaction_record(
&mut self,
app: &mut H,
record: &InteractionRecord,
) {
let Some(window) = self.window else {
return;
};
if !record.can_scroll_descendant_into_view {
return;
}
let Some(inputs) =
crate::declarative::frame::element_record_for_node(&mut *app, window, record.node)
.and_then(|element_record| match &element_record.instance {
crate::declarative::frame::ElementInstance::VirtualList(props) => {
Some(VirtualListPrepaintInputs {
element: element_record.element,
axis: props.axis,
len: props.len,
items_revision: props.items_revision,
measure_mode: props.measure_mode,
overscan: props.overscan,
estimate_row_height: props.estimate_row_height,
gap: props.gap,
scroll_margin: props.scroll_margin,
scroll_handle: props.scroll_handle.clone(),
})
}
_ => None,
})
else {
return;
};
let viewport = match inputs.axis {
fret_core::Axis::Vertical => Px(record.bounds.size.height.0.max(0.0)),
fret_core::Axis::Horizontal => Px(record.bounds.size.width.0.max(0.0)),
};
if viewport.0 <= 0.0 || inputs.len == 0 {
return;
}
let offset_point = inputs.scroll_handle.offset();
let offset_axis = match inputs.axis {
fret_core::Axis::Vertical => offset_point.y,
fret_core::Axis::Horizontal => offset_point.x,
};
let deferred_scroll_to_item = inputs.scroll_handle.deferred_scroll_to_item().is_some()
|| inputs
.scroll_handle
.scroll_to_item_consumed_in_frame(app.frame_id());
let retained_host = crate::elements::with_window_state(&mut *app, window, |window_state| {
window_state.has_state::<crate::windowed_surface_host::RetainedVirtualListHostMarker>(
inputs.element,
)
});
let view_cache_active = self.view_cache_active();
let update = crate::elements::with_element_state(
&mut *app,
window,
inputs.element,
crate::element::VirtualListState::default,
|state| {
let prev_items_revision = state.items_revision;
let prev_viewport = match inputs.axis {
fret_core::Axis::Vertical => state.viewport_h,
fret_core::Axis::Horizontal => state.viewport_w,
};
let prev_offset = match inputs.axis {
fret_core::Axis::Vertical => state.offset_y,
fret_core::Axis::Horizontal => state.offset_x,
};
let prev_window_range = state.window_range;
let render_window_range = state.render_window_range;
state.metrics.ensure_with_mode(
inputs.measure_mode,
inputs.len,
inputs.estimate_row_height,
inputs.gap,
inputs.scroll_margin,
);
state.items_revision = inputs.items_revision;
let content_extent = state.metrics.total_height();
let offset_axis = state.metrics.clamp_offset(offset_axis, viewport);
match inputs.axis {
fret_core::Axis::Vertical => {
state.offset_y = offset_axis;
state.viewport_h = viewport;
}
fret_core::Axis::Horizontal => {
state.offset_x = offset_axis;
state.viewport_w = viewport;
}
}
if viewport.0 > 0.0 {
state.has_final_viewport = true;
}
let visible_range = state.metrics.visible_range(offset_axis, viewport, 0);
let ideal_window_range =
state
.metrics
.visible_range(offset_axis, viewport, inputs.overscan);
let window_mismatch = if let Some(visible) = visible_range {
match render_window_range.filter(|r| {
r.count == inputs.len
&& r.overscan == inputs.overscan
&& r.start_index <= r.end_index
&& r.end_index < r.count
}) {
None => {
// Without a render-derived window, the current declarative subtree may
// not reflect the post-layout visible range. Treat this as a mismatch
// so prepaint can schedule the initial one-shot rerender (Track B) or
// retained-host reconcile (Track A).
true
}
Some(rendered) => {
let rendered_start =
rendered.start_index.saturating_sub(rendered.overscan);
let rendered_end = (rendered.end_index + rendered.overscan)
.min(rendered.count.saturating_sub(1));
visible.start_index < rendered_start || visible.end_index > rendered_end
}
}
} else {
false
};
let mut window_shift_kind = crate::tree::UiDebugVirtualListWindowShiftKind::None;
let allow_preemptive_prefetch = retained_host || !view_cache_active;
let window_range = if window_mismatch {
window_shift_kind = crate::tree::UiDebugVirtualListWindowShiftKind::Escape;
if retained_host {
match (render_window_range, visible_range) {
(Some(rendered), Some(visible))
if rendered.count == inputs.len
&& rendered.overscan == inputs.overscan
&& rendered.start_index <= rendered.end_index
&& rendered.end_index < rendered.count =>
{
Some(crate::virtual_list::shift_virtual_range_minimally(
rendered, visible,
))
}
_ => ideal_window_range,
}
} else {
ideal_window_range
}
} else if inputs.overscan > 0 && !deferred_scroll_to_item {
match (render_window_range, visible_range) {
(Some(rendered), Some(visible))
if rendered.count == inputs.len
&& rendered.overscan == inputs.overscan
&& rendered.start_index <= rendered.end_index
&& rendered.end_index < rendered.count =>
{
let forced_prefetch = if let Some(desired) = ideal_window_range {
let rendered_visible_len = rendered
.end_index
.saturating_sub(rendered.start_index)
.saturating_add(1);
let visible_len = visible
.end_index
.saturating_sub(visible.start_index)
.saturating_add(1);
// If the render-derived window was computed under a smaller
// viewport (e.g. during intrinsic probes), its visible span may
// be shorter than the current visible span. While we can still
// be within the rendered overscan envelope, we should stage a
// one-shot prefetch to the ideal window so the next frame can
// rebuild the correct visible-items set.
if visible_len > rendered_visible_len {
window_shift_kind =
crate::tree::UiDebugVirtualListWindowShiftKind::Prefetch;
// For the non-retained VirtualList path, we will schedule a cache
// root rerender on prefetch so the next frame can rebuild
// `visible_items` against the updated window. Clear the
// render-derived window to ensure the rerender consumes the
// prepaint-derived window.
if !retained_host && view_cache_active {
state.render_window_range = None;
}
Some(desired)
} else {
None
}
} else {
None
};
if let Some(prefetch) = forced_prefetch {
Some(prefetch)
} else if !allow_preemptive_prefetch {
// For non-retained + view-cache hosts, prefetch shifts translate
// directly into cache-root rerenders. Keep the current rendered
// window while it still covers the visible range, and rely on
// escape-driven shifts when we truly leave overscan.
ideal_window_range
} else {
let prefetch_margin = (inputs.overscan / 6).max(1);
// Shift by a slightly larger step than the “near-edge” margin so we
// don’t prefetch on every frame during slow scroll, while still keeping
// each prefetch rerender bounded.
let prefetch_step = if retained_host {
// Retained hosts (ADR 0177) can apply window shifts via reconcile
// without rerendering a parent cache root, so prefer smaller, more
// frequent prefetches. This bounds single-frame attach/detach bursts.
//
// Note: shifting the window by `step` can detach ~`step` items and
// attach ~`step` items, so the attach+detach delta scales like
// `2*step` (see `--check-retained-vlist-attach-detach-max`).
(inputs.overscan / 2).clamp(1, RETAINED_HOST_PREFETCH_STEP_MAX)
} else {
// Non-retained VirtualList pays a full cache-root rerender per shift,
// so prefer fewer, larger shifts while we still have overscan coverage.
//
// This is a deliberate trade-off:
// - larger steps reduce how often we schedule a rerender during smooth scroll,
// - but each rerender may rebuild more rows.
//
// For v1 (non-retained), reducing rerender frequency generally wins because
// we cannot attach/detach rows on cache-hit frames without a retained host
// boundary (ADR 0177).
inputs.overscan.saturating_mul(8)
}
.max(prefetch_margin)
.max(1);
let prefer_forward = if offset_axis.0 > prev_offset.0 + 0.01 {
state.last_scroll_direction_forward = Some(true);
Some(true)
} else if offset_axis.0 + 0.01 < prev_offset.0 {
state.last_scroll_direction_forward = Some(false);
Some(false)
} else {
state.last_scroll_direction_forward
};
if let Some(prefetch) =
crate::virtual_list::prefetch_virtual_range_step(
rendered,
visible,
prefetch_margin,
prefetch_step,
prefer_forward,
)
{
window_shift_kind =
crate::tree::UiDebugVirtualListWindowShiftKind::Prefetch;
// For the non-retained VirtualList path, we will schedule a cache
// root rerender on prefetch so the next frame can rebuild
// `visible_items` against the prefetched window. Clear the
// render-derived window to ensure the rerender consumes the
// prepaint-derived window.
if !retained_host && view_cache_active {
state.render_window_range = None;
}
Some(prefetch)
} else {
ideal_window_range
}
}
}
_ => ideal_window_range,
}
} else {
ideal_window_range
};
state.window_range = window_range;
VirtualListPrepaintWindowUpdate {
prev_items_revision,
prev_viewport,
prev_offset,
prev_window_range,
render_window_range,
window_range,
viewport,
offset: offset_axis,
deferred_scroll_to_item,
window_mismatch,
window_shift_kind,
content_extent,
}
},
);
inputs
.scroll_handle
.set_viewport_size_internal(record.bounds.size);
let content_size = match inputs.axis {
fret_core::Axis::Vertical => Size::new(record.bounds.size.width, update.content_extent),
fret_core::Axis::Horizontal => {
Size::new(update.content_extent, record.bounds.size.height)
}
};
inputs.scroll_handle.set_content_size_internal(content_size);
let prev_offset_point = inputs.scroll_handle.offset();
match inputs.axis {
fret_core::Axis::Vertical => {
inputs
.scroll_handle
.set_offset_internal(fret_core::Point::new(prev_offset_point.x, update.offset));
}
fret_core::Axis::Horizontal => {
inputs
.scroll_handle
.set_offset_internal(fret_core::Point::new(update.offset, prev_offset_point.y));
}
}
if self.debug_enabled {
let policy_key = {
let mut b = CacheKeyBuilder::new();
b.write_u32(inputs.axis as u32);
b.write_u32(inputs.measure_mode as u32);
b.write_u64(inputs.overscan as u64);
b.write_px(inputs.estimate_row_height);
b.write_px(inputs.gap);
b.write_px(inputs.scroll_margin);
b.finish()
};
let inputs_key = {
let mut b = CacheKeyBuilder::new();
b.write_u64(policy_key);
b.write_u64(inputs.len as u64);
b.write_u64(inputs.items_revision);
b.write_px(update.viewport);
b.write_px(update.offset);
b.write_px(update.content_extent);
b.finish()
};
let (window_shift_reason, window_shift_apply_mode, window_shift_invalidation_detail) =
if update.window_shift_kind != crate::tree::UiDebugVirtualListWindowShiftKind::None
{
let reason = if update.deferred_scroll_to_item {
crate::tree::UiDebugVirtualListWindowShiftReason::ScrollToItem
} else if inputs.items_revision != update.prev_items_revision {
crate::tree::UiDebugVirtualListWindowShiftReason::ItemsRevision
} else if (update.viewport.0 - update.prev_viewport.0).abs() > 0.01 {
crate::tree::UiDebugVirtualListWindowShiftReason::ViewportResize
} else if (update.offset.0 - update.prev_offset.0).abs() > 0.01 {
crate::tree::UiDebugVirtualListWindowShiftReason::ScrollOffset
} else if update.prev_window_range.map(|r| (r.count, r.overscan))
!= update.window_range.map(|r| (r.count, r.overscan))
{
crate::tree::UiDebugVirtualListWindowShiftReason::InputsChange
} else {
crate::tree::UiDebugVirtualListWindowShiftReason::Unknown
};
let mode = if retained_host {
crate::tree::UiDebugVirtualListWindowShiftApplyMode::RetainedReconcile
} else {
crate::tree::UiDebugVirtualListWindowShiftApplyMode::NonRetainedRerender
};
let invalidation_detail = if self.view_cache_active() && !retained_host {
match reason {
crate::tree::UiDebugVirtualListWindowShiftReason::ScrollToItem => Some(
crate::tree::UiDebugInvalidationDetail::ScrollHandleScrollToItemWindowUpdate,
),
crate::tree::UiDebugVirtualListWindowShiftReason::ViewportResize => Some(
crate::tree::UiDebugInvalidationDetail::ScrollHandleViewportResizeWindowUpdate,
),
crate::tree::UiDebugVirtualListWindowShiftReason::ItemsRevision => Some(
crate::tree::UiDebugInvalidationDetail::ScrollHandleItemsRevisionWindowUpdate,
),
_ => match update.window_shift_kind {
crate::tree::UiDebugVirtualListWindowShiftKind::None => None,
crate::tree::UiDebugVirtualListWindowShiftKind::Prefetch => Some(
crate::tree::UiDebugInvalidationDetail::ScrollHandlePrefetchWindowUpdate,
),
crate::tree::UiDebugVirtualListWindowShiftKind::Escape => Some(
crate::tree::UiDebugInvalidationDetail::ScrollHandleWindowUpdate,
),
},
}
} else {
None
};
(Some(reason), Some(mode), invalidation_detail)
} else {
(None, None, None)
};
if update.window_shift_kind != crate::tree::UiDebugVirtualListWindowShiftKind::None {
self.debug_record_prepaint_action(crate::tree::UiDebugPrepaintAction {
node: record.node,
target: None,
kind: crate::tree::UiDebugPrepaintActionKind::VirtualListWindowShift,
invalidation: None,
element: Some(inputs.element),
virtual_list_window_shift_kind: Some(update.window_shift_kind),
virtual_list_window_shift_reason: window_shift_reason,
chart_sampling_window_key: None,
node_graph_cull_window_key: None,
frame_id: app.frame_id(),
});
}
self.debug_record_virtual_list_window(crate::tree::UiDebugVirtualListWindow {
source: crate::tree::UiDebugVirtualListWindowSource::Prepaint,
node: record.node,
element: inputs.element,
axis: inputs.axis,
is_probe_layout: false,
items_len: inputs.len,
items_revision: inputs.items_revision,
prev_items_revision: update.prev_items_revision,
measure_mode: inputs.measure_mode,
overscan: inputs.overscan,
estimate_row_height: inputs.estimate_row_height,
gap: inputs.gap,
scroll_margin: inputs.scroll_margin,
viewport: update.viewport,
prev_viewport: update.prev_viewport,
offset: update.offset,
prev_offset: update.prev_offset,
content_extent: update.content_extent,
policy_key,
inputs_key,
window_range: update.window_range,
prev_window_range: update.prev_window_range,
render_window_range: update.render_window_range,
deferred_scroll_to_item: update.deferred_scroll_to_item,
deferred_scroll_consumed: false,
window_mismatch: update.window_mismatch,
window_shift_kind: update.window_shift_kind,
window_shift_reason,
window_shift_apply_mode,
window_shift_invalidation_detail,
});
}
if self.view_cache_active()
&& update.window_shift_kind != crate::tree::UiDebugVirtualListWindowShiftKind::None
{
if retained_host {
let kind = match update.window_shift_kind {
crate::tree::UiDebugVirtualListWindowShiftKind::Prefetch => {
crate::tree::UiDebugRetainedVirtualListReconcileKind::Prefetch
}
crate::tree::UiDebugVirtualListWindowShiftKind::Escape => {
crate::tree::UiDebugRetainedVirtualListReconcileKind::Escape
}
crate::tree::UiDebugVirtualListWindowShiftKind::None => {
unreachable!("window_shift_kind checked above")
}
};
crate::elements::with_window_state(&mut *app, window, |window_state| {
window_state.mark_retained_virtual_list_needs_reconcile(inputs.element, kind);
});
self.request_redraw_coalesced(app);
} else {
let detail = match update.window_shift_kind {
crate::tree::UiDebugVirtualListWindowShiftKind::None => {
unreachable!("window_shift_kind checked above")
}
crate::tree::UiDebugVirtualListWindowShiftKind::Prefetch => {
UiDebugInvalidationDetail::ScrollHandlePrefetchWindowUpdate
}
crate::tree::UiDebugVirtualListWindowShiftKind::Escape => {
UiDebugInvalidationDetail::ScrollHandleWindowUpdate
}
};
self.mark_nearest_view_cache_root_needs_rerender(
record.node,
UiDebugInvalidationSource::Other,
if update.deferred_scroll_to_item {
UiDebugInvalidationDetail::ScrollHandleScrollToItemWindowUpdate
} else {
detail
},
);
self.request_redraw_coalesced(app);
}
}
}
}