1//! A list element that can be used to render a large number of differently sized elements
2//! efficiently. Clients of this API need to ensure that elements outside of the scrolled
3//! area do not change their height for this element to function correctly. If your elements
4//! do change height, notify the list element via [`ListState::splice`] or [`ListState::reset`].
5//! In order to minimize re-renders, this element's state is stored intrusively
6//! on your own views, so that your code can coordinate directly with the list element's cached state.
7//!
8//! If all of your elements are the same height, see [`crate::UniformList`] for a simpler API
9
10use crate::{
11 AnyElement, App, AvailableSpace, Bounds, ContentMask, DispatchPhase, Edges, Element, EntityId,
12 FocusHandle, GlobalElementId, Hitbox, HitboxBehavior, InspectorElementId, IntoElement,
13 Overflow, Pixels, Point, ScrollDelta, ScrollWheelEvent, Size, Style, StyleRefinement, Styled,
14 Window, point, px, size,
15};
16use collections::VecDeque;
17use refineable::Refineable as _;
18use std::{cell::RefCell, ops::Range, rc::Rc};
19use sum_tree::{Bias, Dimensions, SumTree};
20
21type RenderItemFn = dyn FnMut(usize, &mut Window, &mut App) -> AnyElement + 'static;
22
23/// Construct a new list element
24pub fn list(
25 state: ListState,
26 render_item: impl FnMut(usize, &mut Window, &mut App) -> AnyElement + 'static,
27) -> List {
28 List {
29 state,
30 render_item: Box::new(render_item),
31 style: StyleRefinement::default(),
32 sizing_behavior: ListSizingBehavior::default(),
33 }
34}
35
36/// A list element
37pub struct List {
38 state: ListState,
39 render_item: Box<RenderItemFn>,
40 style: StyleRefinement,
41 sizing_behavior: ListSizingBehavior,
42}
43
44impl List {
45 /// Set the sizing behavior for the list.
46 pub fn with_sizing_behavior(mut self, behavior: ListSizingBehavior) -> Self {
47 self.sizing_behavior = behavior;
48 self
49 }
50}
51
52/// The list state that views must hold on behalf of the list element.
53#[derive(Clone)]
54pub struct ListState(Rc<RefCell<StateInner>>);
55
56impl std::fmt::Debug for ListState {
57 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
58 f.write_str("ListState")
59 }
60}
61
62struct StateInner {
63 last_layout_bounds: Option<Bounds<Pixels>>,
64 last_padding: Option<Edges<Pixels>>,
65 items: SumTree<ListItem>,
66 logical_scroll_top: Option<ListOffset>,
67 alignment: ListAlignment,
68 overdraw: Pixels,
69 reset: bool,
70 #[allow(clippy::type_complexity)]
71 scroll_handler: Option<Box<dyn FnMut(&ListScrollEvent, &mut Window, &mut App)>>,
72 scrollbar_drag_start_height: Option<Pixels>,
73 measuring_behavior: ListMeasuringBehavior,
74 pending_scroll: Option<PendingScrollFraction>,
75}
76
77/// Keeps track of a fractional scroll position within an item for restoration
78/// after remeasurement.
79struct PendingScrollFraction {
80 /// The index of the item to scroll within.
81 item_ix: usize,
82 /// Fractional offset (0.0 to 1.0) within the item's height.
83 fraction: f32,
84}
85
86/// Whether the list is scrolling from top to bottom or bottom to top.
87#[derive(Clone, Copy, Debug, Eq, PartialEq)]
88pub enum ListAlignment {
89 /// The list is scrolling from top to bottom, like most lists.
90 Top,
91 /// The list is scrolling from bottom to top, like a chat log.
92 Bottom,
93}
94
95/// A scroll event that has been converted to be in terms of the list's items.
96pub struct ListScrollEvent {
97 /// The range of items currently visible in the list, after applying the scroll event.
98 pub visible_range: Range<usize>,
99
100 /// The number of items that are currently visible in the list, after applying the scroll event.
101 pub count: usize,
102
103 /// Whether the list has been scrolled.
104 pub is_scrolled: bool,
105}
106
107/// The sizing behavior to apply during layout.
108#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
109pub enum ListSizingBehavior {
110 /// The list should calculate its size based on the size of its items.
111 Infer,
112 /// The list should not calculate a fixed size.
113 #[default]
114 Auto,
115}
116
117/// The measuring behavior to apply during layout.
118#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
119pub enum ListMeasuringBehavior {
120 /// Measure all items in the list.
121 /// Note: This can be expensive for the first frame in a large list.
122 Measure(bool),
123 /// Only measure visible items
124 #[default]
125 Visible,
126}
127
128impl ListMeasuringBehavior {
129 fn reset(&mut self) {
130 match self {
131 ListMeasuringBehavior::Measure(has_measured) => *has_measured = false,
132 ListMeasuringBehavior::Visible => {}
133 }
134 }
135}
136
137/// The horizontal sizing behavior to apply during layout.
138#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
139pub enum ListHorizontalSizingBehavior {
140 /// List items' width can never exceed the width of the list.
141 #[default]
142 FitList,
143 /// List items' width may go over the width of the list, if any item is wider.
144 Unconstrained,
145}
146
147struct LayoutItemsResponse {
148 max_item_width: Pixels,
149 scroll_top: ListOffset,
150 item_layouts: VecDeque<ItemLayout>,
151}
152
153struct ItemLayout {
154 index: usize,
155 element: AnyElement,
156 size: Size<Pixels>,
157}
158
159/// Frame state used by the [List] element after layout.
160pub struct ListPrepaintState {
161 hitbox: Hitbox,
162 layout: LayoutItemsResponse,
163}
164
165#[derive(Clone)]
166enum ListItem {
167 Unmeasured {
168 focus_handle: Option<FocusHandle>,
169 },
170 Measured {
171 size: Size<Pixels>,
172 focus_handle: Option<FocusHandle>,
173 },
174}
175
176impl ListItem {
177 fn size(&self) -> Option<Size<Pixels>> {
178 if let ListItem::Measured { size, .. } = self {
179 Some(*size)
180 } else {
181 None
182 }
183 }
184
185 fn focus_handle(&self) -> Option<FocusHandle> {
186 match self {
187 ListItem::Unmeasured { focus_handle } | ListItem::Measured { focus_handle, .. } => {
188 focus_handle.clone()
189 }
190 }
191 }
192
193 fn contains_focused(&self, window: &Window, cx: &App) -> bool {
194 match self {
195 ListItem::Unmeasured { focus_handle } | ListItem::Measured { focus_handle, .. } => {
196 focus_handle
197 .as_ref()
198 .is_some_and(|handle| handle.contains_focused(window, cx))
199 }
200 }
201 }
202}
203
204#[derive(Clone, Debug, Default, PartialEq)]
205struct ListItemSummary {
206 count: usize,
207 rendered_count: usize,
208 unrendered_count: usize,
209 height: Pixels,
210 has_focus_handles: bool,
211}
212
213#[derive(Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
214struct Count(usize);
215
216#[derive(Clone, Debug, Default)]
217struct Height(Pixels);
218
219impl ListState {
220 /// Construct a new list state, for storage on a view.
221 ///
222 /// The overdraw parameter controls how much extra space is rendered
223 /// above and below the visible area. Elements within this area will
224 /// be measured even though they are not visible. This can help ensure
225 /// that the list doesn't flicker or pop in when scrolling.
226 pub fn new(item_count: usize, alignment: ListAlignment, overdraw: Pixels) -> Self {
227 let this = Self(Rc::new(RefCell::new(StateInner {
228 last_layout_bounds: None,
229 last_padding: None,
230 items: SumTree::default(),
231 logical_scroll_top: None,
232 alignment,
233 overdraw,
234 scroll_handler: None,
235 reset: false,
236 scrollbar_drag_start_height: None,
237 measuring_behavior: ListMeasuringBehavior::default(),
238 pending_scroll: None,
239 })));
240 this.splice(0..0, item_count);
241 this
242 }
243
244 /// Set the list to measure all items in the list in the first layout phase.
245 ///
246 /// This is useful for ensuring that the scrollbar size is correct instead of based on only rendered elements.
247 pub fn measure_all(self) -> Self {
248 self.0.borrow_mut().measuring_behavior = ListMeasuringBehavior::Measure(false);
249 self
250 }
251
252 /// Reset this instantiation of the list state.
253 ///
254 /// Note that this will cause scroll events to be dropped until the next paint.
255 pub fn reset(&self, element_count: usize) {
256 let old_count = {
257 let state = &mut *self.0.borrow_mut();
258 state.reset = true;
259 state.measuring_behavior.reset();
260 state.logical_scroll_top = None;
261 state.scrollbar_drag_start_height = None;
262 state.items.summary().count
263 };
264
265 self.splice(0..old_count, element_count);
266 }
267
268 /// Remeasure all items while preserving proportional scroll position.
269 ///
270 /// Use this when item heights may have changed (e.g., font size changes)
271 /// but the number and identity of items remains the same.
272 pub fn remeasure(&self) {
273 let state = &mut *self.0.borrow_mut();
274
275 let new_items = state.items.iter().map(|item| ListItem::Unmeasured {
276 focus_handle: item.focus_handle(),
277 });
278
279 // If there's a `logical_scroll_top`, we need to keep track of it as a
280 // `PendingScrollFraction`, so we can later preserve that scroll
281 // position proportionally to the item, in case the item's height
282 // changes.
283 if let Some(scroll_top) = state.logical_scroll_top {
284 let mut cursor = state.items.cursor::<Count>(());
285 cursor.seek(&Count(scroll_top.item_ix), Bias::Right);
286
287 if let Some(item) = cursor.item() {
288 if let Some(size) = item.size() {
289 let fraction = if size.height.0 > 0.0 {
290 (scroll_top.offset_in_item.0 / size.height.0).clamp(0.0, 1.0)
291 } else {
292 0.0
293 };
294
295 state.pending_scroll = Some(PendingScrollFraction {
296 item_ix: scroll_top.item_ix,
297 fraction,
298 });
299 }
300 }
301 }
302
303 state.items = SumTree::from_iter(new_items, ());
304 state.measuring_behavior.reset();
305 }
306
307 /// The number of items in this list.
308 pub fn item_count(&self) -> usize {
309 self.0.borrow().items.summary().count
310 }
311
312 /// Inform the list state that the items in `old_range` have been replaced
313 /// by `count` new items that must be recalculated.
314 pub fn splice(&self, old_range: Range<usize>, count: usize) {
315 self.splice_focusable(old_range, (0..count).map(|_| None))
316 }
317
318 /// Register with the list state that the items in `old_range` have been replaced
319 /// by new items. As opposed to [`Self::splice`], this method allows an iterator of optional focus handles
320 /// to be supplied to properly integrate with items in the list that can be focused. If a focused item
321 /// is scrolled out of view, the list will continue to render it to allow keyboard interaction.
322 pub fn splice_focusable(
323 &self,
324 old_range: Range<usize>,
325 focus_handles: impl IntoIterator<Item = Option<FocusHandle>>,
326 ) {
327 let state = &mut *self.0.borrow_mut();
328
329 let mut old_items = state.items.cursor::<Count>(());
330 let mut new_items = old_items.slice(&Count(old_range.start), Bias::Right);
331 old_items.seek_forward(&Count(old_range.end), Bias::Right);
332
333 let mut spliced_count = 0;
334 new_items.extend(
335 focus_handles.into_iter().map(|focus_handle| {
336 spliced_count += 1;
337 ListItem::Unmeasured { focus_handle }
338 }),
339 (),
340 );
341 new_items.append(old_items.suffix(), ());
342 drop(old_items);
343 state.items = new_items;
344
345 if let Some(ListOffset {
346 item_ix,
347 offset_in_item,
348 }) = state.logical_scroll_top.as_mut()
349 {
350 if old_range.contains(item_ix) {
351 *item_ix = old_range.start;
352 *offset_in_item = px(0.);
353 } else if old_range.end <= *item_ix {
354 *item_ix = *item_ix - (old_range.end - old_range.start) + spliced_count;
355 }
356 }
357 }
358
359 /// Set a handler that will be called when the list is scrolled.
360 pub fn set_scroll_handler(
361 &self,
362 handler: impl FnMut(&ListScrollEvent, &mut Window, &mut App) + 'static,
363 ) {
364 self.0.borrow_mut().scroll_handler = Some(Box::new(handler))
365 }
366
367 /// Get the current scroll offset, in terms of the list's items.
368 pub fn logical_scroll_top(&self) -> ListOffset {
369 self.0.borrow().logical_scroll_top()
370 }
371
372 /// Scroll the list by the given offset
373 pub fn scroll_by(&self, distance: Pixels) {
374 if distance == px(0.) {
375 return;
376 }
377
378 let current_offset = self.logical_scroll_top();
379 let state = &mut *self.0.borrow_mut();
380 let mut cursor = state.items.cursor::<ListItemSummary>(());
381 cursor.seek(&Count(current_offset.item_ix), Bias::Right);
382
383 let start_pixel_offset = cursor.start().height + current_offset.offset_in_item;
384 let new_pixel_offset = (start_pixel_offset + distance).max(px(0.));
385 if new_pixel_offset > start_pixel_offset {
386 cursor.seek_forward(&Height(new_pixel_offset), Bias::Right);
387 } else {
388 cursor.seek(&Height(new_pixel_offset), Bias::Right);
389 }
390
391 state.logical_scroll_top = Some(ListOffset {
392 item_ix: cursor.start().count,
393 offset_in_item: new_pixel_offset - cursor.start().height,
394 });
395 }
396
397 /// Scroll the list to the given offset
398 pub fn scroll_to(&self, mut scroll_top: ListOffset) {
399 let state = &mut *self.0.borrow_mut();
400 let item_count = state.items.summary().count;
401 if scroll_top.item_ix >= item_count {
402 scroll_top.item_ix = item_count;
403 scroll_top.offset_in_item = px(0.);
404 }
405
406 state.logical_scroll_top = Some(scroll_top);
407 }
408
409 /// Scroll the list to the given item, such that the item is fully visible.
410 pub fn scroll_to_reveal_item(&self, ix: usize) {
411 let state = &mut *self.0.borrow_mut();
412
413 let mut scroll_top = state.logical_scroll_top();
414 let height = state
415 .last_layout_bounds
416 .map_or(px(0.), |bounds| bounds.size.height);
417 let padding = state.last_padding.unwrap_or_default();
418
419 if ix <= scroll_top.item_ix {
420 scroll_top.item_ix = ix;
421 scroll_top.offset_in_item = px(0.);
422 } else {
423 let mut cursor = state.items.cursor::<ListItemSummary>(());
424 cursor.seek(&Count(ix + 1), Bias::Right);
425 let bottom = cursor.start().height + padding.top;
426 let goal_top = px(0.).max(bottom - height + padding.bottom);
427
428 cursor.seek(&Height(goal_top), Bias::Left);
429 let start_ix = cursor.start().count;
430 let start_item_top = cursor.start().height;
431
432 if start_ix >= scroll_top.item_ix {
433 scroll_top.item_ix = start_ix;
434 scroll_top.offset_in_item = goal_top - start_item_top;
435 }
436 }
437
438 state.logical_scroll_top = Some(scroll_top);
439 }
440
441 /// Get the bounds for the given item in window coordinates, if it's
442 /// been rendered.
443 pub fn bounds_for_item(&self, ix: usize) -> Option<Bounds<Pixels>> {
444 let state = &*self.0.borrow();
445
446 let bounds = state.last_layout_bounds.unwrap_or_default();
447 let scroll_top = state.logical_scroll_top();
448 if ix < scroll_top.item_ix {
449 return None;
450 }
451
452 let mut cursor = state.items.cursor::<Dimensions<Count, Height>>(());
453 cursor.seek(&Count(scroll_top.item_ix), Bias::Right);
454
455 let scroll_top = cursor.start().1.0 + scroll_top.offset_in_item;
456
457 cursor.seek_forward(&Count(ix), Bias::Right);
458 if let Some(&ListItem::Measured { size, .. }) = cursor.item() {
459 let &Dimensions(Count(count), Height(top), _) = cursor.start();
460 if count == ix {
461 let top = bounds.top() + top - scroll_top;
462 return Some(Bounds::from_corners(
463 point(bounds.left(), top),
464 point(bounds.right(), top + size.height),
465 ));
466 }
467 }
468 None
469 }
470
471 /// Call this method when the user starts dragging the scrollbar.
472 ///
473 /// This will prevent the height reported to the scrollbar from changing during the drag
474 /// as items in the overdraw get measured, and help offset scroll position changes accordingly.
475 pub fn scrollbar_drag_started(&self) {
476 let mut state = self.0.borrow_mut();
477 state.scrollbar_drag_start_height = Some(state.items.summary().height);
478 }
479
480 /// Called when the user stops dragging the scrollbar.
481 ///
482 /// See `scrollbar_drag_started`.
483 pub fn scrollbar_drag_ended(&self) {
484 self.0.borrow_mut().scrollbar_drag_start_height.take();
485 }
486
487 /// Set the offset from the scrollbar
488 pub fn set_offset_from_scrollbar(&self, point: Point<Pixels>) {
489 self.0.borrow_mut().set_offset_from_scrollbar(point);
490 }
491
492 /// Returns the maximum scroll offset according to the items we have measured.
493 /// This value remains constant while dragging to prevent the scrollbar from moving away unexpectedly.
494 pub fn max_offset_for_scrollbar(&self) -> Size<Pixels> {
495 let state = self.0.borrow();
496 let bounds = state.last_layout_bounds.unwrap_or_default();
497
498 let height = state
499 .scrollbar_drag_start_height
500 .unwrap_or_else(|| state.items.summary().height);
501
502 Size::new(Pixels::ZERO, Pixels::ZERO.max(height - bounds.size.height))
503 }
504
505 /// Returns the current scroll offset adjusted for the scrollbar
506 pub fn scroll_px_offset_for_scrollbar(&self) -> Point<Pixels> {
507 let state = &self.0.borrow();
508 let logical_scroll_top = state.logical_scroll_top();
509
510 let mut cursor = state.items.cursor::<ListItemSummary>(());
511 let summary: ListItemSummary =
512 cursor.summary(&Count(logical_scroll_top.item_ix), Bias::Right);
513 let content_height = state.items.summary().height;
514 let drag_offset =
515 // if dragging the scrollbar, we want to offset the point if the height changed
516 content_height - state.scrollbar_drag_start_height.unwrap_or(content_height);
517 let offset = summary.height + logical_scroll_top.offset_in_item - drag_offset;
518
519 Point::new(px(0.), -offset)
520 }
521
522 /// Return the bounds of the viewport in pixels.
523 pub fn viewport_bounds(&self) -> Bounds<Pixels> {
524 self.0.borrow().last_layout_bounds.unwrap_or_default()
525 }
526}
527
528impl StateInner {
529 fn visible_range(&self, height: Pixels, scroll_top: &ListOffset) -> Range<usize> {
530 let mut cursor = self.items.cursor::<ListItemSummary>(());
531 cursor.seek(&Count(scroll_top.item_ix), Bias::Right);
532 let start_y = cursor.start().height + scroll_top.offset_in_item;
533 cursor.seek_forward(&Height(start_y + height), Bias::Left);
534 scroll_top.item_ix..cursor.start().count + 1
535 }
536
537 fn scroll(
538 &mut self,
539 scroll_top: &ListOffset,
540 height: Pixels,
541 delta: Point<Pixels>,
542 current_view: EntityId,
543 window: &mut Window,
544 cx: &mut App,
545 ) {
546 // Drop scroll events after a reset, since we can't calculate
547 // the new logical scroll top without the item heights
548 if self.reset {
549 return;
550 }
551
552 let padding = self.last_padding.unwrap_or_default();
553 let scroll_max =
554 (self.items.summary().height + padding.top + padding.bottom - height).max(px(0.));
555 let new_scroll_top = (self.scroll_top(scroll_top) - delta.y)
556 .max(px(0.))
557 .min(scroll_max);
558
559 if self.alignment == ListAlignment::Bottom && new_scroll_top == scroll_max {
560 self.logical_scroll_top = None;
561 } else {
562 let (start, ..) =
563 self.items
564 .find::<ListItemSummary, _>((), &Height(new_scroll_top), Bias::Right);
565 let item_ix = start.count;
566 let offset_in_item = new_scroll_top - start.height;
567 self.logical_scroll_top = Some(ListOffset {
568 item_ix,
569 offset_in_item,
570 });
571 }
572
573 if self.scroll_handler.is_some() {
574 let visible_range = self.visible_range(height, scroll_top);
575 self.scroll_handler.as_mut().unwrap()(
576 &ListScrollEvent {
577 visible_range,
578 count: self.items.summary().count,
579 is_scrolled: self.logical_scroll_top.is_some(),
580 },
581 window,
582 cx,
583 );
584 }
585
586 cx.notify(current_view);
587 }
588
589 fn logical_scroll_top(&self) -> ListOffset {
590 self.logical_scroll_top
591 .unwrap_or_else(|| match self.alignment {
592 ListAlignment::Top => ListOffset {
593 item_ix: 0,
594 offset_in_item: px(0.),
595 },
596 ListAlignment::Bottom => ListOffset {
597 item_ix: self.items.summary().count,
598 offset_in_item: px(0.),
599 },
600 })
601 }
602
603 fn scroll_top(&self, logical_scroll_top: &ListOffset) -> Pixels {
604 let (start, ..) = self.items.find::<ListItemSummary, _>(
605 (),
606 &Count(logical_scroll_top.item_ix),
607 Bias::Right,
608 );
609 start.height + logical_scroll_top.offset_in_item
610 }
611
612 fn layout_all_items(
613 &mut self,
614 available_width: Pixels,
615 render_item: &mut RenderItemFn,
616 window: &mut Window,
617 cx: &mut App,
618 ) {
619 match &mut self.measuring_behavior {
620 ListMeasuringBehavior::Visible => {
621 return;
622 }
623 ListMeasuringBehavior::Measure(has_measured) => {
624 if *has_measured {
625 return;
626 }
627 *has_measured = true;
628 }
629 }
630
631 let mut cursor = self.items.cursor::<Count>(());
632 let available_item_space = size(
633 AvailableSpace::Definite(available_width),
634 AvailableSpace::MinContent,
635 );
636
637 let mut measured_items = Vec::default();
638
639 for (ix, item) in cursor.enumerate() {
640 let size = item.size().unwrap_or_else(|| {
641 let mut element = render_item(ix, window, cx);
642 element.layout_as_root(available_item_space, window, cx)
643 });
644
645 measured_items.push(ListItem::Measured {
646 size,
647 focus_handle: item.focus_handle(),
648 });
649 }
650
651 self.items = SumTree::from_iter(measured_items, ());
652 }
653
654 fn layout_items(
655 &mut self,
656 available_width: Option<Pixels>,
657 available_height: Pixels,
658 padding: &Edges<Pixels>,
659 render_item: &mut RenderItemFn,
660 window: &mut Window,
661 cx: &mut App,
662 ) -> LayoutItemsResponse {
663 let old_items = self.items.clone();
664 let mut measured_items = VecDeque::new();
665 let mut item_layouts = VecDeque::new();
666 let mut rendered_height = padding.top;
667 let mut max_item_width = px(0.);
668 let mut scroll_top = self.logical_scroll_top();
669 let mut rendered_focused_item = false;
670
671 let available_item_space = size(
672 available_width.map_or(AvailableSpace::MinContent, |width| {
673 AvailableSpace::Definite(width)
674 }),
675 AvailableSpace::MinContent,
676 );
677
678 let mut cursor = old_items.cursor::<Count>(());
679
680 // Render items after the scroll top, including those in the trailing overdraw
681 cursor.seek(&Count(scroll_top.item_ix), Bias::Right);
682 for (ix, item) in cursor.by_ref().enumerate() {
683 let visible_height = rendered_height - scroll_top.offset_in_item;
684 if visible_height >= available_height + self.overdraw {
685 break;
686 }
687
688 // Use the previously cached height and focus handle if available
689 let mut size = item.size();
690
691 // If we're within the visible area or the height wasn't cached, render and measure the item's element
692 if visible_height < available_height || size.is_none() {
693 let item_index = scroll_top.item_ix + ix;
694 let mut element = render_item(item_index, window, cx);
695 let element_size = element.layout_as_root(available_item_space, window, cx);
696 size = Some(element_size);
697
698 // If there's a pending scroll adjustment for the scroll-top
699 // item, apply it, ensuring proportional scroll position is
700 // maintained after re-measuring.
701 if ix == 0 {
702 if let Some(pending_scroll) = self.pending_scroll.take() {
703 if pending_scroll.item_ix == scroll_top.item_ix {
704 scroll_top.offset_in_item =
705 Pixels(pending_scroll.fraction * element_size.height.0);
706 self.logical_scroll_top = Some(scroll_top);
707 }
708 }
709 }
710
711 if visible_height < available_height {
712 item_layouts.push_back(ItemLayout {
713 index: item_index,
714 element,
715 size: element_size,
716 });
717 if item.contains_focused(window, cx) {
718 rendered_focused_item = true;
719 }
720 }
721 }
722
723 let size = size.unwrap();
724 rendered_height += size.height;
725 max_item_width = max_item_width.max(size.width);
726 measured_items.push_back(ListItem::Measured {
727 size,
728 focus_handle: item.focus_handle(),
729 });
730 }
731 rendered_height += padding.bottom;
732
733 // Prepare to start walking upward from the item at the scroll top.
734 cursor.seek(&Count(scroll_top.item_ix), Bias::Right);
735
736 // If the rendered items do not fill the visible region, then adjust
737 // the scroll top upward.
738 if rendered_height - scroll_top.offset_in_item < available_height {
739 while rendered_height < available_height {
740 cursor.prev();
741 if let Some(item) = cursor.item() {
742 let item_index = cursor.start().0;
743 let mut element = render_item(item_index, window, cx);
744 let element_size = element.layout_as_root(available_item_space, window, cx);
745 let focus_handle = item.focus_handle();
746 rendered_height += element_size.height;
747 measured_items.push_front(ListItem::Measured {
748 size: element_size,
749 focus_handle,
750 });
751 item_layouts.push_front(ItemLayout {
752 index: item_index,
753 element,
754 size: element_size,
755 });
756 if item.contains_focused(window, cx) {
757 rendered_focused_item = true;
758 }
759 } else {
760 break;
761 }
762 }
763
764 scroll_top = ListOffset {
765 item_ix: cursor.start().0,
766 offset_in_item: rendered_height - available_height,
767 };
768
769 match self.alignment {
770 ListAlignment::Top => {
771 scroll_top.offset_in_item = scroll_top.offset_in_item.max(px(0.));
772 self.logical_scroll_top = Some(scroll_top);
773 }
774 ListAlignment::Bottom => {
775 scroll_top = ListOffset {
776 item_ix: cursor.start().0,
777 offset_in_item: rendered_height - available_height,
778 };
779 self.logical_scroll_top = None;
780 }
781 };
782 }
783
784 // Measure items in the leading overdraw
785 let mut leading_overdraw = scroll_top.offset_in_item;
786 while leading_overdraw < self.overdraw {
787 cursor.prev();
788 if let Some(item) = cursor.item() {
789 let size = if let ListItem::Measured { size, .. } = item {
790 *size
791 } else {
792 let mut element = render_item(cursor.start().0, window, cx);
793 element.layout_as_root(available_item_space, window, cx)
794 };
795
796 leading_overdraw += size.height;
797 measured_items.push_front(ListItem::Measured {
798 size,
799 focus_handle: item.focus_handle(),
800 });
801 } else {
802 break;
803 }
804 }
805
806 let measured_range = cursor.start().0..(cursor.start().0 + measured_items.len());
807 let mut cursor = old_items.cursor::<Count>(());
808 let mut new_items = cursor.slice(&Count(measured_range.start), Bias::Right);
809 new_items.extend(measured_items, ());
810 cursor.seek(&Count(measured_range.end), Bias::Right);
811 new_items.append(cursor.suffix(), ());
812 self.items = new_items;
813
814 // If none of the visible items are focused, check if an off-screen item is focused
815 // and include it to be rendered after the visible items so keyboard interaction continues
816 // to work for it.
817 if !rendered_focused_item {
818 let mut cursor = self
819 .items
820 .filter::<_, Count>((), |summary| summary.has_focus_handles);
821 cursor.next();
822 while let Some(item) = cursor.item() {
823 if item.contains_focused(window, cx) {
824 let item_index = cursor.start().0;
825 let mut element = render_item(cursor.start().0, window, cx);
826 let size = element.layout_as_root(available_item_space, window, cx);
827 item_layouts.push_back(ItemLayout {
828 index: item_index,
829 element,
830 size,
831 });
832 break;
833 }
834 cursor.next();
835 }
836 }
837
838 LayoutItemsResponse {
839 max_item_width,
840 scroll_top,
841 item_layouts,
842 }
843 }
844
845 fn prepaint_items(
846 &mut self,
847 bounds: Bounds<Pixels>,
848 padding: Edges<Pixels>,
849 autoscroll: bool,
850 render_item: &mut RenderItemFn,
851 window: &mut Window,
852 cx: &mut App,
853 ) -> Result<LayoutItemsResponse, ListOffset> {
854 window.transact(|window| {
855 match self.measuring_behavior {
856 ListMeasuringBehavior::Measure(has_measured) if !has_measured => {
857 self.layout_all_items(bounds.size.width, render_item, window, cx);
858 }
859 _ => {}
860 }
861
862 let mut layout_response = self.layout_items(
863 Some(bounds.size.width),
864 bounds.size.height,
865 &padding,
866 render_item,
867 window,
868 cx,
869 );
870
871 // Avoid honoring autoscroll requests from elements other than our children.
872 window.take_autoscroll();
873
874 // Only paint the visible items, if there is actually any space for them (taking padding into account)
875 if bounds.size.height > padding.top + padding.bottom {
876 let mut item_origin = bounds.origin + Point::new(px(0.), padding.top);
877 item_origin.y -= layout_response.scroll_top.offset_in_item;
878 for item in &mut layout_response.item_layouts {
879 window.with_content_mask(Some(ContentMask { bounds }), |window| {
880 item.element.prepaint_at(item_origin, window, cx);
881 });
882
883 if let Some(autoscroll_bounds) = window.take_autoscroll()
884 && autoscroll
885 {
886 if autoscroll_bounds.top() < bounds.top() {
887 return Err(ListOffset {
888 item_ix: item.index,
889 offset_in_item: autoscroll_bounds.top() - item_origin.y,
890 });
891 } else if autoscroll_bounds.bottom() > bounds.bottom() {
892 let mut cursor = self.items.cursor::<Count>(());
893 cursor.seek(&Count(item.index), Bias::Right);
894 let mut height = bounds.size.height - padding.top - padding.bottom;
895
896 // Account for the height of the element down until the autoscroll bottom.
897 height -= autoscroll_bounds.bottom() - item_origin.y;
898
899 // Keep decreasing the scroll top until we fill all the available space.
900 while height > Pixels::ZERO {
901 cursor.prev();
902 let Some(item) = cursor.item() else { break };
903
904 let size = item.size().unwrap_or_else(|| {
905 let mut item = render_item(cursor.start().0, window, cx);
906 let item_available_size =
907 size(bounds.size.width.into(), AvailableSpace::MinContent);
908 item.layout_as_root(item_available_size, window, cx)
909 });
910 height -= size.height;
911 }
912
913 return Err(ListOffset {
914 item_ix: cursor.start().0,
915 offset_in_item: if height < Pixels::ZERO {
916 -height
917 } else {
918 Pixels::ZERO
919 },
920 });
921 }
922 }
923
924 item_origin.y += item.size.height;
925 }
926 } else {
927 layout_response.item_layouts.clear();
928 }
929
930 Ok(layout_response)
931 })
932 }
933
934 // Scrollbar support
935
936 fn set_offset_from_scrollbar(&mut self, point: Point<Pixels>) {
937 let Some(bounds) = self.last_layout_bounds else {
938 return;
939 };
940 let height = bounds.size.height;
941
942 let padding = self.last_padding.unwrap_or_default();
943 let content_height = self.items.summary().height;
944 let scroll_max = (content_height + padding.top + padding.bottom - height).max(px(0.));
945 let drag_offset =
946 // if dragging the scrollbar, we want to offset the point if the height changed
947 content_height - self.scrollbar_drag_start_height.unwrap_or(content_height);
948 let new_scroll_top = (point.y - drag_offset).abs().max(px(0.)).min(scroll_max);
949
950 if self.alignment == ListAlignment::Bottom && new_scroll_top == scroll_max {
951 self.logical_scroll_top = None;
952 } else {
953 let (start, _, _) =
954 self.items
955 .find::<ListItemSummary, _>((), &Height(new_scroll_top), Bias::Right);
956
957 let item_ix = start.count;
958 let offset_in_item = new_scroll_top - start.height;
959 self.logical_scroll_top = Some(ListOffset {
960 item_ix,
961 offset_in_item,
962 });
963 }
964 }
965}
966
967impl std::fmt::Debug for ListItem {
968 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
969 match self {
970 Self::Unmeasured { .. } => write!(f, "Unrendered"),
971 Self::Measured { size, .. } => f.debug_struct("Rendered").field("size", size).finish(),
972 }
973 }
974}
975
976/// An offset into the list's items, in terms of the item index and the number
977/// of pixels off the top left of the item.
978#[derive(Debug, Clone, Copy, Default)]
979pub struct ListOffset {
980 /// The index of an item in the list
981 pub item_ix: usize,
982 /// The number of pixels to offset from the item index.
983 pub offset_in_item: Pixels,
984}
985
986impl Element for List {
987 type RequestLayoutState = ();
988 type PrepaintState = ListPrepaintState;
989
990 fn id(&self) -> Option<crate::ElementId> {
991 None
992 }
993
994 fn source_location(&self) -> Option<&'static core::panic::Location<'static>> {
995 None
996 }
997
998 fn request_layout(
999 &mut self,
1000 _id: Option<&GlobalElementId>,
1001 _inspector_id: Option<&InspectorElementId>,
1002 window: &mut Window,
1003 cx: &mut App,
1004 ) -> (crate::LayoutId, Self::RequestLayoutState) {
1005 let layout_id = match self.sizing_behavior {
1006 ListSizingBehavior::Infer => {
1007 let mut style = Style::default();
1008 style.overflow.y = Overflow::Scroll;
1009 style.refine(&self.style);
1010 window.with_text_style(style.text_style().cloned(), |window| {
1011 let state = &mut *self.state.0.borrow_mut();
1012
1013 let available_height = if let Some(last_bounds) = state.last_layout_bounds {
1014 last_bounds.size.height
1015 } else {
1016 // If we don't have the last layout bounds (first render),
1017 // we might just use the overdraw value as the available height to layout enough items.
1018 state.overdraw
1019 };
1020 let padding = style.padding.to_pixels(
1021 state.last_layout_bounds.unwrap_or_default().size.into(),
1022 window.rem_size(),
1023 );
1024
1025 let layout_response = state.layout_items(
1026 None,
1027 available_height,
1028 &padding,
1029 &mut self.render_item,
1030 window,
1031 cx,
1032 );
1033 let max_element_width = layout_response.max_item_width;
1034
1035 let summary = state.items.summary();
1036 let total_height = summary.height;
1037
1038 window.request_measured_layout(
1039 style,
1040 move |known_dimensions, available_space, _window, _cx| {
1041 let width =
1042 known_dimensions
1043 .width
1044 .unwrap_or(match available_space.width {
1045 AvailableSpace::Definite(x) => x,
1046 AvailableSpace::MinContent | AvailableSpace::MaxContent => {
1047 max_element_width
1048 }
1049 });
1050 let height = match available_space.height {
1051 AvailableSpace::Definite(height) => total_height.min(height),
1052 AvailableSpace::MinContent | AvailableSpace::MaxContent => {
1053 total_height
1054 }
1055 };
1056 size(width, height)
1057 },
1058 )
1059 })
1060 }
1061 ListSizingBehavior::Auto => {
1062 let mut style = Style::default();
1063 style.refine(&self.style);
1064 window.with_text_style(style.text_style().cloned(), |window| {
1065 window.request_layout(style, None, cx)
1066 })
1067 }
1068 };
1069 (layout_id, ())
1070 }
1071
1072 fn prepaint(
1073 &mut self,
1074 _id: Option<&GlobalElementId>,
1075 _inspector_id: Option<&InspectorElementId>,
1076 bounds: Bounds<Pixels>,
1077 _: &mut Self::RequestLayoutState,
1078 window: &mut Window,
1079 cx: &mut App,
1080 ) -> ListPrepaintState {
1081 let state = &mut *self.state.0.borrow_mut();
1082 state.reset = false;
1083
1084 let mut style = Style::default();
1085 style.refine(&self.style);
1086
1087 let hitbox = window.insert_hitbox(bounds, HitboxBehavior::Normal);
1088
1089 // If the width of the list has changed, invalidate all cached item heights
1090 if state
1091 .last_layout_bounds
1092 .is_none_or(|last_bounds| last_bounds.size.width != bounds.size.width)
1093 {
1094 let new_items = SumTree::from_iter(
1095 state.items.iter().map(|item| ListItem::Unmeasured {
1096 focus_handle: item.focus_handle(),
1097 }),
1098 (),
1099 );
1100
1101 state.items = new_items;
1102 }
1103
1104 let padding = style
1105 .padding
1106 .to_pixels(bounds.size.into(), window.rem_size());
1107 let layout =
1108 match state.prepaint_items(bounds, padding, true, &mut self.render_item, window, cx) {
1109 Ok(layout) => layout,
1110 Err(autoscroll_request) => {
1111 state.logical_scroll_top = Some(autoscroll_request);
1112 state
1113 .prepaint_items(bounds, padding, false, &mut self.render_item, window, cx)
1114 .unwrap()
1115 }
1116 };
1117
1118 state.last_layout_bounds = Some(bounds);
1119 state.last_padding = Some(padding);
1120 ListPrepaintState { hitbox, layout }
1121 }
1122
1123 fn paint(
1124 &mut self,
1125 _id: Option<&GlobalElementId>,
1126 _inspector_id: Option<&InspectorElementId>,
1127 bounds: Bounds<crate::Pixels>,
1128 _: &mut Self::RequestLayoutState,
1129 prepaint: &mut Self::PrepaintState,
1130 window: &mut Window,
1131 cx: &mut App,
1132 ) {
1133 let current_view = window.current_view();
1134 window.with_content_mask(Some(ContentMask { bounds }), |window| {
1135 for item in &mut prepaint.layout.item_layouts {
1136 item.element.paint(window, cx);
1137 }
1138 });
1139
1140 let list_state = self.state.clone();
1141 let height = bounds.size.height;
1142 let scroll_top = prepaint.layout.scroll_top;
1143 let hitbox_id = prepaint.hitbox.id;
1144 let mut accumulated_scroll_delta = ScrollDelta::default();
1145 window.on_mouse_event(move |event: &ScrollWheelEvent, phase, window, cx| {
1146 if phase == DispatchPhase::Bubble && hitbox_id.should_handle_scroll(window) {
1147 accumulated_scroll_delta = accumulated_scroll_delta.coalesce(event.delta);
1148 let pixel_delta = accumulated_scroll_delta.pixel_delta(px(20.));
1149 list_state.0.borrow_mut().scroll(
1150 &scroll_top,
1151 height,
1152 pixel_delta,
1153 current_view,
1154 window,
1155 cx,
1156 )
1157 }
1158 });
1159 }
1160}
1161
1162impl IntoElement for List {
1163 type Element = Self;
1164
1165 fn into_element(self) -> Self::Element {
1166 self
1167 }
1168}
1169
1170impl Styled for List {
1171 fn style(&mut self) -> &mut StyleRefinement {
1172 &mut self.style
1173 }
1174}
1175
1176impl sum_tree::Item for ListItem {
1177 type Summary = ListItemSummary;
1178
1179 fn summary(&self, _: ()) -> Self::Summary {
1180 match self {
1181 ListItem::Unmeasured { focus_handle } => ListItemSummary {
1182 count: 1,
1183 rendered_count: 0,
1184 unrendered_count: 1,
1185 height: px(0.),
1186 has_focus_handles: focus_handle.is_some(),
1187 },
1188 ListItem::Measured {
1189 size, focus_handle, ..
1190 } => ListItemSummary {
1191 count: 1,
1192 rendered_count: 1,
1193 unrendered_count: 0,
1194 height: size.height,
1195 has_focus_handles: focus_handle.is_some(),
1196 },
1197 }
1198 }
1199}
1200
1201impl sum_tree::ContextLessSummary for ListItemSummary {
1202 fn zero() -> Self {
1203 Default::default()
1204 }
1205
1206 fn add_summary(&mut self, summary: &Self) {
1207 self.count += summary.count;
1208 self.rendered_count += summary.rendered_count;
1209 self.unrendered_count += summary.unrendered_count;
1210 self.height += summary.height;
1211 self.has_focus_handles |= summary.has_focus_handles;
1212 }
1213}
1214
1215impl<'a> sum_tree::Dimension<'a, ListItemSummary> for Count {
1216 fn zero(_cx: ()) -> Self {
1217 Default::default()
1218 }
1219
1220 fn add_summary(&mut self, summary: &'a ListItemSummary, _: ()) {
1221 self.0 += summary.count;
1222 }
1223}
1224
1225impl<'a> sum_tree::Dimension<'a, ListItemSummary> for Height {
1226 fn zero(_cx: ()) -> Self {
1227 Default::default()
1228 }
1229
1230 fn add_summary(&mut self, summary: &'a ListItemSummary, _: ()) {
1231 self.0 += summary.height;
1232 }
1233}
1234
1235impl sum_tree::SeekTarget<'_, ListItemSummary, ListItemSummary> for Count {
1236 fn cmp(&self, other: &ListItemSummary, _: ()) -> std::cmp::Ordering {
1237 self.0.partial_cmp(&other.count).unwrap()
1238 }
1239}
1240
1241impl sum_tree::SeekTarget<'_, ListItemSummary, ListItemSummary> for Height {
1242 fn cmp(&self, other: &ListItemSummary, _: ()) -> std::cmp::Ordering {
1243 self.0.partial_cmp(&other.height).unwrap()
1244 }
1245}
1246
1247#[cfg(test)]
1248mod test {
1249
1250 use gpui::{ScrollDelta, ScrollWheelEvent};
1251 use std::cell::Cell;
1252 use std::rc::Rc;
1253
1254 use crate::{
1255 self as gpui, AppContext, Context, Element, IntoElement, ListState, Render, Styled,
1256 TestAppContext, Window, div, list, point, px, size,
1257 };
1258
1259 #[gpui::test]
1260 fn test_reset_after_paint_before_scroll(cx: &mut TestAppContext) {
1261 let cx = cx.add_empty_window();
1262
1263 let state = ListState::new(5, crate::ListAlignment::Top, px(10.));
1264
1265 // Ensure that the list is scrolled to the top
1266 state.scroll_to(gpui::ListOffset {
1267 item_ix: 0,
1268 offset_in_item: px(0.0),
1269 });
1270
1271 struct TestView(ListState);
1272 impl Render for TestView {
1273 fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
1274 list(self.0.clone(), |_, _, _| {
1275 div().h(px(10.)).w_full().into_any()
1276 })
1277 .w_full()
1278 .h_full()
1279 }
1280 }
1281
1282 // Paint
1283 cx.draw(point(px(0.), px(0.)), size(px(100.), px(20.)), |_, cx| {
1284 cx.new(|_| TestView(state.clone()))
1285 });
1286
1287 // Reset
1288 state.reset(5);
1289
1290 // And then receive a scroll event _before_ the next paint
1291 cx.simulate_event(ScrollWheelEvent {
1292 position: point(px(1.), px(1.)),
1293 delta: ScrollDelta::Pixels(point(px(0.), px(-500.))),
1294 ..Default::default()
1295 });
1296
1297 // Scroll position should stay at the top of the list
1298 assert_eq!(state.logical_scroll_top().item_ix, 0);
1299 assert_eq!(state.logical_scroll_top().offset_in_item, px(0.));
1300 }
1301
1302 #[gpui::test]
1303 fn test_scroll_by_positive_and_negative_distance(cx: &mut TestAppContext) {
1304 let cx = cx.add_empty_window();
1305
1306 let state = ListState::new(5, crate::ListAlignment::Top, px(10.));
1307
1308 struct TestView(ListState);
1309 impl Render for TestView {
1310 fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
1311 list(self.0.clone(), |_, _, _| {
1312 div().h(px(20.)).w_full().into_any()
1313 })
1314 .w_full()
1315 .h_full()
1316 }
1317 }
1318
1319 // Paint
1320 cx.draw(point(px(0.), px(0.)), size(px(100.), px(100.)), |_, cx| {
1321 cx.new(|_| TestView(state.clone()))
1322 });
1323
1324 // Test positive distance: start at item 1, move down 30px
1325 state.scroll_by(px(30.));
1326
1327 // Should move to item 2
1328 let offset = state.logical_scroll_top();
1329 assert_eq!(offset.item_ix, 1);
1330 assert_eq!(offset.offset_in_item, px(10.));
1331
1332 // Test negative distance: start at item 2, move up 30px
1333 state.scroll_by(px(-30.));
1334
1335 // Should move back to item 1
1336 let offset = state.logical_scroll_top();
1337 assert_eq!(offset.item_ix, 0);
1338 assert_eq!(offset.offset_in_item, px(0.));
1339
1340 // Test zero distance
1341 state.scroll_by(px(0.));
1342 let offset = state.logical_scroll_top();
1343 assert_eq!(offset.item_ix, 0);
1344 assert_eq!(offset.offset_in_item, px(0.));
1345 }
1346
1347 #[gpui::test]
1348 fn test_remeasure(cx: &mut TestAppContext) {
1349 let cx = cx.add_empty_window();
1350
1351 // Create a list with 10 items, each 100px tall. We'll keep a reference
1352 // to the item height so we can later change the height and assert how
1353 // `ListState` handles it.
1354 let item_height = Rc::new(Cell::new(100usize));
1355 let state = ListState::new(10, crate::ListAlignment::Top, px(10.));
1356
1357 struct TestView {
1358 state: ListState,
1359 item_height: Rc<Cell<usize>>,
1360 }
1361
1362 impl Render for TestView {
1363 fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
1364 let height = self.item_height.get();
1365 list(self.state.clone(), move |_, _, _| {
1366 div().h(px(height as f32)).w_full().into_any()
1367 })
1368 .w_full()
1369 .h_full()
1370 }
1371 }
1372
1373 let state_clone = state.clone();
1374 let item_height_clone = item_height.clone();
1375 let view = cx.update(|_, cx| {
1376 cx.new(|_| TestView {
1377 state: state_clone,
1378 item_height: item_height_clone,
1379 })
1380 });
1381
1382 // Simulate scrolling 40px inside the element with index 2. Since the
1383 // original item height is 100px, this equates to 40% inside the item.
1384 state.scroll_to(gpui::ListOffset {
1385 item_ix: 2,
1386 offset_in_item: px(40.),
1387 });
1388
1389 cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| {
1390 view.clone()
1391 });
1392
1393 let offset = state.logical_scroll_top();
1394 assert_eq!(offset.item_ix, 2);
1395 assert_eq!(offset.offset_in_item, px(40.));
1396
1397 // Update the `item_height` to be 50px instead of 100px so we can assert
1398 // that the scroll position is proportionally preserved, that is,
1399 // instead of 40px from the top of item 2, it should be 20px, since the
1400 // item's height has been halved.
1401 item_height.set(50);
1402 state.remeasure();
1403
1404 cx.draw(point(px(0.), px(0.)), size(px(100.), px(200.)), |_, _| view);
1405
1406 let offset = state.logical_scroll_top();
1407 assert_eq!(offset.item_ix, 2);
1408 assert_eq!(offset.offset_in_item, px(20.));
1409 }
1410}