1//! A scrollable list of elements with uniform height, optimized for large lists.
2//! Rather than use the full taffy layout system, uniform_list simply measures
3//! the first element and then lays out all remaining elements in a line based on that
4//! measurement. This is much faster than the full layout system, but only works for
5//! elements with uniform height.
6
7use crate::{
8 AnyElement, App, AvailableSpace, Bounds, ContentMask, Element, ElementId, GlobalElementId,
9 Hitbox, InspectorElementId, InteractiveElement, Interactivity, IntoElement, IsZero, LayoutId,
10 ListSizingBehavior, Overflow, Pixels, Point, ScrollHandle, Size, StyleRefinement, Styled,
11 Window, point, size,
12};
13use smallvec::SmallVec;
14use std::{cell::RefCell, cmp, ops::Range, rc::Rc};
15
16use super::ListHorizontalSizingBehavior;
17
18/// uniform_list provides lazy rendering for a set of items that are of uniform height.
19/// When rendered into a container with overflow-y: hidden and a fixed (or max) height,
20/// uniform_list will only render the visible subset of items.
21#[track_caller]
22pub fn uniform_list<R>(
23 id: impl Into<ElementId>,
24 item_count: usize,
25 f: impl 'static + Fn(Range<usize>, &mut Window, &mut App) -> Vec<R>,
26) -> UniformList
27where
28 R: IntoElement,
29{
30 let id = id.into();
31 let mut base_style = StyleRefinement::default();
32 base_style.overflow.y = Some(Overflow::Scroll);
33
34 let render_range = move |range: Range<usize>, window: &mut Window, cx: &mut App| {
35 f(range, window, cx)
36 .into_iter()
37 .map(|component| component.into_any_element())
38 .collect()
39 };
40
41 UniformList {
42 item_count,
43 item_to_measure_index: 0,
44 render_items: Box::new(render_range),
45 decorations: Vec::new(),
46 interactivity: Interactivity {
47 element_id: Some(id),
48 base_style: Box::new(base_style),
49 ..Interactivity::new()
50 },
51 scroll_handle: None,
52 sizing_behavior: ListSizingBehavior::default(),
53 horizontal_sizing_behavior: ListHorizontalSizingBehavior::default(),
54 }
55}
56
57/// A list element for efficiently laying out and displaying a list of uniform-height elements.
58pub struct UniformList {
59 item_count: usize,
60 item_to_measure_index: usize,
61 render_items: Box<
62 dyn for<'a> Fn(Range<usize>, &'a mut Window, &'a mut App) -> SmallVec<[AnyElement; 64]>,
63 >,
64 decorations: Vec<Box<dyn UniformListDecoration>>,
65 interactivity: Interactivity,
66 scroll_handle: Option<UniformListScrollHandle>,
67 sizing_behavior: ListSizingBehavior,
68 horizontal_sizing_behavior: ListHorizontalSizingBehavior,
69}
70
71/// Frame state used by the [UniformList].
72pub struct UniformListFrameState {
73 items: SmallVec<[AnyElement; 32]>,
74 decorations: SmallVec<[AnyElement; 1]>,
75}
76
77/// A handle for controlling the scroll position of a uniform list.
78/// This should be stored in your view and passed to the uniform_list on each frame.
79#[derive(Clone, Debug, Default)]
80pub struct UniformListScrollHandle(pub Rc<RefCell<UniformListScrollState>>);
81
82/// Where to place the element scrolled to.
83#[derive(Clone, Copy, Debug, PartialEq, Eq)]
84pub enum ScrollStrategy {
85 /// Place the element at the top of the list's viewport.
86 Top,
87 /// Attempt to place the element in the middle of the list's viewport.
88 /// May not be possible if there's not enough list items above the item scrolled to:
89 /// in this case, the element will be placed at the closest possible position.
90 Center,
91}
92
93#[derive(Clone, Copy, Debug)]
94#[allow(missing_docs)]
95pub struct DeferredScrollToItem {
96 /// The item index to scroll to
97 pub item_index: usize,
98 /// The scroll strategy to use
99 pub strategy: ScrollStrategy,
100 /// The offset in number of items
101 pub offset: usize,
102}
103
104#[derive(Clone, Debug, Default)]
105#[allow(missing_docs)]
106pub struct UniformListScrollState {
107 pub base_handle: ScrollHandle,
108 pub deferred_scroll_to_item: Option<DeferredScrollToItem>,
109 /// Size of the item, captured during last layout.
110 pub last_item_size: Option<ItemSize>,
111 /// Whether the list was vertically flipped during last layout.
112 pub y_flipped: bool,
113}
114
115#[derive(Copy, Clone, Debug, Default)]
116/// The size of the item and its contents.
117pub struct ItemSize {
118 /// The size of the item.
119 pub item: Size<Pixels>,
120 /// The size of the item's contents, which may be larger than the item itself,
121 /// if the item was bounded by a parent element.
122 pub contents: Size<Pixels>,
123}
124
125impl UniformListScrollHandle {
126 /// Create a new scroll handle to bind to a uniform list.
127 pub fn new() -> Self {
128 Self(Rc::new(RefCell::new(UniformListScrollState {
129 base_handle: ScrollHandle::new(),
130 deferred_scroll_to_item: None,
131 last_item_size: None,
132 y_flipped: false,
133 })))
134 }
135
136 /// Scroll the list to the given item index.
137 pub fn scroll_to_item(&self, ix: usize, strategy: ScrollStrategy) {
138 self.0.borrow_mut().deferred_scroll_to_item = Some(DeferredScrollToItem {
139 item_index: ix,
140 strategy,
141 offset: 0,
142 });
143 }
144
145 /// Scroll the list to the given item index with an offset.
146 ///
147 /// For ScrollStrategy::Top, the item will be placed at the offset position from the top.
148 ///
149 /// For ScrollStrategy::Center, the item will be centered between offset and the last visible item.
150 pub fn scroll_to_item_with_offset(&self, ix: usize, strategy: ScrollStrategy, offset: usize) {
151 self.0.borrow_mut().deferred_scroll_to_item = Some(DeferredScrollToItem {
152 item_index: ix,
153 strategy,
154 offset,
155 });
156 }
157
158 /// Check if the list is flipped vertically.
159 pub fn y_flipped(&self) -> bool {
160 self.0.borrow().y_flipped
161 }
162
163 /// Get the index of the topmost visible child.
164 #[cfg(any(test, feature = "test-support"))]
165 pub fn logical_scroll_top_index(&self) -> usize {
166 let this = self.0.borrow();
167 this.deferred_scroll_to_item
168 .as_ref()
169 .map(|deferred| deferred.item_index)
170 .unwrap_or_else(|| this.base_handle.logical_scroll_top().0)
171 }
172
173 /// Checks if the list can be scrolled vertically.
174 pub fn is_scrollable(&self) -> bool {
175 if let Some(size) = self.0.borrow().last_item_size {
176 size.contents.height > size.item.height
177 } else {
178 false
179 }
180 }
181}
182
183impl Styled for UniformList {
184 fn style(&mut self) -> &mut StyleRefinement {
185 &mut self.interactivity.base_style
186 }
187}
188
189impl Element for UniformList {
190 type RequestLayoutState = UniformListFrameState;
191 type PrepaintState = Option<Hitbox>;
192
193 fn id(&self) -> Option<ElementId> {
194 self.interactivity.element_id.clone()
195 }
196
197 fn source_location(&self) -> Option<&'static core::panic::Location<'static>> {
198 None
199 }
200
201 fn request_layout(
202 &mut self,
203 global_id: Option<&GlobalElementId>,
204 inspector_id: Option<&InspectorElementId>,
205 window: &mut Window,
206 cx: &mut App,
207 ) -> (LayoutId, Self::RequestLayoutState) {
208 let max_items = self.item_count;
209 let item_size = self.measure_item(None, window, cx);
210 let layout_id = self.interactivity.request_layout(
211 global_id,
212 inspector_id,
213 window,
214 cx,
215 |style, window, cx| match self.sizing_behavior {
216 ListSizingBehavior::Infer => {
217 window.with_text_style(style.text_style().cloned(), |window| {
218 window.request_measured_layout(
219 style,
220 move |known_dimensions, available_space, _window, _cx| {
221 let desired_height = item_size.height * max_items;
222 let width = known_dimensions.width.unwrap_or(match available_space
223 .width
224 {
225 AvailableSpace::Definite(x) => x,
226 AvailableSpace::MinContent | AvailableSpace::MaxContent => {
227 item_size.width
228 }
229 });
230 let height = match available_space.height {
231 AvailableSpace::Definite(height) => desired_height.min(height),
232 AvailableSpace::MinContent | AvailableSpace::MaxContent => {
233 desired_height
234 }
235 };
236 size(width, height)
237 },
238 )
239 })
240 }
241 ListSizingBehavior::Auto => window
242 .with_text_style(style.text_style().cloned(), |window| {
243 window.request_layout(style, None, cx)
244 }),
245 },
246 );
247
248 (
249 layout_id,
250 UniformListFrameState {
251 items: SmallVec::new(),
252 decorations: SmallVec::new(),
253 },
254 )
255 }
256
257 fn prepaint(
258 &mut self,
259 global_id: Option<&GlobalElementId>,
260 inspector_id: Option<&InspectorElementId>,
261 bounds: Bounds<Pixels>,
262 frame_state: &mut Self::RequestLayoutState,
263 window: &mut Window,
264 cx: &mut App,
265 ) -> Option<Hitbox> {
266 let style = self
267 .interactivity
268 .compute_style(global_id, None, window, cx);
269 let border = style.border_widths.to_pixels(window.rem_size());
270 let padding = style
271 .padding
272 .to_pixels(bounds.size.into(), window.rem_size());
273
274 let padded_bounds = Bounds::from_corners(
275 bounds.origin + point(border.left + padding.left, border.top + padding.top),
276 bounds.bottom_right()
277 - point(border.right + padding.right, border.bottom + padding.bottom),
278 );
279
280 let can_scroll_horizontally = matches!(
281 self.horizontal_sizing_behavior,
282 ListHorizontalSizingBehavior::Unconstrained
283 );
284
285 let longest_item_size = self.measure_item(None, window, cx);
286 let content_width = if can_scroll_horizontally {
287 padded_bounds.size.width.max(longest_item_size.width)
288 } else {
289 padded_bounds.size.width
290 };
291 let content_size = Size {
292 width: content_width,
293 height: longest_item_size.height * self.item_count + padding.top + padding.bottom,
294 };
295
296 let shared_scroll_offset = self.interactivity.scroll_offset.clone().unwrap();
297 let item_height = longest_item_size.height;
298 let shared_scroll_to_item = self.scroll_handle.as_mut().and_then(|handle| {
299 let mut handle = handle.0.borrow_mut();
300 handle.last_item_size = Some(ItemSize {
301 item: padded_bounds.size,
302 contents: content_size,
303 });
304 handle.deferred_scroll_to_item.take()
305 });
306
307 self.interactivity.prepaint(
308 global_id,
309 inspector_id,
310 bounds,
311 content_size,
312 window,
313 cx,
314 |style, mut scroll_offset, hitbox, window, cx| {
315 let border = style.border_widths.to_pixels(window.rem_size());
316 let padding = style
317 .padding
318 .to_pixels(bounds.size.into(), window.rem_size());
319
320 let padded_bounds = Bounds::from_corners(
321 bounds.origin + point(border.left + padding.left, border.top),
322 bounds.bottom_right() - point(border.right + padding.right, border.bottom),
323 );
324
325 let y_flipped = if let Some(scroll_handle) = self.scroll_handle.as_mut() {
326 let mut scroll_state = scroll_handle.0.borrow_mut();
327 scroll_state.base_handle.set_bounds(bounds);
328 scroll_state.y_flipped
329 } else {
330 false
331 };
332
333 if self.item_count > 0 {
334 let content_height =
335 item_height * self.item_count + padding.top + padding.bottom;
336 let is_scrolled_vertically = !scroll_offset.y.is_zero();
337 let min_vertical_scroll_offset = padded_bounds.size.height - content_height;
338 if is_scrolled_vertically && scroll_offset.y < min_vertical_scroll_offset {
339 shared_scroll_offset.borrow_mut().y = min_vertical_scroll_offset;
340 scroll_offset.y = min_vertical_scroll_offset;
341 }
342
343 let content_width = content_size.width + padding.left + padding.right;
344 let is_scrolled_horizontally =
345 can_scroll_horizontally && !scroll_offset.x.is_zero();
346 if is_scrolled_horizontally && content_width <= padded_bounds.size.width {
347 shared_scroll_offset.borrow_mut().x = Pixels::ZERO;
348 scroll_offset.x = Pixels::ZERO;
349 }
350
351 if let Some(deferred_scroll) = shared_scroll_to_item {
352 let mut ix = deferred_scroll.item_index;
353 if y_flipped {
354 ix = self.item_count.saturating_sub(ix + 1);
355 }
356 let list_height = padded_bounds.size.height;
357 let mut updated_scroll_offset = shared_scroll_offset.borrow_mut();
358 let item_top = item_height * ix + padding.top;
359 let item_bottom = item_top + item_height;
360 let scroll_top = -updated_scroll_offset.y;
361 let offset_pixels = item_height * deferred_scroll.offset;
362 let mut scrolled_to_top = false;
363
364 if item_top < scroll_top + padding.top + offset_pixels {
365 scrolled_to_top = true;
366 updated_scroll_offset.y = -(item_top) + padding.top + offset_pixels;
367 } else if item_bottom > scroll_top + list_height - padding.bottom {
368 scrolled_to_top = true;
369 updated_scroll_offset.y = -(item_bottom - list_height) - padding.bottom;
370 }
371
372 match deferred_scroll.strategy {
373 ScrollStrategy::Top => {}
374 ScrollStrategy::Center => {
375 if scrolled_to_top {
376 let item_center = item_top + item_height / 2.0;
377
378 let viewport_height = list_height - offset_pixels;
379 let viewport_center = offset_pixels + viewport_height / 2.0;
380 let target_scroll_top = item_center - viewport_center;
381
382 if item_top < scroll_top + offset_pixels
383 || item_bottom > scroll_top + list_height
384 {
385 updated_scroll_offset.y = -target_scroll_top
386 .max(Pixels::ZERO)
387 .min(content_height - list_height)
388 .max(Pixels::ZERO);
389 }
390 }
391 }
392 }
393 scroll_offset = *updated_scroll_offset
394 }
395
396 let first_visible_element_ix =
397 (-(scroll_offset.y + padding.top) / item_height).floor() as usize;
398 let last_visible_element_ix = ((-scroll_offset.y + padded_bounds.size.height)
399 / item_height)
400 .ceil() as usize;
401
402 let visible_range = first_visible_element_ix
403 ..cmp::min(last_visible_element_ix, self.item_count);
404
405 let items = if y_flipped {
406 let flipped_range = self.item_count.saturating_sub(visible_range.end)
407 ..self.item_count.saturating_sub(visible_range.start);
408 let mut items = (self.render_items)(flipped_range, window, cx);
409 items.reverse();
410 items
411 } else {
412 (self.render_items)(visible_range.clone(), window, cx)
413 };
414
415 let content_mask = ContentMask { bounds };
416 window.with_content_mask(Some(content_mask), |window| {
417 for (mut item, ix) in items.into_iter().zip(visible_range.clone()) {
418 let item_origin = padded_bounds.origin
419 + point(
420 if can_scroll_horizontally {
421 scroll_offset.x + padding.left
422 } else {
423 scroll_offset.x
424 },
425 item_height * ix + scroll_offset.y + padding.top,
426 );
427 let available_width = if can_scroll_horizontally {
428 padded_bounds.size.width + scroll_offset.x.abs()
429 } else {
430 padded_bounds.size.width
431 };
432 let available_space = size(
433 AvailableSpace::Definite(available_width),
434 AvailableSpace::Definite(item_height),
435 );
436 item.layout_as_root(available_space, window, cx);
437 item.prepaint_at(item_origin, window, cx);
438 frame_state.items.push(item);
439 }
440
441 let bounds = Bounds::new(
442 padded_bounds.origin
443 + point(
444 if can_scroll_horizontally {
445 scroll_offset.x + padding.left
446 } else {
447 scroll_offset.x
448 },
449 scroll_offset.y + padding.top,
450 ),
451 padded_bounds.size,
452 );
453 for decoration in &self.decorations {
454 let mut decoration = decoration.as_ref().compute(
455 visible_range.clone(),
456 bounds,
457 scroll_offset,
458 item_height,
459 self.item_count,
460 window,
461 cx,
462 );
463 let available_space = size(
464 AvailableSpace::Definite(bounds.size.width),
465 AvailableSpace::Definite(bounds.size.height),
466 );
467 decoration.layout_as_root(available_space, window, cx);
468 decoration.prepaint_at(bounds.origin, window, cx);
469 frame_state.decorations.push(decoration);
470 }
471 });
472 }
473
474 hitbox
475 },
476 )
477 }
478
479 fn paint(
480 &mut self,
481 global_id: Option<&GlobalElementId>,
482 inspector_id: Option<&InspectorElementId>,
483 bounds: Bounds<crate::Pixels>,
484 request_layout: &mut Self::RequestLayoutState,
485 hitbox: &mut Option<Hitbox>,
486 window: &mut Window,
487 cx: &mut App,
488 ) {
489 self.interactivity.paint(
490 global_id,
491 inspector_id,
492 bounds,
493 hitbox.as_ref(),
494 window,
495 cx,
496 |_, window, cx| {
497 for item in &mut request_layout.items {
498 item.paint(window, cx);
499 }
500 for decoration in &mut request_layout.decorations {
501 decoration.paint(window, cx);
502 }
503 },
504 )
505 }
506}
507
508impl IntoElement for UniformList {
509 type Element = Self;
510
511 fn into_element(self) -> Self::Element {
512 self
513 }
514}
515
516/// A decoration for a [`UniformList`]. This can be used for various things,
517/// such as rendering indent guides, or other visual effects.
518pub trait UniformListDecoration {
519 /// Compute the decoration element, given the visible range of list items,
520 /// the bounds of the list, and the height of each item.
521 fn compute(
522 &self,
523 visible_range: Range<usize>,
524 bounds: Bounds<Pixels>,
525 scroll_offset: Point<Pixels>,
526 item_height: Pixels,
527 item_count: usize,
528 window: &mut Window,
529 cx: &mut App,
530 ) -> AnyElement;
531}
532
533impl UniformList {
534 /// Selects a specific list item for measurement.
535 pub fn with_width_from_item(mut self, item_index: Option<usize>) -> Self {
536 self.item_to_measure_index = item_index.unwrap_or(0);
537 self
538 }
539
540 /// Sets the sizing behavior, similar to the `List` element.
541 pub fn with_sizing_behavior(mut self, behavior: ListSizingBehavior) -> Self {
542 self.sizing_behavior = behavior;
543 self
544 }
545
546 /// Sets the horizontal sizing behavior, controlling the way list items laid out horizontally.
547 /// With [`ListHorizontalSizingBehavior::Unconstrained`] behavior, every item and the list itself will
548 /// have the size of the widest item and lay out pushing the `end_slot` to the right end.
549 pub fn with_horizontal_sizing_behavior(
550 mut self,
551 behavior: ListHorizontalSizingBehavior,
552 ) -> Self {
553 self.horizontal_sizing_behavior = behavior;
554 match behavior {
555 ListHorizontalSizingBehavior::FitList => {
556 self.interactivity.base_style.overflow.x = None;
557 }
558 ListHorizontalSizingBehavior::Unconstrained => {
559 self.interactivity.base_style.overflow.x = Some(Overflow::Scroll);
560 }
561 }
562 self
563 }
564
565 /// Adds a decoration element to the list.
566 pub fn with_decoration(mut self, decoration: impl UniformListDecoration + 'static) -> Self {
567 self.decorations.push(Box::new(decoration));
568 self
569 }
570
571 fn measure_item(
572 &self,
573 list_width: Option<Pixels>,
574 window: &mut Window,
575 cx: &mut App,
576 ) -> Size<Pixels> {
577 if self.item_count == 0 {
578 return Size::default();
579 }
580
581 let item_ix = cmp::min(self.item_to_measure_index, self.item_count - 1);
582 let mut items = (self.render_items)(item_ix..item_ix + 1, window, cx);
583 let Some(mut item_to_measure) = items.pop() else {
584 return Size::default();
585 };
586 let available_space = size(
587 list_width.map_or(AvailableSpace::MinContent, |width| {
588 AvailableSpace::Definite(width)
589 }),
590 AvailableSpace::MinContent,
591 );
592 item_to_measure.layout_as_root(available_space, window, cx)
593 }
594
595 /// Track and render scroll state of this list with reference to the given scroll handle.
596 pub fn track_scroll(mut self, handle: UniformListScrollHandle) -> Self {
597 self.interactivity.tracked_scroll_handle = Some(handle.0.borrow().base_handle.clone());
598 self.scroll_handle = Some(handle);
599 self
600 }
601
602 /// Sets whether the list is flipped vertically, such that item 0 appears at the bottom.
603 pub fn y_flipped(mut self, y_flipped: bool) -> Self {
604 if let Some(ref scroll_handle) = self.scroll_handle {
605 let mut scroll_state = scroll_handle.0.borrow_mut();
606 let mut base_handle = &scroll_state.base_handle;
607 let offset = base_handle.offset();
608 match scroll_state.last_item_size {
609 Some(last_size) if scroll_state.y_flipped != y_flipped => {
610 let new_y_offset =
611 -(offset.y + last_size.contents.height - last_size.item.height);
612 base_handle.set_offset(point(offset.x, new_y_offset));
613 scroll_state.y_flipped = y_flipped;
614 }
615 // Handle case where list is initially flipped.
616 None if y_flipped => {
617 base_handle.set_offset(point(offset.x, Pixels::MIN));
618 scroll_state.y_flipped = y_flipped;
619 }
620 _ => {}
621 }
622 }
623 self
624 }
625}
626
627impl InteractiveElement for UniformList {
628 fn interactivity(&mut self) -> &mut crate::Interactivity {
629 &mut self.interactivity
630 }
631}