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 point, px, size, AnyElement, AvailableSpace, Bounds, ContentMask, Element, ElementId,
9 GlobalElementId, Hitbox, InteractiveElement, Interactivity, IntoElement, LayoutId,
10 ListSizingBehavior, Pixels, Render, ScrollHandle, Size, StyleRefinement, Styled, View,
11 ViewContext, WindowContext,
12};
13use smallvec::SmallVec;
14use std::{cell::RefCell, cmp, ops::Range, rc::Rc};
15use taffy::style::Overflow;
16
17/// uniform_list provides lazy rendering for a set of items that are of uniform height.
18/// When rendered into a container with overflow-y: hidden and a fixed (or max) height,
19/// uniform_list will only render the visible subset of items.
20#[track_caller]
21pub fn uniform_list<I, R, V>(
22 view: View<V>,
23 id: I,
24 item_count: usize,
25 f: impl 'static + Fn(&mut V, Range<usize>, &mut ViewContext<V>) -> Vec<R>,
26) -> UniformList
27where
28 I: Into<ElementId>,
29 R: IntoElement,
30 V: Render,
31{
32 let id = id.into();
33 let mut base_style = StyleRefinement::default();
34 base_style.overflow.y = Some(Overflow::Scroll);
35
36 let render_range = move |range, cx: &mut WindowContext| {
37 view.update(cx, |this, cx| {
38 f(this, range, cx)
39 .into_iter()
40 .map(|component| component.into_any_element())
41 .collect()
42 })
43 };
44
45 UniformList {
46 item_count,
47 item_to_measure_index: 0,
48 render_items: Box::new(render_range),
49 interactivity: Interactivity {
50 element_id: Some(id),
51 base_style: Box::new(base_style),
52
53 #[cfg(debug_assertions)]
54 location: Some(*core::panic::Location::caller()),
55
56 ..Default::default()
57 },
58 scroll_handle: None,
59 sizing_behavior: ListSizingBehavior::default(),
60 }
61}
62
63/// A list element for efficiently laying out and displaying a list of uniform-height elements.
64pub struct UniformList {
65 item_count: usize,
66 item_to_measure_index: usize,
67 render_items:
68 Box<dyn for<'a> Fn(Range<usize>, &'a mut WindowContext) -> SmallVec<[AnyElement; 64]>>,
69 interactivity: Interactivity,
70 scroll_handle: Option<UniformListScrollHandle>,
71 sizing_behavior: ListSizingBehavior,
72}
73
74/// Frame state used by the [UniformList].
75pub struct UniformListFrameState {
76 item_size: Size<Pixels>,
77 items: SmallVec<[AnyElement; 32]>,
78}
79
80/// A handle for controlling the scroll position of a uniform list.
81/// This should be stored in your view and passed to the uniform_list on each frame.
82#[derive(Clone, Default)]
83pub struct UniformListScrollHandle {
84 base_handle: ScrollHandle,
85 deferred_scroll_to_item: Rc<RefCell<Option<usize>>>,
86}
87
88impl UniformListScrollHandle {
89 /// Create a new scroll handle to bind to a uniform list.
90 pub fn new() -> Self {
91 Self {
92 base_handle: ScrollHandle::new(),
93 deferred_scroll_to_item: Rc::new(RefCell::new(None)),
94 }
95 }
96
97 /// Scroll the list to the given item index.
98 pub fn scroll_to_item(&mut self, ix: usize) {
99 self.deferred_scroll_to_item.replace(Some(ix));
100 }
101
102 /// Get the index of the topmost visible child.
103 pub fn logical_scroll_top_index(&self) -> usize {
104 self.deferred_scroll_to_item
105 .borrow()
106 .unwrap_or_else(|| self.base_handle.logical_scroll_top().0)
107 }
108}
109
110impl Styled for UniformList {
111 fn style(&mut self) -> &mut StyleRefinement {
112 &mut self.interactivity.base_style
113 }
114}
115
116impl Element for UniformList {
117 type RequestLayoutState = UniformListFrameState;
118 type PrepaintState = Option<Hitbox>;
119
120 fn id(&self) -> Option<ElementId> {
121 self.interactivity.element_id.clone()
122 }
123
124 fn request_layout(
125 &mut self,
126 global_id: Option<&GlobalElementId>,
127 cx: &mut WindowContext,
128 ) -> (LayoutId, Self::RequestLayoutState) {
129 let max_items = self.item_count;
130 let item_size = self.measure_item(None, cx);
131 let layout_id = self
132 .interactivity
133 .request_layout(global_id, cx, |style, cx| match self.sizing_behavior {
134 ListSizingBehavior::Infer => {
135 cx.with_text_style(style.text_style().cloned(), |cx| {
136 cx.request_measured_layout(
137 style,
138 move |known_dimensions, available_space, _cx| {
139 let desired_height = item_size.height * max_items;
140 let width = known_dimensions.width.unwrap_or(match available_space
141 .width
142 {
143 AvailableSpace::Definite(x) => x,
144 AvailableSpace::MinContent | AvailableSpace::MaxContent => {
145 item_size.width
146 }
147 });
148 let height = match available_space.height {
149 AvailableSpace::Definite(height) => desired_height.min(height),
150 AvailableSpace::MinContent | AvailableSpace::MaxContent => {
151 desired_height
152 }
153 };
154 size(width, height)
155 },
156 )
157 })
158 }
159 ListSizingBehavior::Auto => cx.with_text_style(style.text_style().cloned(), |cx| {
160 cx.request_layout(style, None)
161 }),
162 });
163
164 (
165 layout_id,
166 UniformListFrameState {
167 item_size,
168 items: SmallVec::new(),
169 },
170 )
171 }
172
173 fn prepaint(
174 &mut self,
175 global_id: Option<&GlobalElementId>,
176 bounds: Bounds<Pixels>,
177 frame_state: &mut Self::RequestLayoutState,
178 cx: &mut WindowContext,
179 ) -> Option<Hitbox> {
180 let style = self.interactivity.compute_style(global_id, None, cx);
181 let border = style.border_widths.to_pixels(cx.rem_size());
182 let padding = style.padding.to_pixels(bounds.size.into(), cx.rem_size());
183
184 let padded_bounds = Bounds::from_corners(
185 bounds.origin + point(border.left + padding.left, border.top + padding.top),
186 bounds.lower_right()
187 - point(border.right + padding.right, border.bottom + padding.bottom),
188 );
189
190 let content_size = Size {
191 width: padded_bounds.size.width,
192 height: frame_state.item_size.height * self.item_count + padding.top + padding.bottom,
193 };
194
195 let shared_scroll_offset = self.interactivity.scroll_offset.clone().unwrap();
196
197 let item_height = self.measure_item(Some(padded_bounds.size.width), cx).height;
198 let shared_scroll_to_item = self
199 .scroll_handle
200 .as_mut()
201 .and_then(|handle| handle.deferred_scroll_to_item.take());
202
203 self.interactivity.prepaint(
204 global_id,
205 bounds,
206 content_size,
207 cx,
208 |style, mut scroll_offset, hitbox, cx| {
209 let border = style.border_widths.to_pixels(cx.rem_size());
210 let padding = style.padding.to_pixels(bounds.size.into(), cx.rem_size());
211
212 let padded_bounds = Bounds::from_corners(
213 bounds.origin + point(border.left + padding.left, border.top),
214 bounds.lower_right() - point(border.right + padding.right, border.bottom),
215 );
216
217 if self.item_count > 0 {
218 let content_height =
219 item_height * self.item_count + padding.top + padding.bottom;
220 let min_scroll_offset = padded_bounds.size.height - content_height;
221 let is_scrolled = scroll_offset.y != px(0.);
222
223 if is_scrolled && scroll_offset.y < min_scroll_offset {
224 shared_scroll_offset.borrow_mut().y = min_scroll_offset;
225 scroll_offset.y = min_scroll_offset;
226 }
227
228 if let Some(ix) = shared_scroll_to_item {
229 let list_height = padded_bounds.size.height;
230 let mut updated_scroll_offset = shared_scroll_offset.borrow_mut();
231 let item_top = item_height * ix + padding.top;
232 let item_bottom = item_top + item_height;
233 let scroll_top = -updated_scroll_offset.y;
234 if item_top < scroll_top + padding.top {
235 updated_scroll_offset.y = -(item_top) + padding.top;
236 } else if item_bottom > scroll_top + list_height - padding.bottom {
237 updated_scroll_offset.y = -(item_bottom - list_height) - padding.bottom;
238 }
239 scroll_offset = *updated_scroll_offset;
240 }
241
242 let first_visible_element_ix =
243 (-(scroll_offset.y + padding.top) / item_height).floor() as usize;
244 let last_visible_element_ix = ((-scroll_offset.y + padded_bounds.size.height)
245 / item_height)
246 .ceil() as usize;
247 let visible_range = first_visible_element_ix
248 ..cmp::min(last_visible_element_ix, self.item_count);
249
250 let mut items = (self.render_items)(visible_range.clone(), cx);
251 let content_mask = ContentMask { bounds };
252 cx.with_content_mask(Some(content_mask), |cx| {
253 for (mut item, ix) in items.into_iter().zip(visible_range) {
254 let item_origin = padded_bounds.origin
255 + point(px(0.), item_height * ix + scroll_offset.y + padding.top);
256 let available_space = size(
257 AvailableSpace::Definite(padded_bounds.size.width),
258 AvailableSpace::Definite(item_height),
259 );
260 item.layout_as_root(available_space, cx);
261 item.prepaint_at(item_origin, cx);
262 frame_state.items.push(item);
263 }
264 });
265 }
266
267 hitbox
268 },
269 )
270 }
271
272 fn paint(
273 &mut self,
274 global_id: Option<&GlobalElementId>,
275 bounds: Bounds<crate::Pixels>,
276 request_layout: &mut Self::RequestLayoutState,
277 hitbox: &mut Option<Hitbox>,
278 cx: &mut WindowContext,
279 ) {
280 self.interactivity
281 .paint(global_id, bounds, hitbox.as_ref(), cx, |_, cx| {
282 for item in &mut request_layout.items {
283 item.paint(cx);
284 }
285 })
286 }
287}
288
289impl IntoElement for UniformList {
290 type Element = Self;
291
292 fn into_element(self) -> Self::Element {
293 self
294 }
295}
296
297impl UniformList {
298 /// Selects a specific list item for measurement.
299 pub fn with_width_from_item(mut self, item_index: Option<usize>) -> Self {
300 self.item_to_measure_index = item_index.unwrap_or(0);
301 self
302 }
303
304 /// Sets the sizing behavior, similar to the `List` element.
305 pub fn with_sizing_behavior(mut self, behavior: ListSizingBehavior) -> Self {
306 self.sizing_behavior = behavior;
307 self
308 }
309
310 fn measure_item(&self, list_width: Option<Pixels>, cx: &mut WindowContext) -> Size<Pixels> {
311 if self.item_count == 0 {
312 return Size::default();
313 }
314
315 let item_ix = cmp::min(self.item_to_measure_index, self.item_count - 1);
316 let mut items = (self.render_items)(item_ix..item_ix + 1, cx);
317 let mut item_to_measure = items.pop().unwrap();
318 let available_space = size(
319 list_width.map_or(AvailableSpace::MinContent, |width| {
320 AvailableSpace::Definite(width)
321 }),
322 AvailableSpace::MinContent,
323 );
324 item_to_measure.layout_as_root(available_space, cx)
325 }
326
327 /// Track and render scroll state of this list with reference to the given scroll handle.
328 pub fn track_scroll(mut self, handle: UniformListScrollHandle) -> Self {
329 self.interactivity.tracked_scroll_handle = Some(handle.base_handle.clone());
330 self.scroll_handle = Some(handle);
331 self
332 }
333}
334
335impl InteractiveElement for UniformList {
336 fn interactivity(&mut self) -> &mut crate::Interactivity {
337 &mut self.interactivity
338 }
339}