tawasuyu/taffy

1//! Alignment of tracks and final positioning of items
2use super::types::GridTrack;
3use crate::compute::common::alignment::compute_alignment_offset;
4use crate::geometry::InBothAbsAxis;
5use crate::geometry::{Line, Point, Rect, Size};
6use crate::style::{AlignContent, AlignItems, AlignSelf, AvailableSpace, Position};
7use crate::tree::{Layout, SizingMode};
8use crate::tree::{LayoutTree, NodeId};
9use crate::util::sys::f32_max;
10use crate::util::MaybeMath;
11use crate::util::{MaybeResolve, ResolveOrZero};
12
13/// Align the grid tracks within the grid according to the align-content (rows) or
14/// justify-content (columns) property. This only does anything if the size of the
15/// grid is not equal to the size of the grid container in the axis being aligned.
16pub(super) fn align_tracks(
17    grid_container_content_box_size: f32,
18    padding: Line<f32>,
19    border: Line<f32>,
20    tracks: &mut [GridTrack],
21    track_alignment_style: AlignContent,
22) {
23    let used_size: f32 = tracks.iter().map(|track| track.base_size).sum();
24    let free_space = grid_container_content_box_size - used_size;
25    let origin = padding.start + border.start;
26
27    // Count the number of non-collapsed tracks (not counting gutters)
28    let num_tracks = tracks.iter().skip(1).step_by(2).filter(|track| !track.is_collapsed).count();
29
30    // Grid layout treats gaps as full tracks rather than applying them at alignment so we
31    // simply pass zero here. Grid layout is never reversed.
32    let gap = 0.0;
33    let layout_is_reversed = false;
34
35    // Compute offsets
36    let mut total_offset = origin;
37    tracks.iter_mut().enumerate().for_each(|(i, track)| {
38        // Odd tracks are gutters (but slices are zero-indexed, so odd tracks have even indicies)
39        let is_gutter = i % 2 == 0;
40
41        // The first non-gutter track is index 1
42        let is_first = i == 1;
43
44        let offset = if is_gutter {
45            0.0
46        } else {
47            compute_alignment_offset(free_space, num_tracks, gap, track_alignment_style, layout_is_reversed, is_first)
48        };
49
50        track.offset = total_offset + offset;
51        total_offset = total_offset + offset + track.base_size;
52    });
53}
54
55/// Align and size a grid item into it's final position
56pub(super) fn align_and_position_item(
57    tree: &mut impl LayoutTree,
58    node: NodeId,
59    order: u32,
60    grid_area: Rect<f32>,
61    container_alignment_styles: InBothAbsAxis<Option<AlignItems>>,
62    baseline_shim: f32,
63) {
64    let grid_area_size = Size { width: grid_area.right - grid_area.left, height: grid_area.bottom - grid_area.top };
65
66    let style = tree.style(node);
67    let aspect_ratio = style.aspect_ratio;
68    let justify_self = style.justify_self;
69    let align_self = style.align_self;
70
71    let position = style.position;
72    let inset_horizontal = style.inset.horizontal_components().map(|size| size.resolve_to_option(grid_area_size.width));
73    let inset_vertical = style.inset.vertical_components().map(|size| size.resolve_to_option(grid_area_size.height));
74    let padding = style.padding.map(|p| p.resolve_or_zero(Some(grid_area_size.width)));
75    let border = style.border.map(|p| p.resolve_or_zero(Some(grid_area_size.width)));
76    let padding_border_size = (padding + border).sum_axes();
77    let inherent_size = style.size.maybe_resolve(grid_area_size).maybe_apply_aspect_ratio(aspect_ratio);
78    let min_size = style
79        .min_size
80        .maybe_resolve(grid_area_size)
81        .or(padding_border_size.map(Some))
82        .maybe_max(padding_border_size)
83        .maybe_apply_aspect_ratio(aspect_ratio);
84    let max_size = style.max_size.maybe_resolve(grid_area_size).maybe_apply_aspect_ratio(aspect_ratio);
85
86    // Resolve default alignment styles if they are set on neither the parent or the node itself
87    // Note: if the child has a preferred aspect ratio but neither width or height are set, then the width is stretched
88    // and the then height is calculated from the width according the aspect ratio
89    // See: https://www.w3.org/TR/css-grid-1/#grid-item-sizing
90    let alignment_styles = InBothAbsAxis {
91        horizontal: justify_self.or(container_alignment_styles.horizontal).unwrap_or_else(|| {
92            if inherent_size.width.is_some() {
93                AlignSelf::Start
94            } else {
95                AlignSelf::Stretch
96            }
97        }),
98        vertical: align_self.or(container_alignment_styles.vertical).unwrap_or_else(|| {
99            if inherent_size.height.is_some() || aspect_ratio.is_some() {
100                AlignSelf::Start
101            } else {
102                AlignSelf::Stretch
103            }
104        }),
105    };
106
107    // Note: This is not a bug. It is part of the CSS spec that both horizontal and vertical margins
108    // resolve against the WIDTH of the grid area.
109    let margin = style.margin.map(|margin| margin.resolve_to_option(grid_area_size.width));
110
111    let grid_area_minus_item_margins_size = Size {
112        width: grid_area_size.width.maybe_sub(margin.left).maybe_sub(margin.right),
113        height: grid_area_size.height.maybe_sub(margin.top).maybe_sub(margin.bottom) - baseline_shim,
114    };
115
116    // If node is absolutely positioned and width is not set explicitly, then deduce it
117    // from left, right and container_content_box if both are set.
118    let width = inherent_size.width.or_else(|| {
119        // Apply width derived from both the left and right properties of an absolutely
120        // positioned element being set
121        if position == Position::Absolute {
122            if let (Some(left), Some(right)) = (inset_horizontal.start, inset_horizontal.end) {
123                return Some(f32_max(grid_area_minus_item_margins_size.width - left - right, 0.0));
124            }
125        }
126
127        // Apply width based on stretch alignment if:
128        //  - Alignment style is "stretch"
129        //  - The node is not absolutely positioned
130        //  - The node does not have auto margins in this axis.
131        if margin.left.is_some()
132            && margin.right.is_some()
133            && alignment_styles.horizontal == AlignSelf::Stretch
134            && position != Position::Absolute
135        {
136            return Some(grid_area_minus_item_margins_size.width);
137        }
138
139        None
140    });
141
142    // Reapply aspect ratio after stretch and absolute position width adjustments
143    let Size { width, height } = Size { width, height: inherent_size.height }.maybe_apply_aspect_ratio(aspect_ratio);
144
145    let height = height.or_else(|| {
146        if position == Position::Absolute {
147            if let (Some(top), Some(bottom)) = (inset_vertical.start, inset_vertical.end) {
148                return Some(f32_max(grid_area_minus_item_margins_size.height - top - bottom, 0.0));
149            }
150        }
151
152        // Apply height based on stretch alignment if:
153        //  - Alignment style is "stretch"
154        //  - The node is not absolutely positioned
155        //  - The node does not have auto margins in this axis.
156        if margin.top.is_some()
157            && margin.bottom.is_some()
158            && alignment_styles.vertical == AlignSelf::Stretch
159            && position != Position::Absolute
160        {
161            return Some(grid_area_minus_item_margins_size.height);
162        }
163
164        None
165    });
166    // Reapply aspect ratio after stretch and absolute position height adjustments
167    let Size { width, height } = Size { width, height }.maybe_apply_aspect_ratio(aspect_ratio);
168
169    // Clamp size by min and max width/height
170    let Size { width, height } = Size { width, height }.maybe_clamp(min_size, max_size);
171
172    // Layout node
173    let measured_size_and_baselines = tree.perform_child_layout(
174        node,
175        Size { width, height },
176        grid_area_size.map(Option::Some),
177        grid_area_minus_item_margins_size.map(AvailableSpace::Definite),
178        SizingMode::InherentSize,
179        Line::FALSE,
180    );
181
182    // Resolve final size
183    let Size { width, height } =
184        Size { width, height }.unwrap_or(measured_size_and_baselines.size).maybe_clamp(min_size, max_size);
185
186    let x = align_item_within_area(
187        Line { start: grid_area.left, end: grid_area.right },
188        justify_self.unwrap_or(alignment_styles.horizontal),
189        width,
190        position,
191        inset_horizontal,
192        margin.horizontal_components(),
193        0.0,
194    );
195    let y = align_item_within_area(
196        Line { start: grid_area.top, end: grid_area.bottom },
197        align_self.unwrap_or(alignment_styles.vertical),
198        height,
199        position,
200        inset_vertical,
201        margin.vertical_components(),
202        baseline_shim,
203    );
204
205    *tree.layout_mut(node) = Layout { order, size: Size { width, height }, location: Point { x, y } };
206}
207
208/// Align and size a grid item along a single axis
209pub(super) fn align_item_within_area(
210    grid_area: Line<f32>,
211    alignment_style: AlignSelf,
212    resolved_size: f32,
213    position: Position,
214    inset: Line<Option<f32>>,
215    margin: Line<Option<f32>>,
216    baseline_shim: f32,
217) -> f32 {
218    // Calculate grid area dimension in the axis
219    let non_auto_margin = Line { start: margin.start.unwrap_or(0.0) + baseline_shim, end: margin.end.unwrap_or(0.0) };
220    let grid_area_size = f32_max(grid_area.end - grid_area.start, 0.0);
221    let free_space = f32_max(grid_area_size - resolved_size - non_auto_margin.sum(), 0.0);
222
223    // Expand auto margins to fill available space
224    let auto_margin_count = margin.start.is_none() as u8 + margin.end.is_none() as u8;
225    let auto_margin_size = if auto_margin_count > 0 { free_space / auto_margin_count as f32 } else { 0.0 };
226    let resolved_margin = Line {
227        start: margin.start.unwrap_or(auto_margin_size) + baseline_shim,
228        end: margin.end.unwrap_or(auto_margin_size),
229    };
230
231    // Compute offset in the axis
232    let alignment_based_offset = match alignment_style {
233        AlignSelf::Start | AlignSelf::FlexStart => resolved_margin.start,
234        AlignSelf::End | AlignSelf::FlexEnd => grid_area_size - resolved_size - resolved_margin.end,
235        AlignSelf::Center => (grid_area_size - resolved_size + resolved_margin.start - resolved_margin.end) / 2.0,
236        // TODO: Add support for baseline alignment. For now we treat it as "start".
237        AlignSelf::Baseline => resolved_margin.start,
238        AlignSelf::Stretch => resolved_margin.start,
239    };
240
241    let offset_within_area = if position == Position::Absolute {
242        if let Some(start) = inset.start {
243            start + non_auto_margin.start
244        } else if let Some(end) = inset.end {
245            grid_area_size - end - resolved_size - non_auto_margin.end
246        } else {
247            alignment_based_offset
248        }
249    } else {
250        alignment_based_offset
251    };
252
253    let mut start = grid_area.start + offset_within_area;
254    if position == Position::Relative {
255        start += inset.start.or(inset.end.map(|pos| -pos)).unwrap_or(0.0);
256    }
257
258    start
259}