path: root/core/src/layout/flex.rs

//! Distribute elements using a flex-based layout.
// This code is heavily inspired by the [`druid`] codebase.
//
// [`druid`]: https://github.com/xi-editor/druid
//
// Copyright 2018 The xi-editor Authors, Héctor Ramón
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
use crate::Element;

use crate::layout::{Limits, Node};
use crate::widget;
use crate::{Alignment, Length, Padding, Point, Size};

/// The main axis of a flex layout.
#[derive(Debug)]
pub enum Axis {
    /// The horizontal axis
    Horizontal,

    /// The vertical axis
    Vertical,
}

impl Axis {
    fn main(&self, size: Size) -> f32 {
        match self {
            Axis::Horizontal => size.width,
            Axis::Vertical => size.height,
        }
    }

    fn cross(&self, size: Size) -> f32 {
        match self {
            Axis::Horizontal => size.height,
            Axis::Vertical => size.width,
        }
    }

    fn pack<T>(&self, main: T, cross: T) -> (T, T) {
        match self {
            Axis::Horizontal => (main, cross),
            Axis::Vertical => (cross, main),
        }
    }
}

/// Computes the flex layout with the given axis and limits, applying spacing,
/// padding and alignment to the items as needed.
///
/// It returns a new layout [`Node`].
pub fn resolve<Message, Theme, Renderer>(
    axis: Axis,
    renderer: &Renderer,
    limits: &Limits,
    width: Length,
    height: Length,
    padding: Padding,
    spacing: f32,
    align_items: Alignment,
    items: &[Element<'_, Message, Theme, Renderer>],
    trees: &mut [widget::Tree],
) -> Node
where
    Renderer: crate::Renderer,
{
    let limits = limits.width(width).height(height).shrink(padding);
    let total_spacing = spacing * items.len().saturating_sub(1) as f32;
    let max_cross = axis.cross(limits.max());

    let mut fill_main_sum = 0;
    let mut some_fill_cross = false;
    let (mut cross, cross_compress) = match axis {
        Axis::Vertical if width == Length::Shrink => (0.0, true),
        Axis::Horizontal if height == Length::Shrink => (0.0, true),
        _ => (max_cross, false),
    };

    let mut available = axis.main(limits.max()) - total_spacing;

    let mut nodes: Vec<Node> = Vec::with_capacity(items.len());
    nodes.resize(items.len(), Node::default());

    // FIRST PASS
    // We lay out non-fluid elements in the main axis.
    // If we need to compress the cross axis, then we skip any of these elements
    // that are also fluid in the cross axis.
    for (i, (child, tree)) in items.iter().zip(trees.iter_mut()).enumerate() {
        let (fill_main_factor, fill_cross_factor) = {
            let size = child.as_widget().size();

            axis.pack(size.width.fill_factor(), size.height.fill_factor())
        };

        if fill_main_factor == 0 && (!cross_compress || fill_cross_factor == 0)
        {
            let (max_width, max_height) = axis.pack(
                available,
                if fill_cross_factor == 0 {
                    max_cross
                } else {
                    cross
                },
            );

            let child_limits =
                Limits::new(Size::ZERO, Size::new(max_width, max_height));

            let layout =
                child.as_widget().layout(tree, renderer, &child_limits);
            let size = layout.size();

            available -= axis.main(size);
            cross = cross.max(axis.cross(size));

            nodes[i] = layout;
        } else {
            fill_main_sum += fill_main_factor;
            some_fill_cross = some_fill_cross || fill_cross_factor != 0;
        }
    }

    // SECOND PASS (conditional)
    // If we must compress the cross axis and there are fluid elements in the
    // cross axis, we lay out any of these elements that are also non-fluid in
    // the main axis (i.e. the ones we deliberately skipped in the first pass).
    //
    // We use the maximum cross length obtained in the first pass as the maximum
    // cross limit.
    if cross_compress && some_fill_cross {
        for (i, (child, tree)) in items.iter().zip(trees.iter_mut()).enumerate()
        {
            let (fill_main_factor, fill_cross_factor) = {
                let size = child.as_widget().size();

                axis.pack(size.width.fill_factor(), size.height.fill_factor())
            };

            if fill_main_factor == 0 && fill_cross_factor != 0 {
                let (max_width, max_height) = axis.pack(available, cross);

                let child_limits =
                    Limits::new(Size::ZERO, Size::new(max_width, max_height));

                let layout =
                    child.as_widget().layout(tree, renderer, &child_limits);
                let size = layout.size();

                available -= axis.main(size);
                cross = cross.max(axis.cross(size));

                nodes[i] = layout;
            }
        }
    }

    let remaining = match axis {
        Axis::Horizontal => match width {
            Length::Shrink => 0.0,
            _ => available.max(0.0),
        },
        Axis::Vertical => match height {
            Length::Shrink => 0.0,
            _ => available.max(0.0),
        },
    };

    // THIRD PASS
    // We only have the elements that are fluid in the main axis left.
    // We use the remaining space to evenly allocate space based on fill factors.
    for (i, (child, tree)) in items.iter().zip(trees).enumerate() {
        let (fill_main_factor, fill_cross_factor) = {
            let size = child.as_widget().size();

            axis.pack(size.width.fill_factor(), size.height.fill_factor())
        };

        if fill_main_factor != 0 {
            let max_main =
                remaining * fill_main_factor as f32 / fill_main_sum as f32;

            let max_main = if max_main.is_nan() {
                f32::INFINITY
            } else {
                max_main
            };

            let min_main = if max_main.is_infinite() {
                0.0
            } else {
                max_main
            };

            let (min_width, min_height) = axis.pack(min_main, 0.0);
            let (max_width, max_height) = axis.pack(
                max_main,
                if fill_cross_factor == 0 {
                    max_cross
                } else {
                    cross
                },
            );

            let child_limits = Limits::new(
                Size::new(min_width, min_height),
                Size::new(max_width, max_height),
            );

            let layout =
                child.as_widget().layout(tree, renderer, &child_limits);
            cross = cross.max(axis.cross(layout.size()));

            nodes[i] = layout;
        }
    }

    let pad = axis.pack(padding.left, padding.top);
    let mut main = pad.0;

    // FOURTH PASS
    // We align all the laid out nodes in the cross axis, if needed.
    for (i, node) in nodes.iter_mut().enumerate() {
        if i > 0 {
            main += spacing;
        }

        let (x, y) = axis.pack(main, pad.1);

        node.move_to_mut(Point::new(x, y));

        match axis {
            Axis::Horizontal => {
                node.align_mut(
                    Alignment::Start,
                    align_items,
                    Size::new(0.0, cross),
                );
            }
            Axis::Vertical => {
                node.align_mut(
                    align_items,
                    Alignment::Start,
                    Size::new(cross, 0.0),
                );
            }
        }

        let size = node.size();

        main += axis.main(size);
    }

    let (intrinsic_width, intrinsic_height) = axis.pack(main - pad.0, cross);
    let size = limits.resolve(
        width,
        height,
        Size::new(intrinsic_width, intrinsic_height),
    );

    Node::with_children(size.expand(padding), nodes)
}