typst/src/library/layout/align.rs

use crate::library::prelude::*;
use crate::library::text::{HorizontalAlign, ParNode};

/// Align a node along the layouting axes.
#[derive(Debug, Hash)]
pub struct AlignNode {
    /// How to align the node horizontally and vertically.
    pub aligns: Axes<Option<RawAlign>>,
    /// The node to be aligned.
    pub child: Content,
}

#[node(Layout)]
impl AlignNode {
    fn construct(_: &mut Vm, args: &mut Args) -> SourceResult<Content> {
        let aligns: Axes<Option<RawAlign>> = args.find()?.unwrap_or_default();
        let body: Content = args.expect("body")?;

        if let Axes { x: Some(x), y: None } = aligns {
            if body
                .to::<dyn Layout>()
                .map_or(true, |node| node.level() == Level::Inline)
            {
                return Ok(body.styled(ParNode::ALIGN, HorizontalAlign(x)));
            }
        }

        Ok(body.aligned(aligns))
    }
}

impl Layout for AlignNode {
    fn layout(
        &self,
        world: Tracked<dyn World>,
        regions: &Regions,
        styles: StyleChain,
    ) -> SourceResult<Vec<Frame>> {
        // The child only needs to expand along an axis if there's no alignment.
        let mut pod = regions.clone();
        pod.expand &= self.aligns.as_ref().map(Option::is_none);

        // Align paragraphs inside the child.
        let mut passed = StyleMap::new();
        if let Some(align) = self.aligns.x {
            passed.set(ParNode::ALIGN, HorizontalAlign(align));
        }

        // Layout the child.
        let mut frames = self.child.layout_block(world, &pod, passed.chain(&styles))?;
        for (region, frame) in regions.iter().zip(&mut frames) {
            // Align in the target size. The target size depends on whether we
            // should expand.
            let target = regions.expand.select(region, frame.size());
            let aligns = self
                .aligns
                .map(|align| align.resolve(styles))
                .unwrap_or(Axes::new(Align::Left, Align::Top));

            frame.resize(target, aligns);
        }

        Ok(frames)
    }

    fn level(&self) -> Level {
        Level::Block
    }
}