cranpose_ui/
layout_direction.rs

1//! Which way the interface reads.
2//!
3//! Layout direction is what turns "start" and "end" into "left" and "right".
4//! It is a composition local rather than a global so a screen can pin a
5//! direction — a code block, a phone number, a language picker — without
6//! reversing the rest of the interface with it.
7
8use cranpose_core::{CompositionLocal, CompositionLocalProvider, compositionLocalOf};
9
10/// Which side of the interface is the start.
11#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
12pub enum LayoutDirection {
13    /// Start is the left edge — Latin, Cyrillic, CJK.
14    #[default]
15    Ltr,
16    /// Start is the right edge — Arabic, Hebrew.
17    Rtl,
18}
19
20impl LayoutDirection {
21    /// Whether the start edge is the right one.
22    pub fn is_rtl(self) -> bool {
23        matches!(self, LayoutDirection::Rtl)
24    }
25
26    /// The direction that reads the other way.
27    pub fn reversed(self) -> Self {
28        match self {
29            LayoutDirection::Ltr => LayoutDirection::Rtl,
30            LayoutDirection::Rtl => LayoutDirection::Ltr,
31        }
32    }
33
34    /// Turns start/end values into physical left/right values.
35    pub fn resolve(self, start: f32, end: f32) -> (f32, f32) {
36        match self {
37            LayoutDirection::Ltr => (start, end),
38            LayoutDirection::Rtl => (end, start),
39        }
40    }
41}
42
43/// The [`CompositionLocal`] carrying the current layout direction.
44pub fn local_layout_direction() -> CompositionLocal<LayoutDirection> {
45    thread_local! {
46        static LOCAL: std::cell::RefCell<Option<CompositionLocal<LayoutDirection>>> =
47            const { std::cell::RefCell::new(None) };
48    }
49    LOCAL.with(|cell| {
50        cell.borrow_mut()
51            .get_or_insert_with(|| compositionLocalOf(LayoutDirection::default))
52            .clone()
53    })
54}
55
56/// The layout direction in force here.
57pub fn layout_direction() -> LayoutDirection {
58    local_layout_direction().current()
59}
60
61/// Runs `content` in `direction`.
62#[allow(non_snake_case)]
63#[track_caller]
64pub fn ProvideLayoutDirection(direction: LayoutDirection, content: impl FnOnce()) {
65    CompositionLocalProvider(vec![local_layout_direction().provides(direction)], content);
66}
67
68#[cfg(test)]
69mod tests {
70    use super::*;
71
72    #[test]
73    fn resolving_start_and_end_follows_the_direction() {
74        assert_eq!(LayoutDirection::Ltr.resolve(4.0, 12.0), (4.0, 12.0));
75        assert_eq!(LayoutDirection::Rtl.resolve(4.0, 12.0), (12.0, 4.0));
76    }
77
78    #[test]
79    fn a_direction_knows_its_opposite() {
80        assert_eq!(LayoutDirection::Ltr.reversed(), LayoutDirection::Rtl);
81        assert!(!LayoutDirection::Ltr.is_rtl());
82        assert!(LayoutDirection::Rtl.is_rtl());
83    }
84
85    #[test]
86    fn a_provided_direction_reaches_the_content_and_ends_with_it() {
87        use std::{cell::Cell, rc::Rc};
88
89        use cranpose_core::{Composition, MemoryApplier, location_key};
90
91        let mut composition = Composition::new(MemoryApplier::new());
92        let outer = Rc::new(Cell::new(LayoutDirection::Rtl));
93        let inside = Rc::new(Cell::new(LayoutDirection::Ltr));
94        let nested = Rc::new(Cell::new(LayoutDirection::Rtl));
95        let after = Rc::new(Cell::new(LayoutDirection::Rtl));
96
97        let key = location_key(file!(), line!(), column!());
98        {
99            let (outer, inside, nested, after) = (
100                Rc::clone(&outer),
101                Rc::clone(&inside),
102                Rc::clone(&nested),
103                Rc::clone(&after),
104            );
105            let mut render = move || {
106                // Nothing provided: the interface reads the default way.
107                outer.set(layout_direction());
108                ProvideLayoutDirection(LayoutDirection::Rtl, || {
109                    inside.set(layout_direction());
110                    // A screen can pin a direction back the other way without
111                    // reversing what surrounds it.
112                    ProvideLayoutDirection(LayoutDirection::Ltr, || nested.set(layout_direction()));
113                });
114                // The provision is scoped to the content, not to what follows.
115                after.set(layout_direction());
116            };
117            composition.render(key, &mut render).expect("render");
118        }
119
120        assert_eq!(outer.get(), LayoutDirection::Ltr);
121        assert_eq!(inside.get(), LayoutDirection::Rtl);
122        assert_eq!(nested.get(), LayoutDirection::Ltr);
123        assert_eq!(after.get(), LayoutDirection::Ltr);
124    }
125
126    #[test]
127    fn the_composition_local_is_one_instance_per_thread() {
128        use std::{cell::Cell, rc::Rc};
129
130        use cranpose_core::{Composition, MemoryApplier, location_key};
131
132        // Two calls that returned different locals would each carry their own
133        // value, and a provision made through one would be invisible to the
134        // other -- which is what a lazily-created local gets wrong.
135        let mut composition = Composition::new(MemoryApplier::new());
136        let seen = Rc::new(Cell::new(LayoutDirection::Ltr));
137        let recorder = Rc::clone(&seen);
138        let key = location_key(file!(), line!(), column!());
139        let mut render = move || {
140            CompositionLocalProvider(
141                vec![local_layout_direction().provides(LayoutDirection::Rtl)],
142                || recorder.set(local_layout_direction().current()),
143            );
144        };
145        composition.render(key, &mut render).expect("render");
146
147        assert_eq!(seen.get(), LayoutDirection::Rtl);
148    }
149}
cranpose_ui/layout_direction.rs

cranpose_ui/
layout_direction.rs
