Implement textual hit testing

glow/src/backend.rs

@@ -7,7 +7,9 @@ use iced_graphics::font;
 use iced_graphics::Layer;
 use iced_graphics::Primitive;
 use iced_native::mouse;
-use iced_native::{Font, HorizontalAlignment, Size, VerticalAlignment};
+use iced_native::{
+    Font, HitTestResult, HorizontalAlignment, Size, VerticalAlignment,
+};
 
 /// A [`glow`] graphics backend for [`iced`].
 ///
@@ -211,6 +213,25 @@ impl backend::Text for Backend {
     ) -> (f32, f32) {
         self.text_pipeline.measure(contents, size, font, bounds)
     }
+
+    fn hit_test(
+        &self,
+        contents: &str,
+        size: f32,
+        font: Font,
+        bounds: Size,
+        point: iced_native::Point,
+        nearest_only: bool,
+    ) -> HitTestResult {
+        self.text_pipeline.hit_test(
+            contents,
+            size,
+            font,
+            bounds,
+            point,
+            nearest_only,
+        )
+    }
 }
 
 #[cfg(feature = "image")]

glow/src/text.rs

@@ -1,6 +1,7 @@
 use crate::Transformation;
 use glow_glyph::ab_glyph;
 use iced_graphics::font;
+use iced_native::HitTestResult;
 use std::{cell::RefCell, collections::HashMap};
 
 #[derive(Debug)]
@@ -109,6 +110,97 @@ impl Pipeline {
         }
     }
 
+    pub fn hit_test(
+        &self,
+        content: &str,
+        size: f32,
+        font: iced_native::Font,
+        bounds: iced_native::Size,
+        point: iced_native::Point,
+        nearest_only: bool,
+    ) -> HitTestResult {
+        use glow_glyph::GlyphCruncher;
+
+        let glow_glyph::FontId(font_id) = self.find_font(font);
+
+        let section = glow_glyph::Section {
+            bounds: (bounds.width, bounds.height),
+            text: vec![glow_glyph::Text {
+                text: content,
+                scale: size.into(),
+                font_id: glow_glyph::FontId(font_id),
+                extra: glow_glyph::Extra::default(),
+            }],
+            ..Default::default()
+        };
+
+        let mut mb = self.measure_brush.borrow_mut();
+
+        // The underlying type is FontArc, so clones are cheap.
+        use ab_glyph::{Font, ScaleFont};
+        let font = mb.fonts()[font_id].clone().into_scaled(size);
+
+        // Implements an iterator over the glyph bounding boxes.
+        let bounds = mb.glyphs(section).map(
+            |glow_glyph::SectionGlyph {
+                 byte_index, glyph, ..
+             }| {
+                (
+                    *byte_index,
+                    iced_native::Rectangle::new(
+                        iced_native::Point::new(
+                            glyph.position.x - font.h_side_bearing(glyph.id),
+                            glyph.position.y - font.ascent(),
+                        ),
+                        iced_native::Size::new(
+                            font.h_advance(glyph.id),
+                            font.ascent() - font.descent(),
+                        ),
+                    ),
+                )
+            },
+        );
+
+        // Implements computation of the character index based on the byte index
+        // within the input string.
+        let char_index = |byte_index| {
+            let mut b_count = 0;
+            for (i, utf8_len) in
+                content.chars().map(|c| c.len_utf8()).enumerate()
+            {
+                if byte_index < (b_count + utf8_len) {
+                    return i;
+                }
+                b_count += utf8_len;
+            }
+            return byte_index;
+        };
+
+        if !nearest_only {
+            for (idx, bounds) in bounds.clone() {
+                if bounds.contains(point) {
+                    return HitTestResult::CharOffset(char_index(idx));
+                }
+            }
+        }
+
+        let (idx, nearest) = bounds.fold(
+            (0usize, iced_native::Point::ORIGIN),
+            |acc: (usize, iced_native::Point), (idx, bounds)| {
+                if bounds.center().distance(point) < acc.1.distance(point) {
+                    (idx, bounds.center())
+                } else {
+                    acc
+                }
+            },
+        );
+
+        HitTestResult::NearestCharOffset(
+            char_index(idx),
+            (point - nearest).into(),
+        )
+    }
+
     pub fn trim_measurement_cache(&mut self) {
         // TODO: We should probably use a `GlyphCalculator` for this. However,
         // it uses a lifetimed `GlyphCalculatorGuard` with side-effects on drop.