Auto merge of #26697 - utsavoza:ugo/issue-11681/22-05-2020, r=jdm

Implement CanvasRenderingContext2d.fillText

The PR consists of broadly two main changes:
- Implementation of Canvas2dRenderingContext.font
- Basic implementation of Canvas2dRenderingContext.fillText

Although I am not fully sure about the long term goals for the canvas backend in Servo, I assumed limited scope for font and text handling (should support simple text drawing with font selection) in the current implementation as I believe a more complete implementation would eventually be brought in as a part of #22957.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #11681
- [x] There are tests for these changes

1 files changed, 226 insertions, 8 deletions

diff --git a/components/canvas/canvas_data.rs b/components/canvas/canvas_data.rs
index 33da592c235..402c2690257 100644
--- a/components/canvas/canvas_data.rs
+++ b/components/canvas/canvas_data.rs
@@ -7,12 +7,24 @@ use crate::raqote_backend::Repetition;
 use canvas_traits::canvas::*;
 use cssparser::RGBA;
 use euclid::default::{Point2D, Rect, Size2D, Transform2D, Vector2D};
+use euclid::{point2, vec2};
+use font_kit::family_name::FamilyName;
+use font_kit::font::Font;
+use font_kit::metrics::Metrics;
+use font_kit::properties::Properties;
+use font_kit::source::SystemSource;
+use gfx::font::FontHandleMethods;
+use gfx::font_cache_thread::FontCacheThread;
+use gfx::font_context::FontContext;
 use ipc_channel::ipc::{IpcSender, IpcSharedMemory};
 use num_traits::ToPrimitive;
+use servo_arc::Arc as ServoArc;
+use std::cell::RefCell;
 #[allow(unused_imports)]
 use std::marker::PhantomData;
 use std::mem;
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
+use style::properties::style_structs::Font as FontStyleStruct;
 use webrender_api::units::RectExt as RectExt_;
 
 /// The canvas data stores a state machine for the current status of
@@ -264,6 +276,15 @@ pub trait GenericDrawTarget {
         operator: CompositionOp,
     );
     fn fill(&mut self, path: &Path, pattern: Pattern, draw_options: &DrawOptions);
+    fn fill_text(
+        &mut self,
+        font: &Font,
+        point_size: f32,
+        text: &str,
+        start: Point2D<f32>,
+        pattern: &Pattern,
+        draw_options: &DrawOptions,
+    );
     fn fill_rect(&mut self, rect: &Rect<f32>, pattern: Pattern, draw_options: Option<&DrawOptions>);
     fn get_format(&self) -> SurfaceFormat;
     fn get_size(&self) -> Size2D<i32>;
@@ -360,6 +381,21 @@ pub enum Filter {
     Point,
 }
 
+pub(crate) type CanvasFontContext = FontContext<FontCacheThread>;
+
+thread_local!(static FONT_CONTEXT: RefCell<Option<CanvasFontContext>> = RefCell::new(None));
+
+pub(crate) fn with_thread_local_font_context<F, R>(canvas_data: &CanvasData, f: F) -> R
+where
+    F: FnOnce(&mut CanvasFontContext) -> R,
+{
+    FONT_CONTEXT.with(|font_context| {
+        f(font_context.borrow_mut().get_or_insert_with(|| {
+            FontContext::new(canvas_data.font_cache_thread.lock().unwrap().clone())
+        }))
+    })
+}
+
 pub struct CanvasData<'a> {
     backend: Box<dyn Backend>,
     drawtarget: Box<dyn GenericDrawTarget>,
@@ -372,7 +408,7 @@ pub struct CanvasData<'a> {
     old_image_key: Option<webrender_api::ImageKey>,
     /// An old webrender image key that can be deleted when the current epoch ends.
     very_old_image_key: Option<webrender_api::ImageKey>,
-    pub canvas_id: CanvasId,
+    font_cache_thread: Mutex<FontCacheThread>,
 }
 
 fn create_backend() -> Box<dyn Backend> {
@@ -384,7 +420,7 @@ impl<'a> CanvasData<'a> {
         size: Size2D<u64>,
         webrender_api: Box<dyn WebrenderApi>,
         antialias: AntialiasMode,
-        canvas_id: CanvasId,
+        font_cache_thread: FontCacheThread,
     ) -> CanvasData<'a> {
         let backend = create_backend();
         let draw_target = backend.create_drawtarget(size);
@@ -398,7 +434,7 @@ impl<'a> CanvasData<'a> {
             image_key: None,
             old_image_key: None,
             very_old_image_key: None,
-            canvas_id: canvas_id,
+            font_cache_thread: Mutex::new(font_cache_thread),
         }
     }
 
@@ -456,11 +492,114 @@ impl<'a> CanvasData<'a> {
         }
     }
 
-    pub fn fill_text(&self, text: String, x: f64, y: f64, max_width: Option<f64>) {
-        error!(
-            "Unimplemented canvas2d.fillText. Values received: {}, {}, {}, {:?}.",
-            text, x, y, max_width
+    // https://html.spec.whatwg.org/multipage/#text-preparation-algorithm
+    pub fn fill_text(
+        &mut self,
+        text: String,
+        x: f64,
+        y: f64,
+        max_width: Option<f64>,
+        is_rtl: bool,
+    ) {
+        // Step 2.
+        let text = replace_ascii_whitespace(text);
+
+        // Step 3.
+        let point_size = self
+            .state
+            .font_style
+            .as_ref()
+            .map_or(10., |style| style.font_size.size().px());
+        let font_style = self.state.font_style.as_ref();
+        let font = font_style.map_or_else(
+            || load_system_font_from_style(None),
+            |style| {
+                with_thread_local_font_context(&self, |font_context| {
+                    let font_group = font_context.font_group(ServoArc::new(style.clone()));
+                    let font = font_group
+                        .borrow_mut()
+                        .first(font_context)
+                        .expect("couldn't find font");
+                    let font = font.borrow_mut();
+                    // Retrieving bytes from font template seems to panic for some core text fonts.
+                    // This check avoids having to obtain bytes from the font template data if they
+                    // are not already in the memory.
+                    if let Some(bytes) = font.handle.template().bytes_if_in_memory() {
+                        Font::from_bytes(Arc::new(bytes), 0)
+                            .unwrap_or_else(|_| load_system_font_from_style(Some(style)))
+                    } else {
+                        load_system_font_from_style(Some(style))
+                    }
+                })
+            },
+        );
+        let font_width = font_width(&text, point_size, &font);
+
+        // Step 6.
+        let max_width = max_width.map(|width| width as f32);
+        let (width, scale_factor) = match max_width {
+            Some(max_width) if max_width > font_width => (max_width, 1.),
+            Some(max_width) => (font_width, max_width / font_width),
+            None => (font_width, 1.),
+        };
+
+        // Step 7.
+        let start = self.text_origin(x as f32, y as f32, &font.metrics(), width, is_rtl);
+
+        // TODO: Bidi text layout
+
+        let old_transform = self.get_transform();
+        self.set_transform(
+            &old_transform
+                .pre_translate(vec2(start.x, 0.))
+                .pre_scale(scale_factor, 1.)
+                .pre_translate(vec2(-start.x, 0.)),
         );
+
+        // Step 8.
+        self.drawtarget.fill_text(
+            &font,
+            point_size,
+            &text,
+            start,
+            &self.state.fill_style,
+            &self.state.draw_options,
+        );
+
+        self.set_transform(&old_transform);
+    }
+
+    fn text_origin(
+        &self,
+        x: f32,
+        y: f32,
+        metrics: &Metrics,
+        width: f32,
+        is_rtl: bool,
+    ) -> Point2D<f32> {
+        let text_align = match self.state.text_align {
+            TextAlign::Start if is_rtl => TextAlign::Right,
+            TextAlign::Start => TextAlign::Left,
+            TextAlign::End if is_rtl => TextAlign::Left,
+            TextAlign::End => TextAlign::Right,
+            text_align => text_align,
+        };
+        let anchor_x = match text_align {
+            TextAlign::Center => -width / 2.,
+            TextAlign::Right => -width,
+            _ => 0.,
+        };
+
+        let anchor_y = match self.state.text_baseline {
+            TextBaseline::Top => metrics.ascent,
+            TextBaseline::Hanging => metrics.ascent * HANGING_BASELINE_DEFAULT,
+            TextBaseline::Ideographic => -metrics.descent * IDEOGRAPHIC_BASELINE_DEFAULT,
+            TextBaseline::Middle => (metrics.ascent - metrics.descent) / 2.,
+            TextBaseline::Alphabetic => 0.,
+            TextBaseline::Bottom => -metrics.descent,
+        };
+
+        point2(x + anchor_x, y + anchor_y)
     }
 
     pub fn fill_rect(&mut self, rect: &Rect<f32>) {
@@ -1042,6 +1181,18 @@ impl<'a> CanvasData<'a> {
         self.backend.set_shadow_color(value, &mut self.state);
     }
 
+    pub fn set_font(&mut self, font_style: FontStyleStruct) {
+        self.state.font_style = Some(font_style)
+    }
+
+    pub fn set_text_align(&mut self, text_align: TextAlign) {
+        self.state.text_align = text_align;
+    }
+
+    pub fn set_text_baseline(&mut self, text_baseline: TextBaseline) {
+        self.state.text_baseline = text_baseline;
+    }
+
     // https://html.spec.whatwg.org/multipage/#when-shadows-are-drawn
     fn need_to_draw_shadow(&self) -> bool {
         self.backend.need_to_draw_shadow(&self.state.shadow_color) &&
@@ -1121,6 +1272,9 @@ impl<'a> Drop for CanvasData<'a> {
     }
 }
 
+const HANGING_BASELINE_DEFAULT: f32 = 0.8;
+const IDEOGRAPHIC_BASELINE_DEFAULT: f32 = 0.5;
+
 #[derive(Clone)]
 pub struct CanvasPaintState<'a> {
     pub draw_options: DrawOptions,
@@ -1133,6 +1287,9 @@ pub struct CanvasPaintState<'a> {
     pub shadow_offset_y: f64,
     pub shadow_blur: f64,
     pub shadow_color: Color,
+    pub font_style: Option<FontStyleStruct>,
+    pub text_align: TextAlign,
+    pub text_baseline: TextBaseline,
 }
 
 /// It writes an image to the destination target
@@ -1214,3 +1371,64 @@ impl RectExt for Rect<u32> {
         self.cast()
     }
 }
+
+fn load_system_font_from_style(font_style: Option<&FontStyleStruct>) -> Font {
+    let mut properties = Properties::new();
+    let style = match font_style {
+        Some(style) => style,
+        None => return load_default_system_fallback_font(&properties),
+    };
+    let family_names = style
+        .font_family
+        .families
+        .iter()
+        .map(|family_name| family_name.into())
+        .collect::<Vec<FamilyName>>();
+    let properties = properties
+        .style(style.font_style.into())
+        .weight(style.font_weight.into())
+        .stretch(style.font_stretch.into());
+    let font_handle = match SystemSource::new().select_best_match(&family_names, &properties) {
+        Ok(handle) => handle,
+        Err(e) => {
+            error!("error getting font handle for style {:?}: {}", style, e);
+            return load_default_system_fallback_font(&properties);
+        },
+    };
+    font_handle.load().unwrap_or_else(|e| {
+        error!("error loading font for style {:?}: {}", style, e);
+        load_default_system_fallback_font(&properties)
+    })
+}
+
+fn load_default_system_fallback_font(properties: &Properties) -> Font {
+    SystemSource::new()
+        .select_best_match(&[FamilyName::SansSerif], properties)
+        .expect("error getting font handle for default system font")
+        .load()
+        .expect("error loading default system font")
+}
+
+fn replace_ascii_whitespace(text: String) -> String {
+    text.chars()
+        .map(|c| match c {
+            ' ' | '\t' | '\n' | '\r' | '\x0C' => '\x20',
+            _ => c,
+        })
+        .collect()
+}
+
+// TODO: This currently calculates the width using just advances and doesn't
+// determine the fallback font in case a character glyph isn't found.
+fn font_width(text: &str, point_size: f32, font: &Font) -> f32 {
+    let metrics = font.metrics();
+    let mut width = 0.;
+    for c in text.chars() {
+        if let Some(glyph_id) = font.glyph_for_char(c) {
+            if let Ok(advance) = font.advance(glyph_id) {
+                width += advance.x() * point_size / metrics.units_per_em as f32;
+            }
+        }
+    }
+    width
+}