1 files changed, 92 insertions, 0 deletions
diff --git a/servo/components/style/font_metrics.rs b/servo/components/style/font_metrics.rs
new file mode 100644
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+++ b/servo/components/style/font_metrics.rs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Access to font metrics from the style system.
+
+#![deny(missing_docs)]
+
+use crate::context::SharedStyleContext;
+use crate::values::computed::Length;
+use crate::Atom;
+
+/// Represents the font metrics that style needs from a font to compute the
+/// value of certain CSS units like `ex`.
+#[derive(Clone, Debug, Default, PartialEq)]
+pub struct FontMetrics {
+    /// The x-height of the font.
+    pub x_height: Option<Length>,
+    /// The zero advance. This is usually writing mode dependent
+    pub zero_advance_measure: Option<Length>,
+}
+
+/// Type of font metrics to retrieve.
+#[derive(Clone, Debug, PartialEq)]
+pub enum FontMetricsOrientation {
+    /// Get metrics for horizontal or vertical according to the Context's
+    /// writing mode.
+    MatchContext,
+    /// Force getting horizontal metrics.
+    Horizontal,
+}
+
+/// A trait used to represent something capable of providing us font metrics.
+pub trait FontMetricsProvider {
+    /// Obtain the metrics for given font family.
+    fn query(
+        &self,
+        _context: &crate::values::computed::Context,
+        _base_size: crate::values::specified::length::FontBaseSize,
+        _orientation: FontMetricsOrientation,
+    ) -> FontMetrics {
+        Default::default()
+    }
+
+    /// Get default size of a given language and generic family.
+    fn get_size(
+        &self,
+        font_name: &Atom,
+        font_family: crate::values::computed::font::GenericFontFamily,
+    ) -> Length;
+
+    /// Construct from a shared style context
+    fn create_from(context: &SharedStyleContext) -> Self
+    where
+        Self: Sized;
+}
+
+// TODO: Servo's font metrics provider will probably not live in this crate, so this will
+// have to be replaced with something else (perhaps a trait method on TElement)
+// when we get there
+#[derive(Debug)]
+#[cfg(feature = "servo")]
+/// Dummy metrics provider for Servo. Knows nothing about fonts and does not provide
+/// any metrics.
+pub struct ServoMetricsProvider;
+
+#[cfg(feature = "servo")]
+impl FontMetricsProvider for ServoMetricsProvider {
+    fn create_from(_: &SharedStyleContext) -> Self {
+        ServoMetricsProvider
+    }
+
+    fn get_size(&self, _: &Atom, _: crate::values::computed::font::GenericFontFamily) -> Length {
+        unreachable!("Dummy provider should never be used to compute font size")
+    }
+}
+
+// Servo's font metrics provider will probably not live in this crate, so this will
+// have to be replaced with something else (perhaps a trait method on TElement)
+// when we get there
+
+#[cfg(feature = "gecko")]
+/// Construct a font metrics provider for the current product
+pub fn get_metrics_provider_for_product() -> crate::gecko::wrapper::GeckoFontMetricsProvider {
+    crate::gecko::wrapper::GeckoFontMetricsProvider::new()
+}
+
+#[cfg(feature = "servo")]
+/// Construct a font metrics provider for the current product
+pub fn get_metrics_provider_for_product() -> ServoMetricsProvider {
+    ServoMetricsProvider
+}