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Lazy load fonts in a FontGroup

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This is a step towards fixing #17267. To fix that, we need to be able to
try various different fallback fonts in turn, which would become
unweildy with the prior eager-loading strategy.

Prior to this change, FontGroup loaded up all Font instances, including
the fallback font, before any of them were checked for the presence of
the glyphs we're trying to render.

So for the following CSS:

    font-family: Helvetica, Arial;

The FontGroup would contain a Font instance for Helvetica, and a Font
instance for Arial, and a Font instance for the fallback font.

It may be that Helvetica contains glyphs for every character in the
document, and therefore Arial and the fallback font are not needed at
all.

This change makes the strategy lazy, so that we'll only create a Font
for Arial if we cannot find a glyph within Helvetica. I've also
substantially refactored the existing code in the process and added
some documentation along the way.
This commit is contained in:
Jon Leighton 2018-02-10 10:27:54 +01:00
parent 691f3be24a
commit f22e5ef3bd
7 changed files with 345 additions and 211 deletions
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5
components/gfx/platform/macos/font.rs
View file

@ -18,6 +18,7 @@ use font::{FontHandleMethods, FontMetrics, FontTableMethods, FontTableTag, Fract
use font::{GPOS, GSUB, KERN};
use platform::font_template::FontTemplateData;
use platform::macos::font_context::FontContextHandle;
use servo_atoms::Atom;
use std::{fmt, ptr};
use std::ops::Range;
use std::sync::Arc;
@ -318,4 +319,8 @@ impl FontHandleMethods for FontHandle {
Some(FontTable::wrap(data))
})
}
fn identifier(&self) -> Atom {
self.font_data.identifier.clone()
}
}