Commits on Feb 22, 2018

1. Lazy load fonts in a FontGroup …

   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.

   

   jonleighton committed Feb 22, 2018
   f22e5ef

2. Add test for FontContext/FontGroup functionality …

   Unfortunately, this required quite a bit of changes to the non-test
   code. That's because FontContext depends on a FontCacheThread, which in
   turn depends on a CoreResourceThread and therefore lots of other data
   structures.
   
   It seemed like it would be very difficult to instantiate a FontContext
   as it was, and even if we could it seems like overkill to have all these
   data structures present for a relatively focused test.
   
   Therefore, I created a FontSource trait which represents the interface
   which FontContext uses to talk to FontCacheThread. FontCacheThread then
   implements FontSource. Then, in the test, we can create a dummy
   implementation of FontSource rather than using FontCacheThread.
   
   This actually has the advantage that we can make our dummy
   implementation behave in certain specific way which are useful for
   testing, for example it can count the number of times
   find_font_template() is called, which helps us verify that
   caching/lazy-loading is working as intended.

   

   jonleighton committed Feb 22, 2018

   Loading status checks…
   e4acb3f