An issue with the density API is that bitmaps assumed the old default density,
so new programs would have to explicitly set the correct density for every bitmap
they create.
This is an attempt to fix that situation, by define the default density of bitmaps
to be the main screen's density, except for old apps where it is the original default
density.
Actually implementing this is not so great, though, because the Bitmap constructors
can't really know anything about who is calling them to know which density to use.
So at this level the compatibility mode is defined per-process -- meaning the initial
package loaded into a process defines the default bitmap density, and everyone else
loaded in later on has to live with that.
In practice this shouldn't be much of a problem, there shouldn't be much mixing of
old vs. new apps in a process. It does mean that, going forward, if a developer is
going to use shared user IDs for this, they will need to make sure either that all of
their apps are in the same compatibility mode, or that their code explicitly sets the
density of bitmaps it receives. This isn't all that great, but I think it is worth
the benefit of allowing people who write modern apps to not have to deal with bitmap
densities.
This change also does some cleanup of the density management (making sure to always
copy over bitmap densities, etc) and adds java docs to explain the various ways
density is set and used by the system.
This change allows us to use drawables that match the current screen
density even when being loaded in compatibility mode. In this case,
the bitmap is loaded in the screen density, and the bitmap and
nine-patch drawables take care of accounting for the density difference.
This should be safe for existing applications, for the most part, since
they shouldn't really be pulling the bitmap out of the drawable. For
the small rare chance of them breaking, it worth getting the correct
graphics. Also this will only happen when there is actually a resource
of the matching density, and no existing apps should have resources for
anything besides the default density (though of course all of the
framework resources will be available in the native density).
As part of this, the bitmap density API has been changed to a single
integer provider the DPI unit density.
The Bitmap functions to get the scaled width/height couldn't actually
do the right thing because they didn't know the destination they would
be drawing to. Now there are two forms of them, taking an explicit
parameter specifying the destination.
The function is used to rebuild any caches associated with the bitmap.
In the case of purgeable bitmaps, this call ensures that the pixels
are decoded for drawing, and therefore prefetching techniques
implemented by callers can be leveraged.
