Make encode_ui() and decode_ui() optional in back-ends

The majority of back-ends define encode_ui() to return NULL and
decode_ui() to do nothing.  This commit allows them to instead specify
the relevant function pointers as NULL, in which case the mid-end won't
try to call them.

I'm planning to add a parameter to decode_ui(), and if I'm going to have
to touch every back-end's version of decode_ui(), I may as well ensure
that most of them never need to be touched again.  And obviously
encode_ui() should go the same way for symmetry.

1 files changed, 6 insertions, 0 deletions

diff --git a/devel.but b/devel.but
index 6496aef..9ce8eee 100644
--- a/devel.but
+++ b/devel.but
@@ -885,6 +885,9 @@ user could edit the save file by hand... But if the user is \e{that}
 determined to cheat, they could just as easily modify the game's
 source.)
 
+The \cw{encode_ui()} function is optional.  If a back-end doesn't need
+this function it can just set the pointer to \cw{NULL}.
+
 \S{backend-decode-ui} \cw{decode_ui()}
 
 \c void (*decode_ui)(game_ui *ui, const char *encoding);
@@ -896,6 +899,9 @@ the best it can, which might just mean not changing the \c{game_ui}
 structure at all.  This might happen if a save file is corrupted, or
 simply from a newer version that encodes more \c{game_ui} data.
 
+Like \cw{encode_ui()}, \cw{decode_ui()} is optional.  If a back-end
+doesn't need this function it can just set the pointer to \cw{NULL}.
+
 \S{backend-changed-state} \cw{changed_state()}
 
 \c void (*changed_state)(game_ui *ui, const game_state *oldstate,