New variable set-message-function to show message at the end of the minibuffer

* doc/lispref/display.texi (Displaying Messages): Document
set-message-function and clear-message-function.

* lisp/minibuffer.el (minibuffer-message-clear-timeout): New defcustom.
(minibuffer-message-timer, minibuffer-message-overlay): New variables.
(set-minibuffer-message, clear-minibuffer-message): New functions.
(set-message-function, clear-message-function): Set variables to
set-minibuffer-message and clear-minibuffer-message respectively.

* src/keyboard.c (read_char): Call clear_message when
Vclear_message_function is a function.

* src/xdisp.c (set_message): Call Vset_message_function when it's a function.
(clear_message): Call Vclear_message_function when it's a function.
(syms_of_xdisp): New variables set-message-function and clear-message-function
(bug#38457).

2 files changed, 53 insertions, 4 deletions

diff --git a/src/keyboard.c b/src/keyboard.c
index 5135fd0bc84..4cf1f64b487 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -2990,6 +2990,8 @@ read_char (int commandflag, Lisp_Object map,
           safe_run_hooks (Qecho_area_clear_hook);
           clear_message (1, 0);
         }
+      else if (FUNCTIONP (Vclear_message_function))
+        clear_message (1, 0);
     }
 
  reread_for_input_method:
diff --git a/src/xdisp.c b/src/xdisp.c
index 08c6927052c..8cba5c5028d 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -11706,13 +11706,32 @@ truncate_message_1 (ptrdiff_t nchars, Lisp_Object a2)
 static void
 set_message (Lisp_Object string)
 {
+  Lisp_Object message = Qnil;
+
   eassert (STRINGP (string));
 
-  message_enable_multibyte = STRING_MULTIBYTE (string);
+  if (FUNCTIONP (Vset_message_function))
+    {
+      ptrdiff_t count = SPECPDL_INDEX ();
+      specbind (Qinhibit_quit, Qt);
+      message = safe_call1 (Vset_message_function, string);
+      unbind_to (count, Qnil);
 
-  with_echo_area_buffer (0, -1, set_message_1, 0, string);
-  message_buf_print = false;
-  help_echo_showing_p = false;
+      if (STRINGP (message))
+        {
+          string = message;
+          message = Qnil;
+        }
+    }
+
+  if (NILP (message))
+    {
+      message_enable_multibyte = STRING_MULTIBYTE (string);
+
+      with_echo_area_buffer (0, -1, set_message_1, 0, string);
+      message_buf_print = false;
+      help_echo_showing_p = false;
+    }
 
   if (STRINGP (Vdebug_on_message)
       && STRINGP (string)
@@ -11768,6 +11787,14 @@ clear_message (bool current_p, bool last_displayed_p)
     {
       echo_area_buffer[0] = Qnil;
       message_cleared_p = true;
+
+      if (FUNCTIONP (Vclear_message_function))
+        {
+          ptrdiff_t count = SPECPDL_INDEX ();
+          specbind (Qinhibit_quit, Qt);
+          safe_call (1, Vclear_message_function);
+          unbind_to (count, Qnil);
+        }
     }
 
   if (last_displayed_p)
@@ -34940,6 +34967,26 @@ display table takes effect; in this case, Emacs does not consult
                doc: /* If non-nil, debug if a message matching this regexp is displayed.  */);
   Vdebug_on_message = Qnil;
 
+  DEFVAR_LISP ("set-message-function", Vset_message_function,
+               doc: /* If non-nil, function to show the message.
+The function is called with one argument that is the message.
+When this function returns nil, the message is displayed in the echo
+area as usual.  When the function returns a string, the returned
+string is displayed in the echo area.  When this function returns
+other non-nil values, this means that the message was handled
+specially, so the same message is not displayed in the echo area.
+See also `clear-message-function' that can be used to clear the
+message displayed by this function.  */);
+  Vset_message_function = Qnil;
+
+  DEFVAR_LISP ("clear-message-function", Vclear_message_function,
+               doc: /* If non-nil, function to clear message.
+Usually this function is called when the next input event arrives.
+The function is called without arguments.  It is expected to clear the
+message displayed by its counterpart function specified by
+`set-message-function'.  */);
+  Vclear_message_function = Qnil;
+
   DEFVAR_LISP ("redisplay--all-windows-cause", Vredisplay__all_windows_cause,
                doc: /*  */);
   Vredisplay__all_windows_cause = Fmake_hash_table (0, NULL);