elisp: Event Input Misc
20.8.6 Miscellaneous Event Input Features
-----------------------------------------
This section describes how to peek ahead at events without using them
up, how to check for pending input, and how to discard pending input.
See also the function ‘read-passwd’ (
Reading a Password).
-- Variable: unread-command-events
This variable holds a list of events waiting to be read as command
input. The events are used in the order they appear in the list,
and removed one by one as they are used.
The variable is needed because in some cases a function reads an
event and then decides not to use it. Storing the event in this
variable causes it to be processed normally, by the command loop or
by the functions to read command input.
For example, the function that implements numeric prefix arguments
reads any number of digits. When it finds a non-digit event, it
must unread the event so that it can be read normally by the
command loop. Likewise, incremental search uses this feature to
unread events with no special meaning in a search, because these
events should exit the search and then execute normally.
The reliable and easy way to extract events from a key sequence so
as to put them in ‘unread-command-events’ is to use
‘listify-key-sequence’ (see below).
Normally you add events to the front of this list, so that the
events most recently unread will be reread first.
Events read from this list are not normally added to the current
command’s key sequence (as returned by, e.g., ‘this-command-keys’),
as the events will already have been added once as they were read
for the first time. An element of the form ‘(t . EVENT)’ forces
EVENT to be added to the current command’s key sequence.
-- Function: listify-key-sequence key
This function converts the string or vector KEY to a list of
individual events, which you can put in ‘unread-command-events’.
-- Function: input-pending-p &optional check-timers
This function determines whether any command input is currently
available to be read. It returns immediately, with value ‘t’ if
there is available input, ‘nil’ otherwise. On rare occasions it
may return ‘t’ when no input is available.
If the optional argument CHECK-TIMERS is non-‘nil’, then if no
input is available, Emacs runs any timers which are ready.
Timers.
-- Variable: last-input-event
This variable records the last terminal input event read, whether
as part of a command or explicitly by a Lisp program.
In the example below, the Lisp program reads the character ‘1’,
ASCII code 49. It becomes the value of ‘last-input-event’, while
‘C-e’ (we assume ‘C-x C-e’ command is used to evaluate this
expression) remains the value of ‘last-command-event’.
(progn (print (read-char))
(print last-command-event)
last-input-event)
⊣ 49
⊣ 5
⇒ 49
-- Macro: while-no-input body...
This construct runs the BODY forms and returns the value of the
last one—but only if no input arrives. If any input arrives during
the execution of the BODY forms, it aborts them (working much like
a quit). The ‘while-no-input’ form returns ‘nil’ if aborted by a
real quit, and returns ‘t’ if aborted by arrival of other input.
If a part of BODY binds ‘inhibit-quit’ to non-‘nil’, arrival of
input during those parts won’t cause an abort until the end of that
part.
If you want to be able to distinguish all possible values computed
by BODY from both kinds of abort conditions, write the code like
this:
(while-no-input
(list
(progn . BODY)))
-- Function: discard-input
This function discards the contents of the terminal input buffer
and cancels any keyboard macro that might be in the process of
definition. It returns ‘nil’.
In the following example, the user may type a number of characters
right after starting the evaluation of the form. After the
‘sleep-for’ finishes sleeping, ‘discard-input’ discards any
characters typed during the sleep.
(progn (sleep-for 2)
(discard-input))
⇒ nil
Info Catalog
elisp: Quoted Character Input
elisp: Reading Input