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 (see Reading a Password).
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.
Elements read from this list are normally recorded by the
record-keeping features (see Recording Input) and while defining a
keyboard macro (see Keyboard Macros). However, an element of the
form (no-record . event) causes event to be
processed normally without recording it.
This function converts the string or vector key to a list of
individual events, which you can put in unread-command-events.
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.
See Timers for Delayed Execution.
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
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)))
This variable allow setting which special events while-no-input
should ignore. It is a list of event symbols (see Event Examples).
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