Emacs normally tries to redisplay the screen whenever it waits for input. With the following function, you can request an immediate attempt to redisplay, in the middle of Lisp code, without actually waiting for input.
This function tries immediately to redisplay. The optional argument
force, if non-nil, forces the redisplay to be performed,
instead of being preempted if input is pending.
The function returns t if it actually tried to redisplay, and
nil otherwise. A value of t does not mean that
redisplay proceeded to completion; it could have been preempted by
newly arriving input.
Although redisplay tries immediately to redisplay, it does
not change how Emacs decides which parts of its frame(s) to redisplay.
By contrast, the following function adds certain windows to the
pending redisplay work (as if their contents had completely changed),
but does not immediately try to perform redisplay.
This function forces some or all windows to be updated the next time
Emacs does a redisplay. If object is a window, that window is
to be updated. If object is a buffer or buffer name, all
windows displaying that buffer are to be updated. If object is
nil (or omitted), all windows are to be updated.
This function does not do a redisplay immediately; Emacs does that as
it waits for input, or when the function redisplay is called.
A function run just before redisplay. It is called with one argument,
the set of windows to be redisplayed. The set can be nil,
meaning only the selected window, or t, meaning all the
windows.
This hook is run just before redisplay. It is called once in each
window that is about to be redisplayed, with current-buffer set
to the buffer displayed in that window.