28.18 Hooks for Window Scrolling and Changes

This section describes how a Lisp program can take action whenever a window displays a different part of its buffer or a different buffer. There are three actions that can change this: scrolling the window, switching buffers in the window, and changing the size of the window. The first two actions run window-scroll-functions; the last runs window-size-change-functions. The paradigmatic use of these hooks is in the implementation of Lazy Lock mode; see section `Font Lock Support Modes' in The GNU Emacs Manual.

Variable: window-scroll-functions

This variable holds a list of functions that Emacs should call before redisplaying a window with scrolling. It is not a normal hook, because each function is called with two arguments: the window, and its new display-start position.

Displaying a different buffer in the window also runs these functions.

These functions must be careful in using window-end (see section 28.10 The Window Start Position); if you need an up-to-date value, you must use the update argument to ensure you get it.

Variable: window-size-change-functions

This variable holds a list of functions to be called if the size of any window changes for any reason. The functions are called just once per redisplay, and just once for each frame on which size changes have occurred.

Each function receives the frame as its sole argument. There is no direct way to find out which windows on that frame have changed size, or precisely how. However, if a size-change function records, at each call, the existing windows and their sizes, it can also compare the present sizes and the previous sizes.

Creating or deleting windows counts as a size change, and therefore causes these functions to be called. Changing the frame size also counts, because it changes the sizes of the existing windows.

It is not a good idea to use save-window-excursion (see section 28.17 Window Configurations) in these functions, because that always counts as a size change, and it would cause these functions to be called over and over. In most cases, save-selected-window (see section 28.4 Selecting Windows) is what you need here.

Variable: redisplay-end-trigger-functions

This abnormal hook is run whenever redisplay in a window uses text that extends past a specified end trigger position. You set the end trigger position with the function set-window-redisplay-end-trigger. The functions are called with two arguments: the window, and the end trigger position. Storing nil for the end trigger position turns off the feature, and the trigger value is automatically reset to nil just after the hook is run.

Function: set-window-redisplay-end-trigger window position

This function sets window's end trigger position at position.

Function: window-redisplay-end-trigger &optional window

This function returns window's current end trigger position.

Variable: window-configuration-change-hook

A normal hook that is run every time you change the window configuration of an existing frame. This includes splitting or deleting windows, changing the sizes of windows, or displaying a different buffer in a window. The frame whose window configuration has changed is the selected frame when this hook runs.