Info Catalog elisp: Windows elisp: Windows and Frames

elisp: Basic Windows

 
 27.1 Basic Concepts of Emacs Windows
 ====================================
 
 A “window” is an area of the screen that is used to display a buffer
 (Buffers).  In Emacs Lisp, windows are represented by a special
 Lisp object type.
 
    Windows are grouped into frames (Frames).  Each frame
 contains at least one window; the user can subdivide it into multiple,
 non-overlapping windows to view several buffers at once.  Lisp programs
 can use multiple windows for a variety of purposes.  In Rmail, for
 example, you can view a summary of message titles in one window, and the
 contents of the selected message in another window.
 
    Emacs uses the word “window” with a different meaning than in
 graphical desktop environments and window systems, such as the X Window
 System.  When Emacs is run on X, each of its graphical X windows is an
 Emacs frame (containing one or more Emacs windows).  When Emacs is run
 on a text terminal, the frame fills the entire terminal screen.
 
    Unlike X windows, Emacs windows are “tiled”; they never overlap
 within the area of the frame.  When a window is created, resized, or
 deleted, the change in window space is taken from or given to the
 adjacent windows, so that the total area of the frame is unchanged.
 
  -- Function: windowp object
      This function returns ‘t’ if OBJECT is a window (whether or not it
      displays a buffer).  Otherwise, it returns ‘nil’.
 
    A “live window” is one that is actually displaying a buffer in a
 frame.
 
  -- Function: window-live-p object
      This function returns ‘t’ if OBJECT is a live window and ‘nil’
      otherwise.  A live window is one that displays a buffer.
 
    The windows in each frame are organized into a “window tree”.  
 Windows and Frames.  The leaf nodes of each window tree are live
 windows—the ones actually displaying buffers.  The internal nodes of the
 window tree are “internal windows”, which are not live.
 
    A “valid window” is one that is either live or internal.  A valid
 window can be “deleted”, i.e., removed from its frame (Deleting
 Windows); then it is no longer valid, but the Lisp object representing
 it might be still referenced from other Lisp objects.  A deleted window
 may be made valid again by restoring a saved window configuration (
 Window Configurations).
 
    You can distinguish valid windows from deleted windows with
 ‘window-valid-p’.
 
  -- Function: window-valid-p object
      This function returns ‘t’ if OBJECT is a live window, or an
      internal window in a window tree.  Otherwise, it returns ‘nil’,
      including for the case where OBJECT is a deleted window.
 
    In each frame, at any time, exactly one Emacs window is designated as
 “selected within the frame”.  For the selected frame, that window is
 called the “selected window”—the one in which most editing takes place,
 and in which the cursor for selected windows appears (Cursor
 Parameters).  The selected window’s buffer is usually also the current
 buffer, except when ‘set-buffer’ has been used (Current Buffer).
 As for non-selected frames, the window selected within the frame becomes
 the selected window if the frame is ever selected.  Selecting
 Windows.
 
  -- Function: selected-window
      This function returns the selected window (which is always a live
      window).
 
    Sometimes several windows collectively and cooperatively display a
 buffer, for example, under the management of Follow Mode (
 (emacs)Follow Mode), where the windows together display a bigger
 portion of the buffer than one window could alone.  It is often useful
 to consider such a “window group” as a single entity.  Several functions
 such as ‘window-group-start’ (Window Start and End) allow you to
 do this by supplying, as an argument, one of the windows as a stand in
 for the whole group.
 
  -- Function: selected-window-group
      When the selected window is a member of a group of windows, this
      function returns a list of the windows in the group, ordered such
      that the first window in the list is displaying the earliest part
      of the buffer, and so on.  Otherwise the function returns a list
      containing just the selected window.
 
      The selected window is considered part of a group when the buffer
      local variable ‘selected-window-group-function’ is set to a
      function.  In this case, ‘selected-window-group’ calls it with no
      arguments and returns its result (which should be the list of
      windows in the group).
 

Info Catalog elisp: Windows elisp: Windows and Frames