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You can make characters invisible, so that they do not appear on
the screen, with the invisible property. This can be either a
text property (see section 32.19 Text Properties) or a property of an overlay
(see section 38.9 Overlays).
In the simplest case, any non-nil invisible property makes
a character invisible. This is the default case--if you don't alter
the default value of buffer-invisibility-spec, this is how the
invisible property works.
More generally, you can use the variable buffer-invisibility-spec
to control which values of the invisible property make text
invisible. This permits you to classify the text into different subsets
in advance, by giving them different invisible values, and
subsequently make various subsets visible or invisible by changing the
value of buffer-invisibility-spec.
Controlling visibility with buffer-invisibility-spec is
especially useful in a program to display the list of entries in a
database. It permits the implementation of convenient filtering
commands to view just a part of the entries in the database. Setting
this variable is very fast, much faster than scanning all the text in
the buffer looking for properties to change.
invisible properties
actually make a character invisible.
t
invisible property is
non-nil. This is the default.
invisible property fits any one of these criteria,
the character is invisible. The list can have two kinds of elements:
atom
invisible property value
is atom or if it is a list with atom as a member.
(atom . t)
invisible property value
is atom or if it is a list with atom as a member.
Moreover, if this character is at the end of a line and is followed
by a visible newline, it displays an ellipsis.
Two functions are specifically provided for adding elements to
buffer-invisibility-spec and removing elements from it.
buffer-invisibility-spec
(if it is not already present in that list).
buffer-invisibility-spec.
This does nothing if element is not in the list.
One convention about the use of buffer-invisibility-spec is
that a major mode should use the mode's own name as an element of
buffer-invisibility-spec and as the value of the invisible
property:
;; If you want to display an ellipsis:
(add-to-invisibility-spec '(my-symbol . t))
;; If you don't want ellipsis:
(add-to-invisibility-spec 'my-symbol)
(overlay-put (make-overlay beginning end)
'invisible 'my-symbol)
;; When done with the overlays:
(remove-from-invisibility-spec '(my-symbol . t))
;; Or respectively:
(remove-from-invisibility-spec 'my-symbol)
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Ordinarily, commands that operate on text or move point do not care
whether the text is invisible. The user-level line motion commands
explicitly ignore invisible newlines if
line-move-ignore-invisible is non-nil, but only because
they are explicitly programmed to do so.
Incremental search can make invisible overlays visible temporarily
and/or permanently when a match includes invisible text. To enable
this, the overlay should have a non-nil
isearch-open-invisible property. The property value should be a
function to be called with the overlay as an argument. This function
should make the overlay visible permanently; it is used when the match
overlaps the overlay on exit from the search.
During the search, such overlays are made temporarily visible by
temporarily modifying their invisible and intangible properties. If you
want this to be done differently for a certain overlay, give it an
isearch-open-invisible-temporary property which is a function.
The function is called with two arguments: the first is the overlay, and
the second is nil to make the overlay visible, or t to
make it invisible again.
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