38.6 Invisible Text

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 Text Properties) or a property of an overlay (see Overlays). Cursor motion also partly ignores these characters; if the command loop finds point within them, it moves point to the other side of them.

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. You should normally use t as the value of the invisible property if you don't plan to set buffer-invisibility-spec yourself.

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.

Two functions are specifically provided for adding elements to buffer-invisibility-spec and removing elements from it.

A convention for 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)

You can check for invisibility using the following function:

Ordinarily, functions that operate on text or move point do not care whether the text is invisible. The user-level line motion commands ignore invisible newlines if line-move-ignore-invisible is non-nil (the default), but only because they are explicitly programmed to do so.

However, if a command ends with point inside or immediately before invisible text, the main editing loop moves point further forward or further backward (in the same direction that the command already moved it) until that condition is no longer true. Thus, if the command moved point back into an invisible range, Emacs moves point back to the beginning of that range, and then back one more character. If the command moved point forward into an invisible range, Emacs moves point forward up to the first visible character that follows the invisible text.

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.