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(elisp)Lazy Properties

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Lazy Computation of Text Properties
-----------------------------------

   Instead of computing text properties for all the text in the buffer,
you can arrange to compute the text properties for parts of the text
when and if something depends on them.

   The primitive that extracts text from the buffer along with its
properties is `buffer-substring'.  Before examining the properties,
this function runs the abnormal hook `buffer-access-fontify-functions'.

 - Variable: buffer-access-fontify-functions
     This variable holds a list of functions for computing text
     properties.  Before `buffer-substring' copies the text and text
     properties for a portion of the buffer, it calls all the functions
     in this list.  Each of the functions receives two arguments that
     specify the range of the buffer being accessed.  (The buffer
     itself is always the current buffer.)

   The function `buffer-substring-no-properties' does not call these
functions, since it ignores text properties anyway.

   In order to prevent the hook functions from being called more than
once for the same part of the buffer, you can use the variable
`buffer-access-fontified-property'.

 - Variable: buffer-access-fontified-property
     If this value's variable is non-`nil', it is a symbol which is used
     as a text property name.  A non-`nil' value for that text property
     means, "the other text properties for this character have already
     been computed."

     If all the characters in the range specified for `buffer-substring'
     have a non-`nil' value for this property, `buffer-substring' does
     not call the `buffer-access-fontify-functions' functions.  It
     assumes these characters already have the right text properties,
     and just copies the properties they already have.

     The normal way to use this feature is that the
     `buffer-access-fontify-functions' functions add this property, as
     well as others, to the characters they operate on.  That way, they
     avoid being called over and over for the same text.