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Re: Multiline font lock questions
Re: Multiline font lock questions
Sun, 19 Nov 2006 14:21:42 -0500
Gnus/5.11 (Gnus v5.11) Emacs/22.0.90 (gnu/linux)
[ Since this refers to doc and vars that are specific to Emacs-22, it might
be better to discuss it on emacs-devel. ]
> There are three ways to ensure correct identification of multiline
> * Place a `font-lock-multiline' property on the construct when it is
> added to the buffer.
> I don't understand how that can help.
Indeed, it's a bit awkwardly placed and written. This is basically saying
"you can do it yourself manually somehow".
> * Use `font-lock-fontify-region-function' hook to extend the scan so
> that the scanned text never starts or ends in the middle of a
> multiline construct.
> That's understandable; I suppose I can just change the region
> boundaries to encompass any paragraphs that intersect the region and
> then forward on to font-lock-default-fontify-region (?)
> * Add a function to `font-lock-extend-region-functions' that does
> the _identification_ and extends the scan so that the scanned text
> never starts or ends in the middle of a multiline construct.
> This is presumably just a way to do the same thing as the above, only
> more cooperatively (?)
Indeed. But this new var only exists in Emacs-22, so if you want your code
to work with Emacs-21, you should use
> There are three ways to do rehighlighting of multiline constructs:
> * Place a `font-lock-multiline' property on the construct. This
> will rehighlight the whole construct if any part of it is changed.
> In some cases you can do this automatically by setting the
> `font-lock-multiline' variable.
> In which cases? Oh, I see that if I read the next info page it will
> tell me. I suggest a link or something here.
> * Use `jit-lock-contextually'....
> What does "Use" mean? I suppose "set it to non-nil," but I had to
> guess that jit-lock-contextually was a variable; I suggest a doc tweak.
Problem is: in 99% of the cases, it's already set, so there's nothing to do
really, other than rely on it doing its job. That's why I set "use".
> ...This will only rehighlight the part
> of the construct that follows the actual change, and will do it
> after a short delay. This only works if the highlighting of the
> various parts of your multiline construct never depends on text in
> subsequent lines. Since `jit-lock-contextually' is activated by
> default, this can be an attractive solution.
> That clearly wouldn't work for the example I showed above.
> * Place a `jit-lock-defer-multiline' property on the construct.
> This works only if `jit-lock-contextually' is used, but it can
> handle the case where highlighting depends on subsequent lines.
> That seems like it could make it work. What's the advantage of using
> this method?
Over what? `font-lock-multiline'?
> Doesn't slow down typing as much?
Yes: the re-highlighting is delayed a little bit (see
jit-lock-context-time), so it doesn't happen after every keystroke.
For small multiline elements, it's not a big change, but for larger ones, it
can be significant.
Another difference is that it only works if font-lock uses jit-lock.
> This very page links to another page describing
> font-lock-extend-after-change-region-function, which seems like it
> constitutes a fourth way to "do rehighlighting of multiline constructs."
Indeed. Alan McKenzie likes this, but I would rather discourage