Syntax highlighting allows files of a certain type (for example, Python programs) to have parts of the text highlighted to distinctively show keywords, comments, or other components. When editing, Vim can lose track of the syntax and may highlight text incorrectly. The :syntax sync command controls how Vim synchronizes the syntax state that should apply at a particular point in the text. The most accurate but slowest result occurs from having Vim always rescan the buffer from the start. Often the syntax file (for example, $VIMRUNTIME/syntax/python.vim) uses :syntax sync to specify the synchronization method to be used for a particular file type.
Highlight from start of file[]
For the most accurate but slowest result, set the syntax synchronization method to fromstart. This can be done with an autocmd in your vimrc:
autocmd BufEnter * :syntax sync fromstart
Search backwards for a highlight starting point[]
For languages that supports C-style comments (e.g. C, C++, javascript and more) it may be desirable to set the synchronization method to ccomment. This method detects if the current position belongs to a multiline comment region. By default, it is for the region Comment, but it can use any other region, like javaScriptComment. Then it searches backward for the starting point /*, and starts highlighting from there.
autocmd FileType javascript syn sync ccomment javaScriptComment
For the rest of multiline syntactic units and comment styles, take a look at :h syn-sync-fourth. This example covers script blocks delimited by <script> and </script> in a html document.
First define the region, if not already defined:
autocmd FileType html syn region javaScript start=+<script\_[^>]*>+ keepend end=+</script\_[^>]*>+me=s-1 contains=@htmlJavaScript,htmlCssStyleComment,htmlScriptTag,@htmlPreproc
And then, when vim detects that current position belongs to a javaScript region, search backwards for <script and start highlighting from there.
autocmd FileType html syn sync match htmlHighlight groupthere javaScript "<script"
Highlight from an amount backwards[]
Vim supports highlighting synchronization by searching a variable amount backwards from the current position for a recognized syntax state. An example for C, from $VIMRUNTIME/syntax/c.vim, is:
if exists("c_minlines")
let b:c_minlines = c_minlines
else
if !exists("c_no_if0")
let b:c_minlines = 50 " #if 0 constructs can be long
else
let b:c_minlines = 15 " mostly for () constructs
endif
endif
exec "syn sync ccomment cComment minlines=" . b:c_minlines
Where c_minlines is the minimum number of lines that Vim goes backward to try to find the start of a comment for syntax highlighting. If the line which starts a comment is outside that range, highlighting may be incorrect. Your vimrc can define this setting, for example:
let c_minlines=500
Larger values improve accuracy, but slow down syntax highlighting.
Rather than using fromstart syntax highlighting, you may get accurate but faster results with something like:
syntax sync minlines=200
Using a mapping to fix when broken[]
The procedures discussed above attempt to ensure that syntax highlighting is always correct, at the cost of slower performance. Another procedure is to tolerate occasional highlighting errors, and correct the problem when it arises by entering a command like:
:syntax sync fromstart
If wanted, mappings can be defined to make entering the command easier:
noremap <F12> <Esc>:syntax sync fromstart<CR> inoremap <F12> <C-o>:syntax sync fromstart<CR>
Now you can press F12 to clean up most syntax highlighting problems. Sometimes, pressing Ctrl-L to redraw the screen helps.
References[]
- :help :syn-sync
- Search for 'sync' in your favorite syntax file in the
$VIMRUNTIME/syntaxdirectory (use:echo expand('$VIMRUNTIME/syntax')to see its location).
Comments[]
I have tweaked the wording, but haven't considered the content much. The autocmd is a bit extreme (applying to all files). It's not clear why the extract from syntax/c.vim is useful. When synching breaks for me, I find that scrolling up/down often fixes it, with no need for the more elaborate ideas here. This doesn't happen often, so I'm not sure what is actually needed, or whether anything helpful can be said about scrolling. JohnBeckett 10:06, May 1, 2011 (UTC)
Surprisingly, I've actually found :syntax sync fromstart to be faster than only reading back a few hundred lines. This is on a fairly long and complex HTML file with embedded CSS and lots of Javascript.
Recently an issue was brought up where syntax highlighting fails for a big file after jump or search on vim 8.
More information here:
☀https://github.com/vim/vim/issues/2790
One of the main reasons for this is that the redraw time is insufficient. To ensure syntax highlighting always works on large files, simply increase the redraw time in your .vimrc file.
set redrawtime=10000