vim custom syntax, highlight class occurrences with different colors - vim

Question

I'm new to vim syntax highlight customizations. Trying to create a text based call path description file:
// Entry point
Class1#mainMethod
Class1#privateMethod2
Class2#method3
Class3#method4
Class4#method5
In plantUML the equivalent would be:
ExternalActor -> Class1 : mainMethod
Class1 -> Class1 : privateMethod2()
Class1 -> Class2 : method3()
Class2 -> Class3 : method4()
Class1 -> Class4 : method5()
I already have some decent syntax file in place that makes it look like:
current syntax file looks like:
syn keyword celTodo contained TODO FIXME XXX NOTE
syn match celComment "//.*$" oneline contains=celTodo
hi celComment ctermfg=yellow
hi celTodo ctermfg=green
syn match methodCall /\(#\)\#<=\w*/ contained oneline
hi methodCall ctermfg=blue
syn match className /\w*\(#\)\#=/ contained oneline
hi className ctermfg=red
syn region line start='\(\.\)\#<=\w' end='.\($\)\#=' oneline fold transparent contains=celComment,className,methodCall
Problem
I think a different background color for each occurrence of the same class would help understand the sequence better.
Is there any way to achieve this? So that Class1 will have a different background color than Class2, 3, and 4. But each class always the same, consistent color.

Answer 1

Syntax highlighting associates a keyword, pattern match, or region with a syntax group. A corresponding highlight group then (directly or indirectly through linked groups) determines the color and formatting of the text.
In order to have different (background or otherwise) colors for each class name, you'd have to define different syntax groups, and assign different highlight groups, too. Your syntax file would not only have a fixed set of :syntax match commands, but also a loop that extracts matches from the current buffer and builds the corresponding :syntax match and :highlight command as the syntax loads (using :execute).
Then you have the problem of updating, if the user adds or changes class names. Normally, a syntax is static, so once it loads, it's done. In your case, you'd have to define :autocmds that periodically re-scan the buffer, and add new class names (and maybe even recycle unused highlight group names, so you don't run out of colors). The CursorHold event would be a good candidate for it, but there will be a delay until the colors show up. The available colors is another problem if you want to make this syntax available to other users. The number of colors can vary wildly, and coming up with background colors that work well with various colorschemes is difficult.
Summary
It is possible, but it would be unusual for a syntax, and have side effects like delays in updating or poor performance. (I've seen this used for highlighting function names from the tags file, though.) Some users definitely would want to turn this off.
Alternative
For small files with few (or very distinct) class names, this additional highlighting probably isn't necessary. For large files with many classes, having everything light up would make it appear like a Christmas tree, and all the colors could be more distracting than helpful. I'd rather leave it to the user to do such highlighting of some classes of interest, on demand. My Mark plugin provides the generic functionality for this, in a way that does not interfere with syntax highlighting, and it ships with color palettes that look like text marker highlightings. I use this often to have better orientation in log files or legacy code bases. (The plugin page has links to alternative plugins; there are a few.)
Your syntax
Group names typically have a common prefix that's identical to the name of your syntax. If that is cel, use celMethodCall instead of methodCall, and so on.
I would put the :hi commands all at the bottom; most syntax plugins do it like this.
Especially if you intend to later share the syntax, favor :hi linking to existing syntax groups (:help highlight-groups) over defining your own colors. Even for your personal use, defining your colors in your ~/.vimrc has the benefit of having a single place to adapt and reuse it, instead of hunting around various syntax scripts.
By using :hi def, users can customize the syntax, e.g. in their ~/.vimrc. :help 44.12 has more information on writing syntax plugins.