Simplest example of extmark usage?

Neovim
3

As you mentioned extmarks are updated if the text changes. Snippet plugins can use this to figure out how the placeholders shifted.

For example, if you’ve a snippet like this:

for ${1:target_list} in ${2:expression_list}:
  ${3:pass}

The plugin can add extmarks to the placeholder regions:

for ${1:target_list} in ${2:expression_list}:
    ^^^^^^^^^^^^^^^^    ^^^^^^^^^^^^^^^^^^^^
  ${3:pass}
  ^^^^^^^^^

Now if the user replaces the first placeholder with some text, the second placeholder moves:

for foo in ${2:expression_list}:
    ^^^    ^^^^^^^^^^^^^^^^^^^^
  ${3:pass}
  ^^^^^^^^^

The extmarks shifted too and the plugin can use this to figure out the updated position of the placeholders

There’s another example how they can be used here: Need advice on how separate render the UI on lua plugin development - #2 by mfussenegger

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