Adds a watcher to a dictionary. A dictionary watcher is identified by three components:

• A dictionary({dict});
• A key pattern({pattern}).
• A function({callback}).

After this is called, every change on {dict} and on keys matching {pattern} will result in {callback} being invoked.

For example, to watch all global variables:

silent! call dictwatcherdel(g:, '*', 'OnDictChanged')
function! OnDictChanged(d,k,z)
  echomsg string(a:k) string(a:z)
endfunction
call dictwatcheradd(g:, '*', 'OnDictChanged')

For now {pattern} only accepts very simple patterns that can contain a "" at the end of the string, in which case it will match every key that begins with the substring before the "". That means if "*" is not the last character of {pattern}, only keys that are exactly equal as {pattern} will be matched.

The {callback} receives three arguments:

• The dictionary being watched.
• The key which changed.
• A dictionary containing the new and old values for the key.

The type of change can be determined by examining the keys present on the third argument:

• If contains both old and new, the key was updated.
• If it contains only new, the key was added.
• If it contains only old, the key was deleted.

This function can be used by plugins to implement options with validation and parsing logic.