If the specified ``<variable>`` changes and no ``<command>`` is given,

a message will be printed to inform about the change.

If ``<command>`` is given, this command will be executed instead.

``<variable>``

Name of the variable being accessed.

``<access>``

One of ``READ_ACCESS``, ``UNKNOWN_READ_ACCESS``, ``MODIFIED_ACCESS``,

``UNKNOWN_MODIFIED_ACCESS``, or ``REMOVED_ACCESS``. The ``UNKNOWN_``

values are only used when the variable has never been set. Once set,

they are never used again during the same CMake run, even if the

variable is later unset.

``<value>``

The value of the variable. On a modification, this is the new

(modified) value of the variable. On removal, the value is empty.

``<current_list_file>``

Full path to the file doing the access.

``<stack>``

List of absolute paths of all files currently on the stack of file

inclusion, with the bottom-most file first and the currently

processed file (that is, ``current_list_file``) last.

Note that for some accesses such as :command:`list(APPEND)`, the watcher

is executed twice, first with a read access and then with a write one.

Also note that an :command:`if(DEFINED)` query on the variable does not

register as an access and the watcher is not executed.

Only non-cache variables can be watched using this command. Access to

cache variables is never watched. However, the existence of a cache

variable ``var`` causes accesses to the non-cache variable ``var`` to

not use the ``UNKNOWN_`` prefix, even if a non-cache variable ``var``

has never existed.