/CMake 3.19


Watch the CMake variable for change.

variable_watch(<variable> [<command>])

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. The command will receive the following arguments: COMMAND(<variable> <access> <value> <current_list_file> <stack>)


Name of the variable being accessed.


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.


The value of the variable. On a modification, this is the new (modified) value of the variable. On removal, the value is empty.


Full path to the file doing the access.


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 list(APPEND), the watcher is executed twice, first with a read access and then with a write one. Also note that an 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.

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