That is not possible at runtime because there are no interrupts you can attach to a single memory location. And if there would be an extra thread which is continuously watching that specific memory location you would have high CPU usage and you could also miss some events, e.g. the variable is toggling very fast between 0 and 1.
However I think it could be possible with "Runtime" variables but maybe this is not what you want. It would make anything slower because every write access to the variable would need more instructions than usual.
The english grammar is freeware, you can use it freely - But it's not Open Source, i.e. you can not change it or publish it in altered way.