CellProlog

is an option to Cell that gives an expression to evaluate before each ordinary evaluation of the contents of the cell.

Details

CellProlog:>expr evaluates expr for every ordinary kernel evaluation done in the cell, as initiated for example by or by Evaluate in Place.
CellProlog is not evaluated for evaluations associated with Dynamic objects.
The default setting is CellProlog->None.
CellProlog is often set at a style or notebook level, rather than for individual cells.