Extending Traces with OAT: An Object Attribute Trace Package for Tcl/Tk

Tcl supports variable traces, which associate arbitrary scripts with variable reads, writes and unsets. We developed OAT (Object Attribute Traces), a protocol for extending traces to attributes of arbitrary Tcl "objects." We wrote several OAT-based extensions including TkOAT, which provides traces on attributes of Tk widgets and canvas items. The OAT protocol and derived extensions bring the benefits of more expressive constraints to Tcl/Tk applications by providing extended traces. OAT requires no changes to the Tcl core and is implemented as a loadable library; OAT-based extended trace packages introduce minimal changes to the code of existing extensions (Tk, CMT, etc.). The new version of our formula manager, TclProp, takes advantage of extended traces provided by OAT.