Traced Variable

A Tcl/Tk Idiom

Problem

Glue Code supports many-to-one interactions between components. How can one support one-to-many, and many-to-many (group-based) interactions between components?

Context

Scripted Components must be interconnected in more general configurations than the one-to-one invocations supported by the use of Glue Code.

Forces

  1. Components must be loosely coupled: they must rely on as few other components as possible.
  2. Components must be reusable: it must be possible to use them in as many situations involving as many other types of component as possible.
  3. Components must be interconnected, in general, in a many-to-many configuration.

Solution

Use a variable as a Broadcaster. Parameterise components with the name of a (usually global) variable. Components that want to receive notifications from other components use the Tcl trace command or the Tcl_TraceVar API functions to register write-callbacks on the variable, so that they are informed when the variable's value has been changed. Components that want to send notifications to multiple other components set the value of the variable, so invoking the trace callbacks.

Consequences

One or more "announcer" components can communicate to one or more "listener" components.

A component can be an "announcer" and a "listener" at the same time by setting a trace on a variable and later setting the variable's value.

One can use a global variable to address a group of components without needing to know the exact identities of the group members.

Traced Variable often have to be global, which can make scripts hard to maintain as they grow in size.

Known Uses

The Tk toolkit uses Traced Variables to allow widgets to act as Observers. Many widgets have a -variable configuration option that takes the name of a global variable which the widget must watch or update. For example. Label, button and message widgets change their displayed text when the value of the named variable is changed while entry widgets both change their contents when the named variable changes and set the value of the variable as the user edits the entry's contents.

Example Code

The following example shows how a global variable can be used to link an entry widget and two button widgets. As the user edits the contents of the entry widget, the button named .b updates its text label. When the user presses the button named .clear, the global variable is set to the empty string, causing the contents of the entry and the label of the button to be cleared. 

set text_var ""

entry .e -textvariable text_var
button .b -textvariable text_var
button .clear -text "Clear" -command {set text_var ""}

pack .e .b .clear