[racket] How to automatically update the value of output widgets?

From: Marijn (hkBst at gentoo.org)
Date: Tue Jan 24 07:30:22 EST 2012

Hash: SHA1

Hi list,

Suppose you're creating a graphical user interface to some external
key-value store. Your interface consists of a number of tabs each of
which contains some input fields and some output fields (potentially
created automatically according to the values of certain fields of the
store which can be changed on a different tab).
For example the store might have entries for the dimensions of a
2-dimensional matrix, its constituent elements and for each row and
column the sum-total of its elements. The user interface might consist
of one tab to enter the dimensions (dimension-tab) and another tab
(element-tab) with input fields for each element and output fields for
the sum-totals of each row and each column. Whenever the element-tab
becomes shown the number and type of its in- and output fields is
recalculated (through an overridden on-superwindow-show method).
Whenever the user changes the value of one of the elements of the
matrix via an input field, the corresponding value in the store is
changed and the row-sums and column-sums are updated automatically by
the store.
The question is now: ``How should the program automatically update the
values of the changed output fields?''. I see a number of
possibilities divided into two philosophies:

Philosophy 1: All output fields have been potentially changed after
any change in an input field.
  Idea 1: Call the tab's refresh method which should inform all its
children to also refresh themselves. Hook into the refresh call of an
output field and have it refresh its value from the store before
refreshing its drawing. Unfortunately I'm a bit fuzzy on the details
of this method override.
  Idea 2: Call the tabs get-children method to find its children.
Examine those recursively with get-children (properly wrapped in an
exception-catcher in case they don't have that method) and for each
leaf child call its update-value-from-store method (again properly

Philosophy 2: Inform output fields when their values have been
definitely changed.
  Idea A: Have each output field register somewhere that it wants to
be notified of any changes to its value. When one of the dimensions
changes the output fields should unregister (because they will be
garbage collected) and the newly created output field should register
in its stead. This can perhaps be accomplished with a collection of
weak references to registered callbacks, though the details are
unclear to me.
  Idea B: Create a FrTime signal for each output value in the store
and hook those up to the output fields. Then FrTime handles the
registering and deregistering of output fields and we only need to
hook up the signals to the store.

Which way would you go if you were creating this user interface?

Thanks for your thoughts,

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