example with callback class methods
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This shows how to use class methods as callbacks.

Note:

There are two class definitions in this example. Both are instantiated - OtherClass around line 84 during
import, and HandlerClass by the get_handlers() function.

HandlerClass.__init__() shows how to define a hal pin which is not associated to any HAL widget.

Passing the builder enables referring to any widget in the tree (see for instance line 33).

Also, a slow (1-second) timer callback is shown which updates the 'value' HAL pin - this pin is
linked to the hal_hbar Horizontal Bar widget through class_callback.hal.
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To run independently:
$  gladevcp -u class_callback.py -H class_callback.hal class_callback.ui