7.3. Adjustment Internals

Ok, you say, that's nice, but what if I want to create my own handlers to respond when the user adjusts a range widget or a spin button, and how do I get at the value of the adjustment in these handlers? To answer these questions and more, let's start by taking a look at the attributes of a GtkAdjustment itself:
 
  lower
  upper
  value
  step_increment
  page_increment
  page_size

Given a GtkAdjustment instance adj, each of the attributes are retrieved by adj.lower, adj.value, etc.

Since, when you set the value of an adjustment, you generally want the change to be reflected by every widget that uses this adjustment, PyGTK provides a method to do this:
 
  adjustment.set_value(value)

As mentioned earlier, Adjustment is a subclass of Object just like all the various widgets, and thus it is able to emit signals. This is, of course, why updates happen automagically when you share an adjustment object between a scrollbar and another adjustable widget; all adjustable widgets connect signal handlers to their adjustment's value_changed signal, as can your program. Here's the definition of this signal callback:
 
def value_changed(adjustment):

The various widgets that use the Adjustment object will emit this signal on an adjustment whenever they change its value. This happens both when user input causes the slider to move on a range widget, as well as when the program explicitly changes the value with the set_value() method. So, for example, if you have a scale widget, and you want to change the rotation of a picture whenever its value changes, you would create a callback like this:
 
def cb_rotate_picture(adj, picture):
  set_picture_rotation (picture, adj.value)
...

and connect it to the scale widget's adjustment like this:
 
adj.connect("value_changed", cb_rotate_picture, picture)

What about when a widget reconfigures the upper or lower fields of its adjustment, such as when a user adds more text to a text widget? In this case, it emits the changed signal, which looks like this:
 
def changed(adjustment):

Range widgets typically connect a handler to this signal, which changes their appearance to reflect the change - for example, the size of the slider in a scrollbar will grow or shrink in inverse proportion to the difference between the lower and upper values of its adjustment.

You probably won't ever need to attach a handler to this signal, unless you're writing a new type of range widget. However, if you change any of the values in a Adjustment directly, you should emit this signal on it to reconfigure whatever widgets are using it, like this:
 
adjustment.emit("changed")

Now go forth and adjust!