== State Parameters and Transitions ==

Something like this gains access to a lot of view parameters:
{{{
for n in dir(chimera.viewer): print n, eval('type(chimera.viewer.%s)' % n)
}}}

Some tips in the [http://www.cgl.ucsf.edu/chimera/docs/ProgrammersGuide/faq.html programming FAQ] are useful (esp, items 4, 5, 7-10).  

==== openState attribute (from FAQ) ====

Get the OpenState attributes of a model: {{{chimera.openModels.openState(id: int, subid: int)}}}

The openState attribute of a Model controls whether that model is active for motion ('.active'), and contains the model's transformation matrix ('.xform') and center of rotation ('.cofr'). Since some models must move in synchrony (e.g. a molecule and its surface), OpenState instances may be shared among multiple models. If you create a model that needs a shared openState with another model, then when adding your model to the list of open models with chimera.openModels.add(), you should use the 'sameAs' keyword to specify the other model.

==== 'Active' models ====

{{{
#
# Working with a specific active model:
# openState(id: int, subid: int)
modelID = chimera.openModels.listIds()[0]
model_openState = chimera.openModels.openState(*modelID)
if model_openState.active:
    # do something useful with it
    print model_openState.xform
#
# looping over all open-active models
#
if chimera.openModels.hasActiveModels():
    om = chimera.openModels.list(all=True)
    for m in om:
        if m.openState.active:
            # do something useful with it
            print m.openState.xform
#
# Setting 'active' models:
# chimera.openModels.setActive(id: int, active: bool)
#
chimera.openModels.setActive(modelID[0], False)
model_openState.active
# False
chimera.openModels.setActive(modelID[0], True)
model_openState.active
# True

}}}


=== Integrity Checking ===

Take a look at the code for {{{chimera.update.checkForChanges()}}}.  This may be helpful in checking the validity of state parameters and in automatically detecting changes for transitions.

Also, {{{chimera.OpenModels.addRemoveHandler(func, data)}}} might be used to add a trigger handler when removing models.  This might be useful in the creation of a state instance as a way to maintain integrity or validity of the saved state.  It may depend on whether the state instance keeps a reference or a copy of models in the saved state.  For efficiency, it is better to keep a reference.  For validity, it could be better to keep a copy.  Perhaps a copy is required only when a model is removed, so the copy action could be triggered then.  If a state instance that is registered as a handler is deleted, then call {{{chimera.OpenModels.deleteRemoveHandler(handler)}}} to delete the trigger handler.