The BigWorld.loadResourceListBG() method is useful for more complicated entities. For example, say you have an entity that can change its look at anytime to any of a large number of combinations. In this case, you can setup your initial look using prerequisites, but there is no way of knowing at which resources will be required in the future.

In order to implement this case, load the model asynchronously when needed, as in the following example:

class PickableFlower( BigWorld.Entity ):
  ASSETS = {
    'lili' : ('models\lili.model', 'effects\sparkles.xml')
    'daisy' : ('models\daisy.model', 'effects\spirits.xml')
    'orchid' : ('models\orchid.model', 'effects\petals.xml')
  }

  PICK_SOUND = 'sounds\pick_flower.wav'

  def __init__(self):
    pass


  # This event handler is called after the entity is initialised,
  # and its initial properties have been set.  We should return
  # a list of resources that must be loaded into memory before
  # onEnterWorld is called.
  def prerequisite(self):
    prereqs = []
    prereqs += PickableFlower.PICK_SOUND
    prereqs += PickableFlower.ASSETS[self.type]
    return prereqs


  # This event handler is called when the entity is first visible to
  # the client, and when the prerequisite resources have been loaded.
  # The prerequisites parameter is a PyResourceRefs instance, and
  # can be used both to access the loaded resources, and to manage
  # their lifetime.
  def onEnterWorld(self, prerequisites):
    self.set_type(-1, prerequisites)


  # This event handler is called whenever the entity type property
  # is changed.  This can occur at anytime.  If we are calling this
  # directly from onEnterWorld, the resources are already loaded and
  # we can use them directly.  Otherwise we must queue up an
  # asynchronous load of the required resources.
  def set_type( self, old_type, prerequisites = None ):
    if self.type != old_type:
        if prerequisites == None:
            resourceList = PickableFlower.ASSETS[self.type]
            BigWorld.loadResourceListBG( resourceList, self.onLoad )
        else:
            self.onLoad( prerequisites )


  # This user-defined callback function is called either when the
  # loadResourceListBg method has finished, or is called by ourselves
  # from enterWorld.
  def onLoad( self, resourceRefs ):
    self.model = resourceRefs[ PickableFlower.ASSETS[self.type][0] ]
    self.particles = resourceRefs[ PickableFlower.ASSETS[self.type][1] ]
    self.model.root.attach( self.particles )


  def onPickEvent(self):
    # picking sound effect is constant between types
    self.model.playSound(PickableFlower.PICK_SOUND)
    self.particles.force(1)

Asynchronously loading the model