async.py

# -*- coding: utf-8 -*-
from celery.task import task


@task
def generate(model, pk, attr):
    instance = model._default_manager.get(pk=pk)
    file = getattr(instance, attr)
    file.generate(save=True)
    return attr


class CeleryCacheStateBackend(object):
    def is_invalid(self, file):
        if not getattr(file, '_file', None):
            # No file on object. Have to check storage.
            return not file.storage.exists(file.name)
        return False

    def validate(self, file):
        if self.is_invalid(file):
            generate.delay(file.instance.__class__, file.instance.pk, file.attname)

    def invalidate(self, file):
        file.delete(save=False)
        generate.delay(file.instance.__class__, file.instance.pk, file.attname) # pre_cache

    def clear(self, file):
        file.detele(save=False)

filename.py

# -*- coding: utf-8 -*-
from django.db.models.signals import post_init


def _post_init_handler(sender, instance, **kwargs):
    value = getattr(instance, _post_init_handler.field)
    setattr(instance, "_%s_state" % _post_init_handler.field, value)


class FilenameCacheStateBackend(object):
    def has_changed(self, file):
        field_name = getattr(file.field, 'image_field', None)
        return getattr(file.instance, "_%s_state" % field_name) != getattr(file.instance, field_name)

    def is_invalid(self, file):
        if not getattr(file, '_file', None):
            # No file on object. Have to check storage.
            return not file.storage.exists(file.name)
        return False

    def validate(self, file):
        if self.is_invalid(file):
            file.generate(save=True)

    def invalidate(self, file):
        if self.has_changed(file):
            file.delete(save=False)
            file.generate(save=True)

    def clear(self, file):
        file.delete(save=False)

    def register_field(self, model_class, field, attname):
        uid = "%s.%s.%s" % (model_class.__module__, model_class.__name__, field.image_field)
        _post_init_handler.field = field.image_field
        post_init.connect(_post_init_handler, sender=model_class, dispatch_uid="%s_init" % uid)