firebase offline-online somethings

Now try to use firebase for design app. But get somethings about :

1. At offline, you can read, but add, delete, edit can not, this "can not" is snapshot that need to online sync, then get data.

2. So firebase example is use models or objects to keep data, but you change activeity or clearn app, then objects data loss. maybe can use Serialization, but not good idea.

3. Can't control local database. This is fun, maybe firebase no local database or other methods for offline. Some app need offline, for  counting data, CURD data, keep then sync, counting age.

mm, I think my java, android app and firebase knowledge, I don't real understand or think error. So just writer this message for keep.