`use_user_state` hook

[Archmonger]

Old Behavior

State is generated on a per-render basis and has no method of being permanently stored.

New Behavior

Allow for persistent state that is databased backed, linked to a User.

Implementation Details

Persistently store/restore state within a database model, linked to a User as a foreign key.

Store context data within a BinaryField.

This will require only serializable data will be allowed within this. Consider using dill.pickle to serialize data more robustly than standard library.

The state should only be fetched from the database once, upon first initialization of the hook's state value. On every set_state call, the value should be synchronized to the database.

We may also want to implement an expires_at parameter to decide when the value gets evicted from the database.

Example Database Model

class IdomUserState(models.Model):
    # One state is stored per user
    # When the user is deleted, the state is also deleted from the database
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    state = models.BinaryField(blank=True, null=True)

The interface may look like this...

# Note: The default state is only used in situations where there is no value in the database
# The websocket is needed in order to grab the user from the `scope`
state, set_state = hooks.use_user_state({"foo":"bar"})
state = {"something":"else"}

# This saves the values to the database
set_state(state)

In the background, the set state is doing the following...

def set_state(state, websocket):
    query = IdomUserState.objects.get_or_create(user=websocket.scope.user)
    query.state = dill.pickle(state)
    query.save()
    ...

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