# Notifications and observers

One key feature of the framework in addition to small memory footprint and type validation is the implementation of the observer pattern. In Anatomy, we introduced the notion of static observers. Here we will discuss them in more details along with dynamic observers. We will also describe in depth the possible signature for notification handlers and the arguments they receive upon invocation.

Note

The point at which notifications are fired has been discussed in Introducing the members.

## Static and dynamic observers

An observer is a callable that is called each time a member changes. For most members it will be:

- when the member get value for the first time either through an assignment or a first access when the default value is used. We will refer to this as a ‘create’ event.

- whenever a different value is assigned to the member. We will refer to this as an ‘update’ event.

- when the value of a member is deleted. We will refer to this as a delete event.

Note

The `ContainerList` member is a special case since it can emit notifications when elements are added or removed from the list. This will be referred to as ‘container’ events.

The distinction between static and dynamic observers comes from the moment at which the binding of the observer to the member is defined. In the case of static observers, this is done at the time of the class definition and hence affects all instances of the class. On the contrary, dynamic observers are bound to a specific instance at a later time.

The next two sections will focus on how to manage static and dynamic observers binding, while the following sections will focus on the signature of the handlers and the content of the notification dictionary passed to the handlers in different situations.

### Static observers

Static observers can be bound to a member in three ways:

- declaring a method matching the name of the member to observe but whose name starts with `_observe_`

- using the `observe` decorator on method. The decorator can take an arbitrary number of arguments which allows to tie the same observer to multiple members. In addition, `observe` accept as argument a dot separated name to automatically observe a member of an atom object stored in a member. Note that this mechanism is limited to a single depth (hence a single dot in the name).

- finally one can manage manually static observer using the following methods defined on the base class of all members: + `add_static_observer` which takes a callable and an optional flag indicating which change types to emit + `remove_static_observer` which takes a single callable as argument

### Dynamic observers

Dynamic observers are managed using the `observe` and `unobserve` methods of the `Atom` class. To observe one needs to pass the name of the member to observe and the callback function. When unobserving, you can either pass just the member name to remove all observers at once or a name and a callback to remove specific observer.

Note

Two specific members have an additional way to manage observers:

- `Event`: expose the methods `bind()` and `unbind()` which takes as single argument the callback to bind.

- `Signal`: similarly `Signal` exposes `connect()` and `disconnect()` which match Qt signals.

## Notification handlers

Now that we discussed all kind of observers and how to manage them, it is more than time to discuss the expected signatures of callback and what information the callback is passed when called.

For observers connected to all members except `Signal`, the callback should accept a single argument which is usually called change. This argument is a dictionary with `str` as keys which are described below:

- `'type'`: A string describing the event that triggered the notification:

    - `'created'`: when accessing or assigning to a member that has no previous value.

    - `'update'`: when assigning a new value to a member with a previous value.

    - `'delete'`: when deleting a member (using `del` or `delattr`)

    - `'container'`: when doing inplace modification on a the of `ContainerList`.

- `'object'`: This is the `Atom` instance that triggered the notification.

- `'name'`: Name of the member from which the notification originate.

- `'value'`: New value of the member (or old value of the member in the case of a delete event).

- `'oldvalue'`: Old value of the member in the case of an update.

Note

As of 0.8.0 `observe` and `add_static_observer` also accepts an optional `ChangeType` flag which can be used to selectively enable or disable which change `type` events are generated.

    from atom.api import Atom, Int, ChangeType

    class Widget(Atom):

        count = Int()

    def on_change(change):
        print(change["type"])

    Widget.count.add_static_observer(on_change, ChangeType.UPDATE | ChangeType.DELETE)

    w = Widget()
    w.count  # Will not emit a "create" event since it was disabled
    w.count += 1 # Will trigger an "update" event
    del w.count # Will trigger a "delete" event

Warning

If you attach twice the same callback function to a member, the second call will override the change type flag of the observer.

In the case of `'container'` events emitted by `ContainerList` the change dictionary can contains additional information (note that `'value'` and `'oldvalue'` are present):

- `'operation'`: a str describing the operation that took place (append, extend, __setitem__, insert, __delitem__, pop, remove, reverse, sort, __imul__, __iadd__)

- `'item'`: the item that was modified if the modification affected a single item.

- `'items'`: the items that were modified if the modification affected multiple items.

  Note

  As mentioned previously, `Signal` emits notifications in a different format. When calling (emitting) the signal, it will pass whatever arguments and keyword arguments it was passed as is to the observers as illustrated below.

      class MyAtom(Atom):

          s = Signal()

      def print_pair(name, value):
          print(name, value)

      a = MyAtom()
      a.s.connect(print_pair)
      a.s('a', 1)

## Suppressing notifications

If for any reason you need to prevent notifications to be propagated you can use the `suppress_notifications` context manager. Inside this context manager, notifications will not be propagated.