2.4.5 Accumulator

Motivation

Accumulator is a notification strategy used to prevent redundant delivery. It combines Lazy Model, Qualified Notification, and Recording Model. When changes are performed on the Model, the intended notification is not delivered to the listeners. Instead, it is recorded once and kept for later delivery. Redundant notifications are discarded. Like in a Lazy Model, the Accumulator then delivers the recorded notifications when instructed from an external request.

The Accumulator is useful when multiple operations must be performed on a Model, and each operation would trigger a notification, forcing unnecessary refreshes of the View. The Accumulator neutralizes redundant notifications, and delivers a single notification at the end of the series of operations.

Design

The accumulator can exist in two different forms: either as a native Model Accumulator, or as an External Accumulator.

A native Model Accumulator is a Model whose design allows storing notifications in a private log, instead of issuing them. Additional notifications are added only if a similar entry is not already present in the log, although for some qualified notifications some merging is needed. For example, if a Model incurs two changes and the notification contains qualifying information about the change, the following scenario would occur:

1. The Model value_a is requested to change from 3 to 5. A notification is added to the log as { "change" : "value_a", "old_value" : 3, "new_value" : 5}
2. The Model value_a is requested to change again from 5 to 7. A notification for the change of value_a is already present in the log, but the new notification cannot be simply discarded.
3. The Model combines the old notification and the new notification so that the log contains the original value (3), and the latest (7) { "change" : "value_a", "old_value" : 3, "new_value" : 7}

On external client request, the Model will then consume the log and deliver the notifications.

The second accumulator strategy, the External Accumulator is instead providing accumulator features to one or more SubModels not supporting the concept natively. The Accumulator listen to its SubModels and logs their notifications. The View is listening to the Accumulator and therefore is not informed of the changes.

Depending on the surrounding design choices, the following strategies can be implemented:

1. additional notifications from any submodel are neutralized.
2. additional notifications from the same submodel are neutralized. notifications from other submodels are recorded.
3. all notifications are enqueued.