Frontend Event queue

Migrated to MD

This article documents the event queue introduced in NAE 5.5.1

The NAE frontend library implements an event queue (EventQueueService) that assures a serial execution of task related events.

Currently events performed by these services are queued:

• AssignTaskService

• CancelTaskService

• DelegateTaskService

• FinishTaskService

• TaskDataService

These services correspond to the assign, cancel, delegate, finish, setData and getData task events.

A queued event consists of 3 parts:

• a condition that determines whether the event can execute

• the execution of the event

• the cancellation of the event

When a scheduled event gets to the front of the queue and is executed its condition is evaluated first. The condition should check if the event can be performed in the current state of the application and its associated task. Examples of such conditions include:

• Is the task we wanted to cancel still assigned to somebody?

• Is the data field we wanted to set still editable?

If the condition evaluates to true, the event will be executed in accordance to the provided callback. An AfterAction is passed to the callback, which signals to the queue, that the next event can be executed. If you want to schedule your own events into the queue, don’t forget to resolve the provided AfterAction. Otherwise the queue will get stuck and no more task events will executed in the entire application!

If the condition evaluates to false, the effects of the event that remain present on the frontend should be cleared. For example:

• A setData event fails the condition, because the associated data field was made visible by a previous event. However the value set by the user still remains on the frontend. The revert operation changes the value of the visible field to the last known backend value.