Problem/Motivation
In Drupal 7 it was very common to temporarily put data on an entity by using an arbitrary property to hold the data. This was abusing the fact that entities were simple objects:
$entity->myproperty = 'my data';
This was intended to keep track of some temporary state until the end of the page request.
Unfortunately this practice is completely unreliable; whenever static caches are cleared, or an unchanged entity is loaded from the database, a new entity will be instantiated and the temporary data is lost.
I see this practice continuing in the wild in D8 code, even though we can now solve keeping track of temporary state by writing a custom service to hold the data. Writing custom services might be overkill though in many cases, and with the lack of an officially supported alternative we will see the continued abuse of properties on the entity.
Proposed resolution
Provide new methods:
EntityInterface::setTemporaryData($key, $value, $persist_on_reload = TRUE)EntityInterface::getTemporaryData($key)EntityInterface::clearTemporaryData($key)
Alongside this we could maybe deprecate the setting of arbitrary properties on entities, so that in D9 or beyond we can disable this completely. This could be as simple as a few lines of documentation on the EntityInterface, or even a deprecated warning being emitted whenever an unsupported property is accessed.
Remaining tasks
Discuss potential pitfalls.
User interface changes
None.
API changes
New methods added.
Data model changes
None.