Classifier: Single Image Viewer should "reset Subject" when component unmounts?

[shaunanoordin]

Lifecycle Improvement

Packages: lib-classifier, app-project (see note-1) Component: Subject Viewer -> Single Image Viewer Follows #2221 (initial issue) and #2241 (initial solution)

Context:

• In #2221, we discovered there's a flaw in the Single Image Viewer lifecycle. If a user on app-project visits the Classifier page, navigates to the project Home page, then returns to the Classifier page, the Subject resource remains fully loaded, but the Subject image file has to be re-loaded.
• This breaks the assumption that when the Classifier loads, the Subject and image file will always be loading, in order.

• 2241 fixes the lifecycle bug by adding a "placeholder image" for the "image file is loading" phase.

While we have a basic solution to the pertinent problem, we can still improve the component lifecycle. One idea is to perform a proper cleanup after the component unmounts.

Recommended Enhancement:

• When the SingleImageViewer component unmounts (i.e. runs componentWillUnmount()), it should run resetSubject() to clean up the state of the Subject.
  • This should ensure the "when the Classifier (re)loads, the Subject and image file should begin loading, in order" assumption built into the component
  • ⚠️ However, we need to double check that the resetSubject() ONLY resets the Subject back to a "loading state". It shouldn't change other parameters (e.g. it shouldn't skip the Subject and move to the next one in the queue, or erase existing annotations!) or otherwise break functionality. We just want to ensure the lifecycle assumptions are followed, is all.

(note 1) the actual code changes need to be made in lib-classifier, but the behaviour can only be tested on app-project/

Status

Recommended enhancement. Low urgency since it's not breaking anything at the moment, that we know if, but a clean lifecycle is the correct way to do things.

[goplayoutside3]

I'm unsure if this is still needed. React no longer uses componentWillUnmount and SingleImageViewer has been refactored to a functional component since this Issue was opened. @shaunanoordin when you have time, will you double check if this Issue is still a recommended refactor?