fix(4610): computed keepAlive_ 'true' by default

[Tzahile]

Code change checklist

• Added/updated unit tests
• Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

This is a PR to fix #4610
keepAlive_ default value is changed to true.
As mentioned in the issue description:

With #1799 I don't see any downsides to having all computed values be keepAlive

This PR also

• changes the warning message and its triggers (trigger now if keepAlive_ is false)
• updates tests for v4 and v5
• updates docs with new behaviour and examples

[changeset-bot]

🦋 Changeset detected

Latest commit: 38c8923

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
mobx	Minor

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[Tzahile]

Modifications

• Added a global flag

• keepAlive is set by this order:

  • explicit set
  • global flag
  • defaults to true

  If a computed has no explicit keepAlive ->
  it will then check for global flag value. if there's no global flag ->
  then it will default to true.

[mweststrate]

Thanks for taking the effort of putting up this PR! However, as discussed in the issue (and happy to elaborate more), the current behavior is very deliberately by design. Apologies for my late follow up on the issue, that might have saved the trouble of setting up the PR. But thanks nonetheless for taking a stab at it!

[mweststrate]

Thanks for taking the effort of putting up this PR! However, as discussed in the issue (and happy to elaborate more), the current behavior is very deliberately by design. Apologies for my late follow up on the issue, that might have saved the trouble of setting up the PR. But thanks nonetheless for taking a stab at it!

In theory the PR could still be useful except for changing the default part. What makes me pause here is that it would alter the behavior of any dependency that would also rely on MobX, and might break it as they would assume the current behavior without any override.

[Tzahile]

Hi @mweststrate
No problem at all, I'm happy to have the opportunity to learn and contribute to a tool I use a lot.

I can make the changes so that my PR is still valuable, could you then please specify what I should do?
Or reverting the default value is the only thing you think should be changed?