Do I need PolarisMode to share state between Ember and React on the same page?

No. An app that renders both Ember and React on the same page can share one store between them while using LegacyMode, the schema mode that emulates Models, rather than switching to PolarisMode (preview). See Setup - React for giving React components access to the store.

We recommend that apps without Ember avoid LegacyMode because legacy features such as Models, Adapters and Serializers are built on EmberObject. Using them means shipping ember-source and Ember's reactivity system, a large cost in bundle size and performance for an app that doesn't otherwise use Ember (see Drawbacks of Using LegacyMode). When Ember is already on the page, the app has already paid that cost, so LegacyMode adds little on top of it.

Watch Out For @computed and @tracked on Models

The fields WarpDrive defines for a Model, such as @attr, @belongsTo and @hasMany, use WarpDrive's own signals, so React components that read them re-render when they change. Properties you add to a Model yourself with Ember's @computed or @tracked don't use those signals. Ember templates still update when they change, but React components that read them won't re-render.

🔭 Coming Soon

WarpDrive uses signal decorators internally to define reactive fields. Watch the RFCs for an upcoming proposal to make them public, so apps can define properties that re-render in any framework.