hydrate doesn't emit events initially

[hiendv]

Which project does this relate to?

Start

Describe the bug

Unlike normal render which comes with router.load(), hydrate from @tanstack/start-client-core won't emit events like 'onLoad', 'onResolved' from router subscriptions when initially rendered.

In my scenario, I need to trigger an action once the router is loaded but the StartClient and hydrate won't allow it.

Your Example Website or App

https://github.com/hiendv/tanstack-start-hydrate-events

Steps to Reproduce the Bug or Issue

1. Go to the reproduction repo
2. Clone and pnpm install
3. pnpm test
4. This test fails: https://github.com/hiendv/tanstack-start-hydrate-events/blob/master/src/client.test.tsx
5. This is the test for the workaround: https://github.com/hiendv/tanstack-start-hydrate-events/blob/master/src/client-promised.test.tsx

Expected behavior

As a user, I expect to be able to subscribe to events like normal router.load()

Screenshots or Videos

No response

Platform

• OS: Ubuntu 24.04.2 LTS
• Browser: Version 134.0.6998.88 (Official Build) (64-bit) / JSDOM / Happy DOM
• Version:
  • @tanstack/start-client-core@1.114.25
  • @tanstack/react-router@1.114.25
  • react-dom@19.0.0
  • react@19.0.0

Additional context

No response

[schiller-manuel]

I can imagine adding a new onHydrate event. would that solve your issue?

[hiendaovinh]

I think we need to somehow make it compatible between router.load and hydrate.

[schiller-manuel]

@hiendaovinh what do you mean by that? can't you subscribe to a new onHydrate event in addition?

[hiendaovinh]

I mean, these are events supported by router.load() https://tanstack.com/router/latest/docs/framework/react/api/router/RouterEventsType
So if we add onHydrate event only for start, it doesn't seem to be compatible. People came from TRouter would expect their listeners to behave the same. I think we should support all of them if possible.

[schiller-manuel]

there is no load happening upon hydration, so why would there be an onLoad event?

what does this mean "People came from TRouter" ?
That `onHydrate" event would be a router event, not specifically tied to Start. You can handle SSR yourself without using Start, so that event would fire there as well.

[hiendaovinh]

I see. My bad, I thought that onHydrate woule be a <StartClient /> prop.

[schiller-manuel]

you can emit your own events or handle anything in router's hydrate method.