RFC for a new Transition.waitFor method

[frederikbosch]

Propose Ember Router public Transition.waitFor method

Rendered

Summary

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[featheredtoast]

The alternative suggestion I had for awaiting routeWillChange methods (as they are written today) is if these methods don't return a promise, there would be nothing to wait for.

It also is similar to how Ember (before/after) model hooks work:
https://guides.emberjs.com/release/routing/asynchronous-routing/#toc_the-router-pauses-for-promises

If the model hook (or the related beforeModel or afterModel hooks) return normal (non-promise) objects or arrays, the transition will complete immediately. But if the model hook (or the related beforeModel or afterModel hooks) returns a promise... the transition will pause until that promise fulfills or rejects.

[frederikbosch]

I have thought of this, the RFC mentions this as alternative solution.

An alternative solution to the waitFor method would be let the router await routeWillChange listeners. But with probably many listeners being used by current users, it is hard to foresee if this would break current applications. A new method allows users to opt-in to an asynchronous switch between old and new route.

And later

Alternatives are already discussed within the design and include awaiting routeWillChange listeners. This alternative has not beed chosen because the author of this RFC cannot foresee if this would break current applications.

If the consensus is going to be that routeWillChange should be awaited, if applicable, then I am happy to create a PR for that. In the RFC I have chosen a new method for aforementioned reasons.

[featheredtoast]

All good, I saw that - I just wanted to mention why it could possibly be a preferred route for RFC reviewers+maintainers.

[evoactivity]

The router and base route class is currently being redesigned, any kind of changes to router.js is going to end up being thrown away in the not so distant future.

Introducing this as a concept, updating guides and teaching it feels like it would be wasted effort as it will all be replaced soon, possibly for something very different, so we risk adding churn here.

I could be convinced it's worth having something in place in the interim, but I do lean more on the side that this could be done in app code for people who really want to use view transitions now without needing to land and shepherd a change at the framework level. A general solution that extends the current base route could be shared as an addon.

I'd be interested in the opinions of @ef4, @mansona and @nickschot.