core(user-flow): start/end navigation functions

[adamraine]

We previously discussed this idea 🔒 here. Using some promise/callback BS we can implement both the callback approach and start/end approach for user triggered navigations.

[connorjclark]

+1, but docs?

[adamraine]

Yeah docs sound good. @paulirish is #14021 ready to go first?

Also, just had an idea. We could add a new smoke test runner for this. I'll do it in a separate PR.

[paulirish]

why can't this just be a promise? looking at the extpromise impl i'm not seeing what's special about it…

[paulirish]

There's a pattern that the playwright guys used to do (and still do).. and i think it'd achieve what you're going for here…

    let fulfill = () => {};
    let reject = () => {};
    const result = new Promise((res, rej) => { fulfill = res; reject = rej; });

    // …
    fulfill(continueNav)

[connorjclark]

That's not any different. It's just encapsulated. I like it.

[connorjclark]

There's a big amount of indirection here that takes a moment to figure out. I tried to help in my comment suggestion.

[adamraine]

The main goal was to make the code easier to read by reducing the nested promises and such. It was a generic pattern that we might user later, but that's just wishful thinking I guess.

I'm fine going with paul's suggestion here.