Warn when returning repo.state, allow viewmodel function

[nhunzaker]

Sometimes a presenter just wants to subscribe to all state changes. We can't just return this.repo.state, because the view model is only recalculated when a presenter receives new props.

So here's my solution: support returning a function that is given the current repo state:

class MyPresenter extends Presenter {
  viewModel() {
    return state => state
  }
  render() {
    return <p>Test</p>
  }
}

Or, to quickly extract several keys:

class MyPresenter extends Presenter {
  viewModel() {
    return ({ dogs, cats }) => ({ dogs, cats })
  }
  render() {
    return <p>Test</p>
  }
}

We'll also now warn if this.repo.state is returned from the view model. What do you think?

[nhunzaker]

I have mixed thoughts on returning a function. On one hand, it's very ergonomic. On the other, it totally invites unexpected rendering inefficiencies. Still I also can't help but wonder if there are other neat things you could do here, like make a view model module.

[cwmanning]

👍

[cwmanning]

@nhunzaker easy to add those alternate code examples from the PR description into the docs?

[nhunzaker]

Yess. I sure can.

[nhunzaker]

@cwmanning done.

[coveralls]

Coverage remained the same at 100.0% when pulling 072734f on nh-warn-on-app-state into 0286dd0 on master.