Allow handleSubmit to optionally return a promise and call setSubmitting automagically

Simple example from docs:

  err => {
    setSubmitting(false);
    setErrors(err);
  }

For now we have few separate setter functions (like setErrors, setStatus, etc.), each with simple notation f(params) => void. API docs says that it is an imperative way to manage formik’s inner state. And I think of them as a set of setState callbacks where each one updates it’s own portion of formik state. This way of thinking conflicts with example above - when you have to call multiple setter callbacks in a row (finally, we’re not executing React’s setState this way).

More idiomatic api may look like this:

    setSubmitting(false)
      .then(() => setErrors(err));

Or React-way callback:

    setSubmitting(false, () => setErrors(err))

But the simplest one still would be a plain f(params) => void function for the whole state:

    setFormikState({ isSubmitting: false, errors: err })

Also, this way we can skip extra re-render.

[abumalick]

Did you have the change to explore the idea @jaredpalmer ?

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[jaredpalmer]

i think this is a good idea. will explore next week