Add value prop to ActionForm

[nhunzaker]

This commit adds a value prop to the ActionForm component of microcosm-dom. When set, the value prop is always passed as the serialized value of the form. This is intended to better support controlled forms.

This also achieves a nice parity with ActionButton, which lets you set a similar value property as the callback value when the button is clicked.

import { login } from '../actions/session'

class Login extends React.Component {
  state = {
    email: '',
    password: ''
  }

  render() {
    let { email, password } = this.state

    return (
      <ActionForm action={login} value={this.state}>
        <input type="email" name="email" value={email} onChange={this.setField} />
        <input type="password" name="password" value={password} onChange={this.setField} />
      </ActionForm>
    )
  }

  setField = event => this.setState({ [event.target.name]: [event.target.value] })
}

Fixes #429

[codecov-io]

Codecov Report

Merging #486 into master will increase coverage by 0.55%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
+ Coverage   97.99%   98.54%   +0.55%     
==========================================
  Files          29       29              
  Lines         698      687      -11     
  Branches      135      135              
==========================================
- Hits          684      677       -7     
+ Misses         12        8       -4     
  Partials        2        2

Impacted Files	Coverage Δ
packages/microcosm-dom/src/action-form.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/domain.js	97.91% <0%> (-0.05%)	:arrow_down:
packages/microcosm/src/subject.js	100% <0%> (ø)	:arrow_up:
packages/microcosm/src/microcosm.js	100% <0%> (ø)	:arrow_up:
packages/microcosm/src/observable.js	100% <0%> (+2.27%)	:arrow_up:
packages/microcosm/src/ledger.js	100% <0%> (+11.11%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 0c66124...f79383e. Read the comment docs.

[nhunzaker]

Ack yes. Good call!

[dce]

When you say

When set, the value prop is always passed as the serialized value of the form.

Do you mean "the value prop is always passed intead of the serialized value of the form"?

[nhunzaker]

@dce yes. That is what I meant to say.

[nhunzaker]

@cwmanning Documentation added!