Models allow code reuse between Components and reducers' action creators

AnyJunk-redux-models-component.js

// @flow
// model Collection
import Maybe from 'folktale/data/maybe'
import Result from 'folktale/data/result'

const MAX_VOLUME = 100

type CollectionState = {
  volume: number,
}

class Collection {

  constructor(data: CollectionState) {
    this._data = data
  }

  safeGetVolume(): Maybe<number> {
    return Maybe.fromNullable(this._data)
      .chain(d => Maybe.fromNullable(d.volume))
  }

  // this is used by the Component and by the reducer's thunk
  validateVolume(): Result<string, number> {
    return this.safeGetVolume()
      .map(volume => {
        if (volume >= MAX_VOLUME) {
          return Result.Error('Volume exceeding the maximum allowed')      
        }
        return Result.Ok(volume)
      })
      .getOrElse(Result.Error('Invalid volume'))
  }
}

// Component Volume
import {Component}, React from 'react'
import {Text} from 'react-native'

type Props = {
  data: CollectionState,
}
class Volume extends Component<Props>{

  constructor(props: Props) {
    super(props)
    this.collection = new Collection(props.data)
  }

  Collection: Collection

  render() {
    return this.collection.validateVolume()
      .matchWith({
        Error: ({value}) => {
          <Text style={styles.error}>{value}</Text>
        },
        Ok: ({value}) => {
          <Text>Volume: {value}yd</Text>
        },
      })
  }
}

// collection reducer

function failedSetVolume(errMsg: string): Action {
  return {
    type: 'SET_VOLUME_FAILURE',
    payload: {
      errMsg,
    },
  }
}

function successSetVolume(volume: number): Action {
  return {
    type: 'SET_VOLUME_SUCCESS',
    payload: {
      volume,
    },
  }
}

// relevant reducer thunk
const setVolume = (collection: Collection) => (dispatch: Dispatch) => Action {
  return collection.validateVolume()
    .matchWith({
      Error: ({value}) => dispatch(failedSetVolume(value)),
      Ok: ({value}) => dispatch(successSetVolume(value)),
    })
}

// reducer implementation omitted