import { createForm, getIn } from 'final-form'
import createDecorator from 'final-form-calculate'

// Create Form
const form = createForm({ onSubmit })

// Create Decorator
const decorator = createDecorator(
  // Calculations:
  {
    field: 'foo', // when the value of foo changes...
    updates: {
      // ...set field "doubleFoo" to twice the value of foo
      doubleFoo: (fooValue, allValues) => fooValue * 2
    }
  },
  {
    field: 'bar', // when the value of bar changes...
    updates: {
      // ...set field "foo" to previous value of bar
      foo: (fooValue, allValues, prevValues) => prevValues.bar
    }
  },
  {
    field: /items\[\d+\]/, // when a field matching this pattern changes...
    updates: {
      // ...sets field "total" to the sum of all items
      total: (itemValue, allValues) =>
        (allValues.items || []).reduce((sum, value) => sum + value, 0)
    }
  },
  {
    field: 'foo', // when the value of foo changes...
    updates: {
      // ...asynchronously set field "doubleFoo" to twice the value using a promise
      doubleFoo: (fooValue, allValues) =>
        new Promise(resolve => {
          setTimeout(() => resolve(fooValue * 2), 100)
        })
    }
  },
  {
    field: /\.timeFrom/, // when a deeper field matching this pattern changes...
    updates: (value, name, allValues) => {
      const toField = name.replace('timeFrom', 'timeTo')
      const toValue = getIn(allValues, toField)
      if (toValue && value > toValue) {
        return {
          [toField]: value
        }
      }

      return {}
    }
  }
)

// Decorate form
const undecorate = decorator(form)

// Use form as normal