24 自动缓存避免重复计算

针对输入的参数做计算，只要所有相关的参数值没变，不再重新计算而是直接使用之前缓存的结果。

import { createSelector } from 'reselect'
const selectShopItems = state => state.shop.items
const selectTaxPercent = state => state.shop.taxPercent
const selectSubtotal = createSelector(selectShopItems, items =>
  items.reduce((subtotal, item) => subtotal + item.value, 0)
)
const selectTax = createSelector(
  selectSubtotal,
  selectTaxPercent,
  (subtotal, taxPercent) => subtotal * (taxPercent / 100)
)
const selectTotal = createSelector(
  selectSubtotal,
  selectTax,
  (subtotal, tax) => ({ total: subtotal + tax })
)
const exampleState = {
  shop: {
    taxPercent: 8,
    items: [
      { name: 'apple', value: 1.2 },
      { name: 'orange', value: 0.95 }
    ]
  }
}
console.log(selectSubtotal(exampleState)) // 2.15
console.log(selectTax(exampleState)) // 0.172
console.log(selectTotal(exampleState)) // { total: 2.322 }