computed: {

  totalAmount () {

    // 計算出 items 數(shù)組中的 amount 總額

    return this.items.reduce((a, b) =>

      ({ amount: a.amount + b.amount })).amount

  }

},

watch: {

  totalAmount (newVal) {

    // 當(dāng)計算屬性變更時觸發(fā)更新

    console.log('amount change to ', newVal)

  }

}