watch 监听

• 普通 watch

watch: {
  value (newValue, oldValue) {
    console.log(newValue)
  }
}

• 数组 watch

watch: {
  arr: {
    handler (newValue, oldValue) {
      console.log(newValue)
    },
    deep: true
  }
}

• 对象 watch

watch: {
  obj: {
    handler (newValue, oldValue) {
      console.log(newValue)
    },
    deep: true
  }
}

• 对象的具体属性 watch

watch: {
   'obj.name' (newValue, oldValue) {
    console.log(newValue)
  }
}