如何通过Vue Composition API / Vue 3观看道具更改?

时间:2019-12-01 13:14:54

标签: javascript typescript vue.js vue-reactivity vue-composition-api

虽然Vue Composition API RFC Reference sitewatch模块具有许多高级使用场景,但没有如何观看道具道具的示例?

Vue Composition API RFC's main pagevuejs/composition-api in Github中都没有提及。

我已经创建了Codesandbox来详细说明此问题。

<template>
  <div id="app">
    <img width="25%" src="./assets/logo.png">
    <br>
    <p>Prop watch demo with select input using v-model:</p>
    <PropWatchDemo :selected="testValue"/>
  </div>
</template>

<script>
import { createComponent, onMounted, ref } from "@vue/composition-api";
import PropWatchDemo from "./components/PropWatchDemo.vue";

export default createComponent({
  name: "App",
  components: {
    PropWatchDemo
  },
  setup: (props, context) => {
    const testValue = ref("initial");

    onMounted(() => {
      setTimeout(() => {
        console.log("Changing input prop value after 3s delay");
        testValue.value = "changed";
        // This value change does not trigger watchers?
      }, 3000);
    });

    return {
      testValue
    };
  }
});
</script>
<template>
  <select v-model="selected">
    <option value="null">null value</option>
    <option value>Empty value</option>
  </select>
</template>

<script>
import { createComponent, watch } from "@vue/composition-api";

export default createComponent({
  name: "MyInput",
  props: {
    selected: {
      type: [String, Number],
      required: true
    }
  },
  setup(props) {
    console.log("Setup props:", props);

    watch((first, second) => {
      console.log("Watch function called with args:", first, second);
      // First arg function registerCleanup, second is undefined
    });

    // watch(props, (first, second) => {
    //   console.log("Watch props function called with args:", first, second);
    //   // Logs error:
    //   // Failed watching path: "[object Object]" Watcher only accepts simple
    //   // dot-delimited paths. For full control, use a function instead.
    // })

    watch(props.selected, (first, second) => {
      console.log(
        "Watch props.selected function called with args:",
        first,
        second
      );
      // Both props are undefined so its just a bare callback func to be run
    });

    return {};
  }
});
</script>

编辑:尽管我的问题和代码示例最初是使用JavaScript编写的,但实际上我正在使用TypeScript。托尼·汤姆(Tony Tom)的第一个答案虽然有效,但会导致类型错误。这由MichalLevý的答案解决。因此,之后我用typescript标记了这个问题。

EDIT2 :这是我针对此自定义选择组件的无功布线的抛光但准系统的版本,位于<b-form-select> bootstrap-vue上方(否则不可知,实现,但此基础组件会根据是通过编程还是通过用户交互来进行更改,都发出@input和@change事件。

<template>
  <b-form-select
    v-model="selected"
    :options="{}"
    @input="handleSelection('input', $event)"
    @change="handleSelection('change', $event)"
  />
</template>

<script lang="ts">
import {
  createComponent, SetupContext, Ref, ref, watch, computed,
} from '@vue/composition-api';

interface Props {
  value?: string | number | boolean;
}

export default createComponent({
  name: 'CustomSelect',
  props: {
    value: {
      type: [String, Number, Boolean],
      required: false, // Accepts null and undefined as well
    },
  },
  setup(props: Props, context: SetupContext) {
    // Create a Ref from prop, as two-way binding is allowed only with sync -modifier,
    // with passing prop in parent and explicitly emitting update event on child:
    // Ref: https://vuejs.org/v2/guide/components-custom-events.html#sync-Modifier
    // Ref: https://medium.com/@jithilmt/vue-js-2-two-way-data-binding-in-parent-and-child-components-1cd271c501ba
    const selected: Ref<Props['value']> = ref(props.value);

    const handleSelection = function emitUpdate(type: 'input' | 'change', value: Props['value']) {
      // For sync -modifier where 'value' is the prop name
      context.emit('update:value', value);
      // For @input and/or @change event propagation
      // @input emitted by the select component when value changed <programmatically>
      // @change AND @input both emitted on <user interaction>
      context.emit(type, value);
    };

    // Watch prop value change and assign to value 'selected' Ref
    watch(() => props.value, (newValue: Props['value']) => {
      selected.value = newValue;
    });

    return {
      selected,
      handleSelection,
    };
  },
});
</script>

5 个答案:

答案 0 :(得分:3)

我只是想在上面的答案中添加更多详细信息。正如Michal所提到的,props是一个对象,整体上是反应性的。但是,props对象中的每个键都不是独立的。

watch值相比,我们需要调整reactive对象中的值的ref签名

// watching value of a reactive object (watching a getter)

watch(() => props.selected, (selection, prevSelection) => { 
   /* ... */ 
})
// directly watching a ref

const selected = ref(props.selected)

watch(selected, (selection, prevSelection) => { 
   /* ... */ 
})

仅提供一些更多信息,即使它不是问题中提到的情况: 如果我们要监视多个属性,则可以传递一个数组而不是单个引用

// Watching Multiple Sources

watch([ref1, ref2, ...], ([refVal1, refVal2, ...],[prevRef1, prevRef2, ...]) => { 
   /* ... */ 
})

答案 1 :(得分:2)

如果您查看watch,并键入here(最后一个项目符号),则可以清楚地看到watch的第一个参数可以是数组,函数或Ref<T>

传递给props函数的

setup是反应性对象(可能由reactive()制造),它的属性是吸气剂。因此,您要做的就是将getter的值作为watch的第一个参数传递-在这种情况下为字符串“ initial”。由于Vue 2 $watch API是在后台使用的,因此您实际上在尝试在组件实例上监视名称为“ initial”的不存在的属性。您的回调仅被调用一次,不再调用。之所以调用它至少一次是因为新的watch API的行为类似于具有$watch选项的当前immediate

因此,偶然地,您做的事情与托尼·汤姆(Tony Tom)所建议的相同,但价值错误。在两种情况下,如果您使用的是TypeScript,那都是无效的代码

您可以改为:

watch(() => props.selected, (first, second) => {
      console.log(
        "Watch props.selected function called with args:",
        first,
        second
      );
    });

这是watch API docs的第二个例子

其他方法是使用toRefs转换props对象,使其属性为Ref<T>类型,您可以将其作为watch的第一个参数传递

答案 2 :(得分:1)

按如下所示更改您的观看方法。

 watch("selected", (first, second) => {
      console.log(
        "Watch props.selected function called with args:",
        first,second
      );
      // Both props are undefined so its just a bare callback func to be run
    });

答案 3 :(得分:1)

这没有解决如何“监视”属性的问题。但是,如果您想知道如何使道具通过Vue的Composition API做出响应,请继续阅读。在大多数情况下,您不必编写大量代码来“监视”事物(除非您在更改后会产生副作用)。

秘密在于:组件props是被动的。一旦您访问特定的道具,它就不会反应。划分或访问对象的一部分的过程称为“解构”。在新的Composition API中,您需要经常思考这一点-这是决定使用reactive()ref()的关键部分。

因此,我建议(下面的代码)是您要保留所需的属性,并将其设置为ref

export default defineComponent({
  name: 'MyAwesomestComponent',
  props: {
    title: {
      type: String,
      required: true,
    },
    todos: {
      type: Array as PropType<Todo[]>,
      default: () => [],
    },
    ...
  },
  setup(props){ // this is important--pass the root props object in!!!
    ...
    // Now I need a reactive reference to my "todos" array...
    var todoRef = toRefs(props).todos
    ...
    // I can pass todoRef anywhere, with reactivity intact--changes from parents will flow automatically.
    // To access the "raw" value again:
    todoRef.value
    // Soon we'll have "unref" or "toRaw" or some official way to unwrap a ref object
    // But for now you can just access the magical ".value" attribute
  }
}

我当然希望Vue向导能够弄清楚如何使它更容易...但是据我所知,这是我们必须使用Composition API编写的代码类型。

这里是link to the official documentation,他们在那里直接警告您不要破坏道具。

答案 4 :(得分:0)

您可以使用路径到达给定的属性:

 watch("props.value", (newValue: Props['value']) => {
      selected.value = newValue;
    });