文章目录
前言
对ref的概念一直有点模糊, 整理一下吧.
一、获取DOM
最常见的用法.
1.获取domElement
以Composition API 写法为例.
引入ref, 在想要获取的DOM元素上定义ref属性赋值一个自定义名称, 然后在setup内声明一个同名变量来存储该DOM元素, 具体如下.
但并不能在setup中直接获取到DOM, setup相当于Vue2中的beforeCreate和create周期, 所以你看到我引入了onMounted.
typescript
<div ref="diva"></div>
<img ref="imga"></img>
<canvas ref="canvasa"></canvas>
typescript
import { defineComponent, ref, onMounted } from 'vue';
export default defineComponent({
name: 'test',
setup() {
const diva = ref<HTMLDivElement | null>();
const imga = ref<HTMLImageElement | null>();
const canvasa = ref<HTMLCanvasElement | null>();
onMounted(() => {
console.log(diva);
console.log(imga);
console.log(canvasa);
})
}
})然后一定要把ref给return出去, 不然怎么搞都是undefined.
如果获取到的元素具备子元素, 子元素也会被一同获取, 所以某些时候可以通过获取父元素的方式来获取子元素, 比如ul内有一堆li, 当然是获取ul再遍历比较好.
关于获取到的DOM如何定义类型, 可以从node_modules/typescript/lib/lib.dom.ts找到该接口:
typescript
interface HTMLElementTagNameMap {
"a": HTMLAnchorElement;
"abbr": HTMLElement;
"address": HTMLElement;
"area": HTMLAreaElement;
"article": HTMLElement;
"aside": HTMLElement;
"audio": HTMLAudioElement;
"b": HTMLElement;
"base": HTMLBaseElement;
"bdi": HTMLElement;
"bdo": HTMLElement;
"blockquote": HTMLQuoteElement;
"body": HTMLBodyElement;
"br": HTMLBRElement;
"button": HTMLButtonElement;
"canvas": HTMLCanvasElement;
"caption": HTMLTableCaptionElement;
"cite": HTMLElement;
"code": HTMLElement;
"col": HTMLTableColElement;
"colgroup": HTMLTableColElement;
"data": HTMLDataElement;
"datalist": HTMLDataListElement;
"dd": HTMLElement;
"del": HTMLModElement;
"details": HTMLDetailsElement;
"dfn": HTMLElement;
"dialog": HTMLDialogElement;
"dir": HTMLDirectoryElement;
"div": HTMLDivElement;
"dl": HTMLDListElement;
"dt": HTMLElement;
"em": HTMLElement;
"embed": HTMLEmbedElement;
"fieldset": HTMLFieldSetElement;
"figcaption": HTMLElement;
"figure": HTMLElement;
"font": HTMLFontElement;
"footer": HTMLElement;
"form": HTMLFormElement;
"frame": HTMLFrameElement;
"frameset": HTMLFrameSetElement;
"h1": HTMLHeadingElement;
"h2": HTMLHeadingElement;
"h3": HTMLHeadingElement;
"h4": HTMLHeadingElement;
"h5": HTMLHeadingElement;
"h6": HTMLHeadingElement;
"head": HTMLHeadElement;
"header": HTMLElement;
"hgroup": HTMLElement;
"hr": HTMLHRElement;
"html": HTMLHtmlElement;
"i": HTMLElement;
"iframe": HTMLIFrameElement;
"img": HTMLImageElement;
"input": HTMLInputElement;
"ins": HTMLModElement;
"kbd": HTMLElement;
"label": HTMLLabelElement;
"legend": HTMLLegendElement;
"li": HTMLLIElement;
"link": HTMLLinkElement;
"main": HTMLElement;
"map": HTMLMapElement;
"mark": HTMLElement;
"marquee": HTMLMarqueeElement;
"menu": HTMLMenuElement;
"meta": HTMLMetaElement;
"meter": HTMLMeterElement;
"nav": HTMLElement;
"noscript": HTMLElement;
"object": HTMLObjectElement;
"ol": HTMLOListElement;
"optgroup": HTMLOptGroupElement;
"option": HTMLOptionElement;
"output": HTMLOutputElement;
"p": HTMLParagraphElement;
"param": HTMLParamElement;
"picture": HTMLPictureElement;
"pre": HTMLPreElement;
"progress": HTMLProgressElement;
"q": HTMLQuoteElement;
"rp": HTMLElement;
"rt": HTMLElement;
"ruby": HTMLElement;
"s": HTMLElement;
"samp": HTMLElement;
"script": HTMLScriptElement;
"section": HTMLElement;
"select": HTMLSelectElement;
"slot": HTMLSlotElement;
"small": HTMLElement;
"source": HTMLSourceElement;
"span": HTMLSpanElement;
"strong": HTMLElement;
"style": HTMLStyleElement;
"sub": HTMLElement;
"summary": HTMLElement;
"sup": HTMLElement;
"table": HTMLTableElement;
"tbody": HTMLTableSectionElement;
"td": HTMLTableCellElement;
"template": HTMLTemplateElement;
"textarea": HTMLTextAreaElement;
"tfoot": HTMLTableSectionElement;
"th": HTMLTableCellElement;
"thead": HTMLTableSectionElement;
"time": HTMLTimeElement;
"title": HTMLTitleElement;
"tr": HTMLTableRowElement;
"track": HTMLTrackElement;
"u": HTMLElement;
"ul": HTMLUListElement;
"var": HTMLElement;
"video": HTMLVideoElement;
"wbr": HTMLElement;
}获取不同的标签就定义为<对应类型 | null>, 在某些周期是null.
2.获取组件
与获取DOM的方法相同, 但类型定义有所区别.
InstanceType<T> 是 ts 自带的类型, 能够直接获取组件完整的实例类型.
typescript
const 自定义 = ref<InstanceType <typeof 组件名>>();
typescript
import { defineComponent, ref, onMounted } from 'vue';
import Footer from "../components/Footer.vue";
export default defineComponent({
name: 'test',
setup() {
const footer = ref<InstanceType <typeof Footer>>();
onMounted(() => {
console.log(footer.value);
});
}
})依然需要在onMounted获取.
3.v-bind与ref
如果用v-bind指令向ref绑定一个函数, 该函数内可从参数获取到DOM:
html
<div :ref="setRefAction"></div>
typescript
const setRefAction = (el: any) => {
console.log(el)
};二、响应式处理
ref() 也可以像 reactive() 一样对值进行响应式处理, 不过ref可以额外接受基础类型的响应式处理, reactive只能接受对象类型.
可以将 ref 看成 reactive 的一个变形版本，这是由于 reactive 内部采用 Proxy 来实现，
而 Proxy 只接受对象作为入参，这才有了 ref 来解决值类型的数据响应，如果传入 ref 的是一个对象，
内部也会调用 reactive 方法进行深层响应转换.看来对象类型用ref做响应式处理会产生额外的工作量.
ref可以接受非对象类型:
typescript
const state = ref<Boolean>(true);但就像访问获取到的DOM一样, 访问ref值需要xxx.value.
接收对象类型时会返回Proxy对象, 因为这还是reactive的处理结果, 尝试直接从这个proxy对象中访问属性会导致TS报错:
typescript
const obj = ref<Object>({ a: 1, b: 2}); // Proxy<Object>{ a: 1, b: 2 };
console.log(obj.value.a)"obj.value中不含有该属性", 但其实是有的, 将obj的类型断言为any可以正常访问.
总结
--