| 1 |
- (window.webpackJsonp=window.webpackJsonp||[]).push([[143],{467:function(t,s,a){"use strict";a.r(s);var n=a(3),e=Object(n.a)({},(function(){var t=this,s=t._self._c;return s("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[s("h1",{attrs:{id:"_02-元素渲染"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_02-元素渲染"}},[t._v("#")]),t._v(" 02. 元素渲染")]),t._v(" "),s("p",[t._v("元素是构成 React 应用的最小砖块，描述了你在屏幕上想看到的内容。")]),t._v(" "),s("div",{staticClass:"language-js line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-js"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" element "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),t._v("h1"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v("Hello"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" world"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("h1"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br")])]),s("p",[t._v("与浏览器的 DOM 元素不同，"),s("strong",[t._v("React 元素是创建开销极小的普通对象")]),t._v("。React DOM 会负责更新 DOM 来与 React 元素保持一致。")]),t._v(" "),s("h2",{attrs:{id:"将一个元素渲染为-dom"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#将一个元素渲染为-dom"}},[t._v("#")]),t._v(" 将一个元素渲染为 DOM")]),t._v(" "),s("p",[t._v("假设你的 HTML 文件某处有一个 "),s("code",[t._v("<div>")]),t._v("：")]),t._v(" "),s("div",{staticClass:"language-html line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-html"}},[s("code",[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("div")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token attr-name"}},[t._v("id")]),s("span",{pre:!0,attrs:{class:"token attr-value"}},[s("span",{pre:!0,attrs:{class:"token punctuation attr-equals"}},[t._v("=")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v('"')]),t._v("root"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v('"')])]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("div")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br")])]),s("p",[t._v("根节点，React应用只有单一根DOM节点")]),t._v(" "),s("blockquote",[s("p",[t._v("但一个html页面可以有多个React应用，每个应用对应一个独立根节点。")])]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" el "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("h1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("Hello")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("h1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v("\nReactDOM"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("render")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("el"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" document"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("getElementById")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'root'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br")])]),s("h2",{attrs:{id:"更新已渲染的元素"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#更新已渲染的元素"}},[t._v("#")]),t._v(" 更新已渲染的元素")]),t._v(" "),s("p",[t._v("React 元素是"),s("a",{attrs:{href:"https://en.wikipedia.org/wiki/Immutable_object",target:"_blank",rel:"noopener noreferrer"}},[t._v("不可变对象"),s("OutboundLink")],1),t._v("。一旦被创建，你就无法更改它的子元素或者属性。一个元素就像电影的单帧：它代表了某个特定时刻的 UI。")]),t._v(" "),s("p",[t._v("根据我们已有的知识，更新 UI 唯一的方式是创建一个全新的元素，并将其传入 "),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/react-dom.html#render",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("ReactDOM.render()")]),s("OutboundLink")],1),t._v("。")]),t._v(" "),s("p",[t._v("考虑一个计时器的例子：")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("tick")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// element是一个React元素")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" element "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("div")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("h1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("Hello, world!")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("h1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("h2")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("It is ")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Date")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("toLocaleTimeString")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("h2")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("div")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n ReactDOM"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("render")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("element"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" document"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("getElementById")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'root'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("setInterval")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("tick"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("1000")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br"),s("span",{staticClass:"line-number"},[t._v("6")]),s("br"),s("span",{staticClass:"line-number"},[t._v("7")]),s("br"),s("span",{staticClass:"line-number"},[t._v("8")]),s("br"),s("span",{staticClass:"line-number"},[t._v("9")]),s("br"),s("span",{staticClass:"line-number"},[t._v("10")]),s("br"),s("span",{staticClass:"line-number"},[t._v("11")]),s("br")])]),s("p",[t._v("注意：在实践中，"),s("strong",[t._v("大多数 React 应用只会调用一次 "),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/react-dom.html#render",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("ReactDOM.render()")]),s("OutboundLink")],1)]),t._v("。在下一个章节，我们将学习如何将这些代码封装到**"),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/state-and-lifecycle.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("有状态组件"),s("OutboundLink")],1),t._v("**中。")]),t._v(" "),s("h2",{attrs:{id:"react-只更新它需要更新的部分"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#react-只更新它需要更新的部分"}},[t._v("#")]),t._v(" React 只更新它需要更新的部分")]),t._v(" "),s("p",[t._v("上个例子中，尽管每一秒我们都会新建一个描述整个 UI 树的元素，但React DOM "),s("strong",[t._v("只会更新实际改变了的内")]),t._v("容。")])])}),[],!1,null,null,null);s.default=e.exports}}]);
|
