前言
早就听说过使用BEM命名管理样式,平时在项目中也有用到,但总感觉写起来太麻烦,最近刚好在看Elementplus源码,发现Elementplus使用BEM的方案非常棒,本文就来结合ElementPlus源码分析一下如何在项目中优雅的使用BEM命名
BEM的介绍
BEM是一种针对css的前端命名规范,是块(Block),元素(Element),**修饰符(Modifier)**的简写。
Block 可以理解为模块,比如:article,dialog,sidebar,form,tab
Element 可以理解为块里的元素,比如form里面的input,submit
modifier 可以理解为对block或者element的修饰,比如修饰form__submit--disable,form--theme-dark
BEM的命名规范
.block { }
.block__element { }
.block--modifier { }
.block__element--modifier { }
使用__来连接block和element,使用--来连接block和modifier
清楚了概念和命名规范,下面我们就来看ElementPlus是怎么实现BEM的
BEM的实践
以实现一个简单dialog组件为例,使用普通的BEM方法实现如下
<template>
<div class="dialog-overlay" v-if="visible">
<section class="dialog">
<header class="dialog__header">
<div class="dialog-title">{{ title }}</div>
<span class="dialog-header__close" @click="visible=false">X</span>
</header>
<div class="dialog__body">
{{ message }}
</div>
<footer class="dialog__footer">
<button class="dialog__btn">{{ cancelBtnTxt }}</button>
<button class="dialog__btn dialog__btn--primary">{{ comfirmBtnTxt }}</button>
</footer>
</section>
</div>
</template>
<script setup lang="ts">
import { ref } from 'vue'
const props = defineProps({
title:{
type:String,
default:'Title'
},
message:String,
cancelBtnTxt:{
type:String,
default:'Cancel'
},
comfirmBtnTxt:{
type:String,
default:'Comfirm'
}
})
const visible = ref(true)
const close = ()=>{
visible.value = false
}
</script>
<style scoped lang="scss">
:global(body){
font-family: PingFang SC,Microsoft YaHei,Arial,sans-serif;
}
.dialog-overlay{
position: fixed;
top: 0;
right: 0;
bottom: 0;
left: 0;
z-index: 2000;
height: 100%;
background-color:rgba(0, 0, 0, .5);
overflow: auto;
}
.dialog {
position: fixed;
left: 50%;
top: 50%;
transform: translate(-50%,-50%);
border-radius:2px;
background: #fff;
width: 559px;
min-height: 200px;
&__header{
padding: 20px 20px 10px;
}
&__title{
color: #333;
font-size: 18px;
}
&__body{
padding: 30px 20px;
font-size: 14px;
color: #333;
}
&-close{
position: absolute;
right: 15px;
top: 15px;
color: #999;
font-size: 18px;
}
&__btn{
min-width: 33px;
padding: 8px 15px;
background: none;
border: 1px solid #ddd;
border-radius: 2px;
color: #666;
margin-left: 10px;
cursor: pointer;
&--primary{
background: #409eff;
color: #fff;
border-color:#409eff;
}
}
&__footer{
padding: 10px 20px 20px;
display: flex;
justify-content: flex-end;
}
}
</style>
代码很简单,一看就懂,不做过多解释,下面看用ElementPlus的方法如何进行改造
useNamespace
首先定义一个工具方法useNamespace,此函数返回了符合BEM命名规则的方法
// useNamespace.ts
import { computed, unref } from 'vue'
// 定义一个组件的命名前缀
const defaultNamespace = 'poliwen'
// 定义一个描述组件状态的变量
const statePrefix = 'is-'
// 定义个_bem方法,此方法返回符合BEM规范的命名
const _bem = (
namespace: string,
block: string,
blockSuffix: string,
element: string,
modifier: string,
) => {
let cls = `${namespace}-${block}`
if (blockSuffix)
cls += `-${blockSuffix}`
if (element)
cls += `__${element}`
if (modifier)
cls += `--${modifier}`
return cls
}
// useNamespace.ts
// 定义一个命名空间,并且返回BEM方法
export const useNamespace = (block: string) => {
const namespace = computed(() => defaultNamespace)
const b = (blockSuffix = '') =>
_bem(unref(namespace), block, blockSuffix, '', '')
const e = (element?: string) =>
element ? _bem(unref(namespace), block, '', element, '') : ''
const m = (modifier?: string) =>
modifier ? _bem(unref(namespace), block, '', '', modifier) : ''
const be = (blockSuffix?: string, element?: string) =>
blockSuffix && element
? _bem(unref(namespace), block, blockSuffix, element, '')
: ''
const em = (element?: string, modifier?: string) =>
element && modifier
? _bem(unref(namespace), block, '', element, modifier)
: ''
const bm = (blockSuffix?: string, modifier?: string) =>
blockSuffix && modifier
? _bem(unref(namespace), block, blockSuffix, '', modifier)
: ''
const bem = (blockSuffix?: string, element?: string, modifier?: string) =>
blockSuffix && element && modifier
? _bem(unref(namespace), block, blockSuffix, element, modifier)
: ''
const is: {
(name: string, state: boolean | undefined): string
(name: string): string
} = (name: string, ...args: [boolean | undefined] | []) => {
const state = args.length >= 1 ? args[0]! : true
return name && state ? `${statePrefix}${name}` : ''
}
return {
namespace,
b,
e,
m,
be,
em,
bm,
bem,
is,
}
}
export type UseNamespaceReturn = ReturnType<typeof useNamespace>
useNamespace库的使用
import { useNamespace } from './compo/useNamespace'
const bs = useNamespace('dialog')
ns.b() // el-dialog
ns.b('overlay') // el-dialog-overlay
ns.e('header') // el-dialog__header
ns.m('theme-dark') // el-dialog--theme-dark
ns.be('header','close') // el-dialog-header__close
ns.em('footer','small') // el-dialog__footer--small
ns.bm('footer','small') // el-dialog-footer--small
ns.bem('footer','btn','primary') // el-dialog-footer__btn--primary
ns.is('closeable') // is-closeable
ns.b() 返回结果为 "el-dialog "
ms.b('overlay')
ns.e('header') 返回结果为 "el-dialog__header"
ns.m('theme-dark') 返回结果为 "el-dialog--theme-dark"
ns.be('header','close') 返回结果为 "el-dialog-header__close" 意思为返回一个block + element
ns.em('foter,'small') 返回结果为 "el-dialog__footer--small" 意思为返回一个element + modifier
ns.bm('footer','small') 返回结果为 "el-dialog-footer--small" 意思为返回一个block + modifier
ns.bem('footer','btn','primary') 返回结果为" el-dialog-footer__btn--primary" 意思为返回一个block+element+modifer
ns.is('closeable') 返回is-closeable 通常用来描述组件的状态,如is-closeable 表示:是否显示关闭按钮
知道了useNamespace库的用法,下面我们来改造dialog.vue的html模板
改造前的代码
<template>
<div class="dialog-overlay" v-if="visible">
<section class="dialog">
<header class="dialog__header">
<div class="dialog__title">{{ title }}</div>
<span class="dialog-header__close" @click="visible=false">X</span>
</header>
<div class="dialog__body">
{{ message ?? 'BEM是一种针对css的前端命名规范,是块(Block),元素(Element),修饰符(Modifier)的简写。' }}
</div>
<footer class="dialog__footer">
<button class="dialog__btn">{{ cancelBtnTxt }}</button>
<button class="dialog__btn dialog__btn--primary">{{ comfirmBtnTxt }}</button>
</footer>
</section>
</div>
</template>
改造后的代码
<template>
<div :class="ns.b('overlay')" v-if="visible">
<section :class="ns.b()">
<header :class="ns.e('header')">
<div :class="ns.e('title')">{{ title }}</div>
<span :class="ns.be('header','close')" @click="visible=false">X</span>
</header>
<div :class="ns.e('body')">
{{ message ?? 'BEM是一种针对css的前端命名规范,是块(Block),元素(Element),修饰符(Modifier)的简写。' }}
</div>
<footer :class="ns.e('footer')">
<button :class="ns.e('btn')">{{ cancelBtnTxt }}</button>
<button :class="[ns.e('btn'), ns.em('btn','primary')]">{{ comfirmBtnTxt }}</button>
</footer>
</section>
</div>
</template>
<script setup lang="ts">
import { ref } from 'vue'
import { useNamespace } from '../composables/useNamespace'
const ns = useNamespace('dialog')
const visible = ref(true)
const close = ()=>{
visible.value = false
}
</script>
scss也需要改造,新增了一个$nameSpace命名空间
<style scoped lang="scss">
:global(body){
font-family: PingFang SC,Microsoft YaHei,Arial,sans-serif;
}
$namespace: 'poliwen';
.#{$namespace}-dialog-overlay{
position: fixed;
top: 0;
right: 0;
bottom: 0;
left: 0;
z-index: 2000;
height: 100%;
background-color:rgba(0, 0, 0, .5);
overflow: auto;
}
.#{$namespace}-dialog {
position: fixed;
left: 50%;
top: 50%;
transform: translate(-50%,-50%);
border-radius:2px;
background: #fff;
width: 559px;
min-height: 200px;
&__header{
padding: 20px 20px 10px;
}
&__title{
color: #333;
font-size: 18px;
}
&__body{
padding: 30px 20px;
font-size: 14px;
color: #333;
}
&-header{
&__close{
position: absolute;
right: 15px;
top: 15px;
color: #999;
font-size: 18px;
}
}
&__btn{
min-width: 33px;
padding: 8px 15px;
background: none;
border: 1px solid #ddd;
border-radius: 2px;
color: #666;
margin-left: 10px;
cursor: pointer;
&--primary{
background: #409eff;
color: #fff;
border-color:#409eff;
}
}
&__footer{
padding: 10px 20px 20px;
display: flex;
justify-content: flex-end;
}
}
</style>
BEM的mixins的封装
我们还可以将BEM的命名封装成scss的mixins方法
$namespace: 'poliwen';
$common-separator: '-';
$element-separator: '__';
$modifier-separator: '--';
$state-prefix: 'is-';
$namespace: 'el';
// BEM support Func
@function selectorToString($selector) {
$selector: inspect($selector);
$selector: str-slice($selector, 2, -2);
@return $selector;
}
@function containsModifier($selector) {
$selector: selectorToString($selector);
@if str-index($selector, $modifier-separator) {
@return true;
} @else {
@return false;
}
}
@function containWhenFlag($selector) {
$selector: selectorToString($selector);
@if str-index($selector, '.' + $state-prefix) {
@return true;
} @else {
@return false;
}
}
@function containPseudoClass($selector) {
$selector: selectorToString($selector);
@if str-index($selector, ':') {
@return true;
} @else {
@return false;
}
}
@function hitAllSpecialNestRule($selector) {
@return containsModifier($selector) or containWhenFlag($selector) or
containPseudoClass($selector);
}
// bem('block', 'element', 'modifier') => 't5-block__element--modifier'
@function bem($block, $element: '', $modifier: '') {
$name: $namespace + $common-separator + $block;
@if $element != '' {
$name: $name + $element-separator + $element;
}
@if $modifier != '' {
$name: $name + $modifier-separator + $modifier;
}
// @debug $name;
@return $name;
}
// BEM
@mixin b($block) {
$B: $namespace + "-" + $block !global;
.#{$B} {
@content;
}
}
@mixin e($element) {
$E: $element !global;
$selector: &;
$currentSelector: "";
@each $unit in $element {
$currentSelector: #{$currentSelector + "." + $B + $element-separator + $unit + ","};
}
@if hitAllSpecialNestRule($selector) {
@at-root {
#{$selector} {
#{$currentSelector} {
@content;
}
}
}
} @else {
@at-root {
#{$currentSelector} {
@content;
}
}
}
}
@mixin m($modifier) {
$selector: &;
$currentSelector: "";
@each $unit in $modifier {
$currentSelector: #{$currentSelector + $selector + $modifier-separator + $unit + ","};
}
@at-root {
#{$currentSelector} {
@content;
}
}
}
以上代码稍稍有点复杂,感兴趣的可以去学习下scss的高级用法
定义好mixins方法之后,我们最终的样式代码如下
<style scoped lang="scss">
@import "../assets/mixins.scss";
:global(body){
font-family: PingFang SC,Microsoft YaHei,Arial,sans-serif;
}
@include b(dialog) {
&-overlay{
position: fixed;
top: 0;
right: 0;
bottom: 0;
left: 0;
z-index: 2000;
height: 100%;
background-color:rgba(0, 0, 0, .5);
overflow: auto;
}
position: fixed;
left: 50%;
top: 50%;
transform: translate(-50%,-50%);
border-radius:2px;
background: #fff;
width: 559px;
min-height: 200px;
@include e(header){
padding: 20px 20px 10px;
}
@include e(title){
color: #333;
font-size: 18px;
}
@include e(body){
padding: 30px 20px;
font-size: 14px;
color: #333;
}
&-header{
&__close{
position: absolute;
right: 15px;
top: 15px;
color: #999;
font-size: 18px;
}
}
@include e(btn){
min-width: 33px;
padding: 8px 15px;
background: none;
border: 1px solid #ddd;
border-radius: 2px;
color: #666;
margin-left: 10px;
cursor: pointer;
&--primary{
background: #409eff;
color: #fff;
border-color:#409eff;
}
}
@include e(footer){
padding: 10px 20px 20px;
display: flex;
justify-content: flex-end;
}
}
</style>
总结
BEM样式命名是各大主流组件库的常用命名方法,因此掌握它的使用很有必要,本文主要结合elementPlus的源码分析了BEM的妙用,通过定义一个useNamespace工具库,来生成符合BEM命名规范的方法,使用scss编写符合BEM命名的mixins,在组件中结合useNamespace和minxins就能很方便的使用BEM命名大法。
本文参考资料:github.com/element-plu…
文中案例代码请访问github.com/PoliWen/myB…