效果
首先看看效果：
本文将介绍如何基于Angular 8和Bootstrap 4来实现上面的主题切换效果。
设计
遵循Bootstrap的设计，我们会使用 bootswatch.com 提供的免费主题来实现上面的效果。Bootswatch为前端程序员提供了多达21种免费的Bootstrap主题，并且提供了API文档 和 实例页面 ，介绍如何在HTML+jQuery的环境中实现主题切换。其实，我们也可以使用Bootstrap官网提供的主题设计工具来设计自己的主题，这些自定义的主题也是可以用在本文介绍的方法里的，只需要替换相关的资源地址就可以。如果你打开Bootswatch的API，你就会看到各种主题的元数据信息，我们可以使用其中的cssMin链接来替换主页的link地址，以达到切换主题的目的。
在开工之前，还是要做一些粗略的设计。为了简单起见，我使用Bootstrap的Navbar来完成这个功能，因为Navbar的代码可以直接从Bootstrap官网拷贝过来，稍微改改就行。不同的是，我将Navbar封装在一个组件（Component）里，这样做的好处是，可以将切换主题的功能封装起来，以实现模块化的设计。下图展示了这一设计：
基本流程如下：
步骤
首先，根据Bootswatch API所返回的数据结构，定义一个数据模型：
计算
安全
数据库
网络和加速
企业服务
export class ThemeDefinition {
name: string;
description: string;
thumbnail: string;
preview: string;
css: string;
cssMin: string;
cssCdn: string;
scss: string;
scssVariables: string;
}
export class Themes {
version: String;
themes: ThemeDefinition[];
}然后，创建theme.service.ts服务，用来调用Bootswatch API：
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';
import { Themes } from '../models/themes';
@Injectable({
providedIn: 'root'
})
export class ThemeService {
constructor(private http: HttpClient) { }
getThemes(): Observable<Themes> {
return this.http.get<Themes>('https://bootswatch.com/api/4.json');
}
}接下来，创建Navbar组件，关键代码部分就是将主题的名称绑定到dropdown上，并根据选择的主题名称决定当前所显示的主题名称是否应该是active的。当然，dropdown的每个item还应该响应用户的点击事件：
<nav class="navbar navbar-expand-lg navbar-dark bg-dark">
<a class="navbar-brand" href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" ><i class="fab fa-acquisitions-incorporated"></i></a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent"
aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto">
<li class="nav-item active">
<a class="nav-link" href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >Link</a>
</li>
<li *ngIf="themes" class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" id="navbarDropdown" role="button" data-toggle="dropdown"
aria-haspopup="true" aria-expanded="false">
主题
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a *ngFor="let theme of themes.themes"
[className]="theme.name === selectedTheme ? 'dropdown-item active' : 'dropdown-item'" href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow"
(click)="onThemeItemSelected($event)">{{theme.name}}</a>
</div>
</li>
</ul>
</div>
</nav>Navbar组件的代码如下：
import { Component, OnInit, Output, EventEmitter, Input } from '@angular/core';
import { Themes } from 'src/app/models/themes';
import { ThemeService } from 'src/app/services/theme.service';
import { ThemeDefinition } from 'src/app/models/theme-definition';
@Component({
selector: 'app-nav-bar',
templateUrl: './nav-bar.component.html',
styleUrls: ['./nav-bar.component.css']
})
export class NavBarComponent implements OnInit {
@Input() themes: Themes;
@Input() selectedTheme:string;
@Output() themeSelectionChanged : EventEmitter<ThemeDefinition> = new EventEmitter();
constructor(private themeService: ThemeService) { }
ngOnInit() {
}
onThemeItemSelected(event: any) {
const selectedThemeName = event.target.text;
const selectedTheme = this.themes.themes.find(t => t.name === selectedThemeName);
this.themeSelectionChanged.emit(selectedTheme);
}
}在onThemeItemSelected事件处理函数中，会读取被点击dropdown item的名称，根据该名称找到所选的主题，然后将其作为事件数据，发起themeSelectionChanged事件，然后，就是app.component.ts来处理这个事件了。在该事件处理函数中，从事件数据获取主题信息，然后调用applyTheme方法来应用主题：
import { Component, OnInit } from '@angular/core';
import { ThemeDefinition } from './models/theme-definition';
import { Themes } from './models/themes';
import { ThemeService } from './services/theme.service';
import { environment } from 'src/environments/environment';
import { StorageMap } from '@ngx-pwa/local-storage';
@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {
title = 'nblogger';
themes: Themes;
selectedTheme: string;
constructor(private themeService: ThemeService,
private storage: StorageMap) {
}
ngOnInit() {
this.themeService.getThemes()
.subscribe(data => {
this.themes = data;
this.storage.get('app-theme-name').subscribe(name => {
const themeName = name ? name : environment.defaultTheme;
const currentTheme = this.themes.themes.find(t => t.name === themeName);
this.applyTheme(currentTheme);
});
});
}
onThemeSelectionChanged(event: ThemeDefinition) {
this.applyTheme(event);
}
private applyTheme(def: ThemeDefinition): void {
this.storage.set('app-theme-name', def.name).subscribe(()=>{});
this.selectedTheme = def.name;
const links = document.getElementsByTagName('link');
for(let i = 0; i < links.length; i++) {
const link = links[i];
if (link.getAttribute('rel').indexOf('style') !== -1 &&
link.getAttribute('type').indexOf('text') !== -1) {
link.setAttribute('href', def.cssMin);
}
}
}
}在applyTheme方法中，首先会将所选主题名称设置到LocalStorage中，以便下次打开页面的时候能够直接应用主题；然后，从当前document中找到所需的link tag，并将其href值替换为所选主题信息的cssMin链接地址（内容可以参考Bootswatch的API结果）以此完成主题替换。
当重新打开页面时，app.component.ts中的ngOnInit初始化方法会被首先调用，它会通过theme.service.ts来读取主题信息，之后判断LocalStorage中是否有已经设置好的主题。如果有，则使用该主题，否则就从environment.ts的默认值中选择主题名称进行设置。
app.component.ts所使用的template就比较简单，主体是对Navbar组件的引用，还可以加一些额外的HTML元素进行效果测试：
<app-nav-bar [themes]="themes" [selectedTheme]="selectedTheme" (themeSelectionChanged)="onThemeSelectionChanged($event)"></app-nav-bar>
<div class="container">
<article>
<h2>Heading 1</h2>
<h3>Heading 2</h3>
<h4>Heading 3</h4>
<h5>Heading 4</h5>
</article>
<div class="alert alert-primary" role="alert">
这是一个警告框
</div>
<div class="alert alert-secondary" role="alert">
A simple secondary alert—check it out!
</div>
<div class="alert alert-success" role="alert">
A simple success alert—check it out!
</div>
<div class="alert alert-danger" role="alert">
A simple danger alert—check it out!
</div>
<div class="alert alert-warning" role="alert">
A simple warning alert—check it out!
</div>
<div class="alert alert-info" role="alert">
A simple info alert—check it out!
</div>
<div class="alert alert-light" role="alert">
A simple light alert—check it out!
</div>
<div class="alert alert-dark" role="alert">
A simple dark alert—check it out!
</div>
<button type="button" class="btn btn-primary">Primary</button>
<button type="button" class="btn btn-secondary">Secondary</button>
<button type="button" class="btn btn-success">成功</button>
<button type="button" class="btn btn-danger">失败</button>
<button type="button" class="btn btn-warning">警告</button>
<button type="button" class="btn btn-info">信息</button>
<button type="button" class="btn btn-light">Light</button>
<button type="button" class="btn btn-dark">Dark</button>
<button type="button" class="btn btn-link">Link</button>
</div>当然，记得在index.html中加入link的占位符，以便上面的applyTheme方法能够找到它：
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Nblogger</title>
<base href="/" rel="external nofollow" >
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="icon" type="image/x-icon" href="favicon.ico" rel="external nofollow" >
<link rel="stylesheet" type="text/css" href="#" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >
</head>
<body>
<app-root></app-root>
</body>
</html>总结
我们可以将Bootswatch的所有主题下载到本地，由本地服务来提供主题的API，这样切换主题会变得更快，也可以自己自定义主题然后扩展这个自制的本地API来提供更丰富的主题，根据需要来定吧。
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