Let's implement a Theme Switch like the Angular Material Site

This article is mainly intended for beginners. I've added a TL;DR; below as a precautionary measure, in case you're a more advanced Angular Developer. Please feel free to navigate to a section that is most interesting to you.

TL;DR;

• Why a Theme Switch?
• The Setup
  • Implementing the HeaderComponent
    
  • Implementing the MenuComponent
• Implementing the Theme Switch
  • How Angular Material Website does it?
  • Our Implementation
• Next Steps
• Closing Notes

Why a Theme Switch?

Alright! So Dark Mode is a not so new cool feature. Most of the websites out there are implementing it as custom themes in their Apps.

And why wouldn't they do that, right? Dark Mode is awesome! It's easier on the eyes, better in terms of power consumption, and can provide an excellent user experience(especially in low-lighting conditions)

Yeah! You heard me. Low-lit ambiances are the best suited for Dark Mode. I mean, nobody's stopping you from changing to Dark Mode even in well-lit environments. But still, Dark Mode is easier on the eyes and makes more sense IN THE DARK.(see what I did there?)

Also, I mentioned Dark Mode as a way to enhance UX right? Well, there are multiple aspects to it. It's always better to offer users a way to customize the look and feel of your Apps, without compromising with your design language. And most of the Web Apps(or Apps, in general), do that via themes.

The other aspect might look more or less like a gimmick to some of you. But you can take the UX of your Web App, to the next level, by intelligently switching themes, based on the ambient lighting conditions that the user is in. I'll get back to this later.

One of my favorite websites, that implement Themes is the Angular Material Site. You might have seen this switch that lets you change the theme on the website.

We'll pretty much replicate the same effect in our Angular App. So without further ado, let's get started.

The Setup

I've set-up Angular Material on StackBlitz that you can use as a starter template:

From here on, let's add a few Angular Material Components that we can use to see something on the UI. I'll add a toolbar, an icon on it, a menu for theme options, and a button.

Since all these Angular Material Components will be used in my AppModule, it would make sense to create a separate AppMaterialModule that re-exports all the Material related modules from it.

...
import { MatButtonModule } from "@angular/material/button";
import { MatIconModule } from "@angular/material/icon";
import { MatMenuModule } from "@angular/material/menu";
import { MatToolbarModule } from "@angular/material/toolbar";
...

@NgModule({
  exports: [
    MatButtonModule,
    MatIconModule,
    MatMenuModule,
    MatToolbarModule,
  ]
})
export class AppMaterialModule {}

And now I can add the AppMaterialModule to the imports array of my AppModule.

...
import { AppMaterialModule } from "./app-material.module";
...

@NgModule({
  imports: [
    ...
    AppMaterialModule,
    ...
  ],
  ...
})
export class AppModule {}

I'm doing this in here because I will be using all the Angular Material Components exposed by these Angular Material Modules in my AppModule. This would not make much sense in a Real-World Application as we would generally not use all the Angular Material Components in all our Modules. So creating a single AppMaterialModule and then importing it in every Angular Module that you create can lead to performance hits. So you might want to avoid that in such cases.

Moving on, I should now be able to use these Angular Material Components in my App. The look that I'm going for is really simple. THIS

Judging from the image above, we need a HeaderComponent, a MenuComponent that opens upon clicking the ? icon and the rest would already be accommodated by our Sample StackBlitz.

Implementing the HeaderComponent:

I plan to make this a smart component. You can learn more about the Smart and Dumb Component Pattern from this video by Stephen Fluin.

Alright, now continuing with our HeaderComponent, it needs to pass on some options for the menu to the MenuComponent. Each option would have things like, backgroundColor, buttonColor, & headingColor for the icon to show on each menu item; and a label, and a value corresponding to each label.

Now we do know that Angular Material has 4 such pre-built themes named:

• deeppurple-amber.css
• indigo-pink.css
• pink-bluegrey.css
• purple-green.css

So we'll need 4 items for options. To avoid hard-coding of these options in the component itself, I'll just expose this data as a json file and store it in the assets folder in a file named options.json. Doing that will allow me to fetch it with path /assets/options.json

This file would look something like this:

[
  {
    "backgroundColor": "#fff",
    "buttonColor": "#ffc107",
    "headingColor": "#673ab7",
    "label": "Deep Purple & Amber",
    "value": "deeppurple-amber"
  },
  {
    "backgroundColor": "#fff",
    "buttonColor": "#ff4081",
    "headingColor": "#3f51b5",
    "label": "Indigo & Pink",
    "value": "indigo-pink"
  },
  {
    "backgroundColor": "#303030",
    "buttonColor": "#607d8b",
    "headingColor": "#e91e63",
    "label": "Pink & Blue Grey",
    "value": "pink-bluegrey"
  },
  {
    "backgroundColor": "#303030",
    "buttonColor": "#4caf50",
    "headingColor": "#9c27b0",
    "label": "Purple & Green",
    "value": "purple-green"
  }
]

Considering that this data was exposed as a REST API, I can then fetch it using HttpClient in my App. And that's what I'll be doing in this article. Alternatively, you can also leverage it as a static asset instead of exposing this data via a REST API. If you do that, you can import it directly in the HeaderComponent using import options from 'path-to-options.json'. You'll just have to set resolveJsonModule, and esModuleInterop to true in the compilerOptions, in your tsconfig.app.json/tsconfig.json. I have an implementation for this version in this sample StackBlitz just in case you're interested.

Okay. Let's carry on. Now, since I also have the structure of the option Object, I can create an interface for static typing. Let's store it in a file named option.model.ts:

export interface Option {
  backgroundColor: string;
  buttonColor: string;
  headingColor: string;
  label: string;
  value: string;
}

Perfect! Now the responsibility of the HeaderComponent is to:

• Render the header(Obviously!)
• Fetch the options and give it to the MenuComponent.

But we do need to also change the theme at some point. So it's better that we abstract the whole business logic related to themes in a service that I'd call ThemeService. So let's implement that first:

import { Injectable } from "@angular/core";
import { HttpClient } from "@angular/common/http";
import { Observable } from "rxjs/Observable";

import { Option } from "./option.model";

@Injectable()
export class ThemeService {
  constructor(
    private http: HttpClient,
  ) {}

  getThemeOptions(): Observable<Array<Option>> {
    return this.http.get<Array<Option>>("assets/options.json");
  }

  setTheme(themeToSet) {
    // TODO(@SiddAjmera): Implement this later
  }
}

Sweet! We can now inject this service as a dependency in the HeaderComponent which would look something like this:

import { Component, OnInit } from "@angular/core";
import { Observable } from "rxjs/Observable";

import { Option } from "../option.model";
import { ThemeService } from "../theme.service";

@Component({
  selector: "app-header",
  templateUrl: "./header.component.html",
  styleUrls: ["./header.component.css"]
})
export class HeaderComponent implements OnInit  {
  options$: Observable<Array<Option>> = this.themeService.getThemeOptions();

  constructor(private readonly themeService: ThemeService) {}

  ngOnInit() {
    this.themeService.setTheme("deeppurple-amber");
  }

  themeChangeHandler(themeToSet) {
    this.themeService.setTheme(themeToSet);
  }
}

As you can see, the HeaderComponent is also responsible for changing the theme now.

And the template would look like this:

<mat-toolbar color="primary">
  <mat-toolbar-row>
    <span>Dora</span>
    <span class="spacer"></span>
    <app-menu 
      [options]="options$ | async"
      (themeChange)="themeChangeHandler($event)">
    </app-menu>
  </mat-toolbar-row>
</mat-toolbar>

Notice how instead of subscribeing to the options$ Observable in the Component Class, we've used the async pipe to unwrap it. This is a pattern that makes Angular reactive and as far as possible, you should follow this pattern. Once we get the options, we can then pass it as an input to the MenuComponent's options @Input property.

Also, since the responsibility of changing the theme is also taken up by the HeaderComponent, we can implement the MenuComponent as a dumb/presentational component. So let's do that now.

Implementing the MenuComponent

So now we can tell that the MenuComponent would accept options as an @Input and then iterate through them to render these options. We can also clearly see that it has a themeChange @Output property that calls the handler with the newly selected theme. So we can implement the MenuComponent Class like this:

import { Component, EventEmitter, Input, Output } from "@angular/core";

import { Option } from "../option.model";
import { ThemeService } from "../theme.service";

@Component({
  selector: "app-menu",
  templateUrl: "./menu.component.html",
  styleUrls: ["./menu.component.css"]
})
export class MenuComponent {
  @Input() options: Array<Option>;
  @Output() themeChange: EventEmitter<string> = new EventEmitter<string>();

  constructor(private themeService: ThemeService) {}

  changeTheme(themeToSet) {
    this.themeChange.emit(themeToSet);
  }
}

And the template would look something like this:

<mat-icon
  class="icon" 
  [matMenuTriggerFor]="menu">
  palette
</mat-icon>
<mat-menu #menu="matMenu">
  <button
    *ngFor="let option of options"
    mat-menu-item
    (click)="changeTheme(option.value)">
    <mat-icon
      role="img" 
      svgicon="theme-example"
      aria-hidden="true">
      <svg
        xmlns="http://www.w3.org/2000/svg" 
        xmlns:xlink="http://www.w3.org/1999/xlink" 
        width="100%" 
        height="100%"
        viewBox="0 0 80 80" 
        fit="" 
        preserveAspectRatio="xMidYMid meet" 
        focusable="false">
        <defs>
          <path
            d="M77.87 0C79.05 0 80 .95 80 2.13v75.74c0 1.17-.95 2.13-2.13 2.13H2.13C.96 80 0 79.04 0 77.87V2.13C0 .95.96 0 2.13 0h75.74z"
            id="a">
          </path>
          <path
            d="M54 40c3.32 0 6 2.69 6 6 0 1.2 0-1.2 0 0 0 3.31-2.68 6-6 6H26c-3.31 0-6-2.69-6-6 0-1.2 0 1.2 0 0 0-3.31 2.69-6 6-6h28z"
            id="b">
          </path>
          <path d="M0 0h80v17.24H0V0z" id="c"></path>
        </defs>
        <use xlink:href="#a" [attr.fill]="option.backgroundColor"></use>
        <use xlink:href="#b" [attr.fill]="option.buttonColor"></use>
        <use xlink:href="#c" [attr.fill]="option.headingColor"></use>
      </svg>
    </mat-icon>
    <span>{{ option.label }}</span>
  </button>
</mat-menu>

Alright! Now we have everything in place. We just need a way to switch themes. How do we do that?

Implementing the Theme Switch

This is the final piece of the puzzle. And we can do this in several different ways. But the Angular Material Website has already implemented this right? And the good thing is, it's open-source. So we do have access to the source code.

So instead of trying to re-invent the wheel, I'm going to cheat a little bit and see how Angular Material Docs App did it.

How Angular Material Website does it?

If you check the actual implementation, they have implemented something called a ThemePicker. This is what we see at the top-right, in the header.

This(as the name suggests) is responsible for switching the theme on the website. This component calls a service called StyleManager.

What does this service do, you might ask. Well, when you change the theme from the ThemePicker it:

• Checks whether there's a link tag on the HTML Document with a class attribute, the value of which is: style-manager-theme:
• If there isn't such a link tag, it adds this link tag to the head of the document, and then set the href property with the selected theme path on it.
• If there is such a link tag, it then, it simply sets the href property on this link tag to the selected theme path.

Great, now that we understand what the StyleManager does, I can just copy the StyleManager service in my project. Once I do that, I can just inject this in my ThemeService and call the setStyle method from it with the appropriate values and it should ideally work.

So let's try it out.

Our Implementation

I'll first copy the style-manager.ts in a file named style-manager.service.ts:

/**
 * Copied from https://github.com/angular/material.angular.io/blob/master/src/app/shared/style-manager/style-manager.ts
 * TODO(@SiddAjmera): Give proper attribution here
 */

import { Injectable } from "@angular/core";

@Injectable()
export class StyleManagerService {
  constructor() {}

  /**
   * Set the stylesheet with the specified key.
   */
  setStyle(key: string, href: string) {
    getLinkElementForKey(key).setAttribute("href", href);
  }

  /**
   * Remove the stylesheet with the specified key.
   */
  removeStyle(key: string) {
    const existingLinkElement = getExistingLinkElementByKey(key);
    if (existingLinkElement) {
      document.head.removeChild(existingLinkElement);
    }
  }
}

function getLinkElementForKey(key: string) {
  return getExistingLinkElementByKey(key) || createLinkElementWithKey(key);
}

function getExistingLinkElementByKey(key: string) {
  return document.head.querySelector(
    `link[rel="stylesheet"].${getClassNameForKey(key)}`
  );
}

function createLinkElementWithKey(key: string) {
  const linkEl = document.createElement("link");
  linkEl.setAttribute("rel", "stylesheet");
  linkEl.classList.add(getClassNameForKey(key));
  document.head.appendChild(linkEl);
  return linkEl;
}

function getClassNameForKey(key: string) {
  return `app-${key}`;
}

Great. So now that I have this service in place, as planned, I'll inject this service as a dependency in my ThemeService and implement the setTheme method:

...
import { StyleManagerService } from "./style-manager.service";

@Injectable()
export class ThemeService {
  constructor(
    ...
    private styleManager: StyleManagerService
  ) {}

  ...

  setTheme(themeToSet) {
    this.styleManager.setStyle(
      "theme",
      `node_modules/@angular/material/prebuilt-themes/${themeToSet}.css`
    );
  }
}

All I'm doing here is calling the setStyle method from the StyleManagerService with the name of the style key(theme in this case), and the value of the href attribute that it has to set.

The setStyle method, again, either creates a new link tag and then sets the href attribute on it; or updates the href attribute on a pre-existing link tag.

And that's pretty much it. This is what our final code looks like.

Perfect! We now have a theme switch, just like the one on the Angular Material Website. And it works as expected.

Next Steps

This is all great. But wouldn't it be awesome if our App could automatically switch themes based on the ambient light? Well, that's exactly what we're going to do in the next article. Check it out here.

I’m grateful to Martina Kraus, and Rajat Badjatya for taking the time to proofread it and providing all the constructive feedback in making this article better. I hope this article has taught you something new related to Angular. If it did share this article with your friends who are new to Angular and want to achieve something similar.

This article was originally published by me under the Angular Publication on DEV.TO