Set sass variable value in Angular 7

Question

I have been working with angular for the last few weeks, and now I have a requirement to dynamically style a public site. The site admin set various color codes as well as a logo image from admin in a database. These will be reflected when the Public Site opens.

As I am from an asp.net background, previously what I would do is on master page load, take values from the DB and write them into a .less file, and let java-script library take care of it. It's simple there.

But for my current situation, I am using sass, and I am not able find a way to write variables into a .scss file.

I just learn a new thing APP_INITIALIZER from here ,but ultimately this post not showing how to write in the .scss file.

I am actually thinking this with my asp.net knowledge,but may be I am wrong ,or there are another way of implementation.

I want a simple solution ,what we do in asp.net I want to achieve this in same way.

1. Take variable value from DB via api,when application loading for first time.

2. Write values in SASS variable file .

3. After that SASS will take care of this and we get result as expected .

Please give some suggestion or example ,to start with .

Thanks.

Answer 1

First, in ASP.NET, it’s common to store CSS rules and other static assets in a database, which makes sense for a server-side rendering framework.

In Angular, however, since it’s primarily client-side (except for Angular Universal), the approach is different. Even for things like translations (i18n or custom), the data is usually stored on the front end (like in .json files) rather than in a database.

In Angular, you should store your themes in a way that allows you to switch between them dynamically. You can store keys or variables for styles and themes, but the actual CSS is still maintained in .css or .scss files.

For a practical example, consider using CSS variables to dynamically set your app’s theme in Angular (CSS vars in use while dynamically setting app theme (Angular)). This is just one approach, and there are various methods depending on your preference.

UPDATE:

There might be some misunderstandings from my previous explanation, but I’ll leave it as is and share a related experience.

I worked on a web app where users could customize their themes via settings. The CSS rules weren’t stored in the database, but the color values were, and they were used to set SASS variables. A special script would compile the CSS with these custom values on demand, which could be slow, but a splash screen helped mitigate this. Similarly, in another project, translations were stored in a database, but a script would generate and update .json files in the assets/i18n folder for each release or deployment.

Answer 2

Since sass is a pre compiled css. we cannot dynamically change the theme without generating a seperate theme.css. This is where JSS comes to play. JSS is a javascript based style inject mechanism, where css are directly injected into the files you are using it.

react-angular-material uses it extensively, where we can pass color variables dynamically to change theme of the application.

for instance this guy has made it with angular.

Docs: jss-angular, jss

links: jss-with-angular

Answer 3

While @Cold Cerberus has suggested a good approach and is right about maintaining style related things at front-end, i am suggesting some ways for this.

As you said you want various colour combination,you can use Conditional CSS of SASS.

body[theme="theme1"] {
   // theme 1 css
}

body[them="theme2"] {
    // theme 2 css
}

You can use sass theme map along with conditional css.

Just update your attribute and theme will be applied automatically.

    themeChange() {
    const dom = document.querySelector('body');
    dom.theme = theme1;    // change theme here
    }

If you are very particular about some element style which should be updated from back-end (like colour code) you can use ng-style along with theme approach.

<some-element [ngStyle]="{'font-style': styleExp}">...</some-element>

You have to use smart combination of above in order to fulfill your requirement.

Answer 4

As other answers explained, it is not possible to set SASS variables and process that on the client, as SASS is converted to plain CSS at build time and when app is running or in APP_INITIALIZER browser can process only CSS.

I see two options to achieve what you want.

Generally, you would have some base css for the app, and then you need to load the additional css based on admin settings. What needs to be considered from css point of view is that all css specificity in additional css should be greater than base css, because otherwise it won't override the base. That requires basic css knowledge so I won't go into details.

Method 1

Generate your additional css on server request. Load it when app is started from server URL. Reload it by js when admin change any settings.

1. Define backend endpoint at address /additional.css (or it could be similar to /api/theme/custom-css) which will generate css out of database. For example you have background=red in db, then the endpoint should return

body {background-color: red;}

2. Add <link id="additionalCss" rel="stylesheet" type="text/css" href="additional.css" /> in <head> of index.html. And that will be enough to make it work.
3. To reload you can use different methods, but I believe this should work

document.getElementById('additionalCss').href = document.getElementById('additionalCss').href;

This will make new request to the server, server will execute DB -> css and return the updated css, which will be applied to the browser.

And if you want to be cool (or need to support big and complex themes) scss can be used. Backend should generate scss variable definitions out of database, then should use some server-side app to compile scss -> css, and then serve compiled css back to the client. But this will be overkill if additional css is simple enough.

One important consideration of this method is browser caching, because content behind additional.css is dynamic, but browser may cache it, not call the backend and serve outdated version.

Method 2

If you don't want or can't mess with the backend. Load settings from DB by some API endpoint in json, then generate css code on the client and apply it.

1. Use HttpClient to get settings JSON and generate css as string out of it. For example server returns

{
  "background": "red"
}

then you convert this to string as

cssCode = 'body {background-color: red}';

2. Use

let additionalCssStyle = document.getElementById('additionalCss');
if (! additionalCssStyle) {
  additionalCssStyle = document.createElement("style");
  additionalCssStyle.id = 'additionalCss';
  document.head.appendChild(additionalCssStyle);
}
additionalCssStyle.innerText = cssCode;

3. To reload - save changed to backend, then repeat 1. and 2.

Answer 5

It is not possible in that way but rather than using the sass variable, you use the value of the sass variable. It may be any value.

Why? because sass is compiled during packaging and in the end, it would still generate plane CSS.

An example of a framework making use of this optional style processor is angular.

---

In your case I would recommend looking into dynamic themeing within angular as what you require definitely needs JavaScript. Look into the guide on medium given by one of the contributors.

https://stackoverflow.com/a/54559350/3070499

Answer 6

I don't think that what you want will be possible to do... Angular processes the SASS files during application build and writes all the common results into a plain old css file. The component-specific stuff will get generated as javascript that, in turn, will apply your styling at run time.

Hence all the SASS variables you need to set up have to be present at compile time.

What you can do, though, is to pre-define your setup in Angular components and then toggle it based on an input (from your DB or wherever else), like so:

// your.component.ts
@Component({
  // ... component stuff
  styles: ['h1.option1 {color: red;}', 'h1.option2 {color: blue;}'],
  template: `
    <h1 *ngIf="optionSelection$ | async as option; else noOption"
        [class.option1]="option == 1" 
        [class.option2]="option == 2">
       Hey there, I'm styled!
    </h1>
    <ng-template #noOption>
      <h1>No option received</h1>
    </ng-template>
  `
})
export class YourComponent {
  optionSelection$: Observable<number>;
  constructor(yourService: YourService){
    this.optionSelection$ = yourService.getYourOption().pipe(startWith(null));
  }
}

Hope this helps a little :-)