StencilJS : How can a StencilJS component utilize external theming?

StencilJS components can use external theming. StencilJS provides a few different ways to apply external styles to components:

1. CSS Variables

StencilJS supports CSS variables, which allows you to define custom properties in your CSS and use them throughout your components. CSS variables provide a way to define and use variables in your CSS stylesheets, making it easier to update your styles across your application.

To define a CSS variable in StencilJS, you can use the -- syntax to prefix the variable name, followed by the value. For example, to define a variable for the background color of a component, you can do the following:

:host {
  --background-color: #f1f1f1;
}

Then, you can use the variable throughout your component's styles by referencing it with the var() function. For example, to apply the variable to the background color of an element, you can do the following:

.element {
  background-color: var(--background-color);
}

You can also set the value of the variable dynamically using JavaScript. To do this, you can use the setProperty() method of the style property of an element. For example:

const element = document.querySelector('.element');
element.style.setProperty('--background-color', 'red');

This will update the value of the --background-color variable to red for the .element element.

2. Shadow DOM

Shadow DOM is a key feature of StencilJS, which is a tool for building web components. Shadow DOM is a way to encapsulate the markup and styles of a component so that they don't conflict with the markup and styles of other components or the main document. This allows you to create reusable components that can be used across your application without causing conflicts or unintended effects.

In StencilJS, you can use the shadow attribute to create a shadow DOM for your component. For example:

@Component({
    tag: 'my-component',
    shadow: true,
    styles: `
    .container {
      background-color: #f1f1f1;
      padding: 10px;
    }
  `,
    template: `
    <div class="container">
      <slot></slot>
    </div>
  `
})
export class MyComponent {}

In this example, the shadow attribute is set to true, which creates a shadow DOM for the component. The styles property defines the CSS styles for the component, which are encapsulated within the shadow DOM. The template property defines the markup for the component, which includes a <slot> element that allows content to be inserted into the component.

When this component is used in the main document, the content inside the component will be inserted into the <slot> element and rendered within the shadow DOM. This ensures that the styles and markup of the component don't interfere with the rest of the document.

3. CSS Imports

StencilJS supports CSS imports, which allow you to include external CSS files in your component's styles. This can be useful for separating out styles into different files, or for reusing styles across different components.

To use CSS imports in StencilJS, you can use the @import rule in your component's styles. For example:

@Component({
    tag: 'my-component',
    styles: ` @import url('https://fonts.googleapis.com/css?family=Roboto');

    .container {
        font-family: 'Roboto', sans-serif;
        background-color: #f1f1f1;
        padding: 10px;
    }

    `,
    template: ` <div class="container" > <slot></slot> </div> `

}) export class MyComponent {}

In this example, the @import rule is used to import the Roboto font from Google Fonts. The font is then used in the component's styles.

Note that there are some performance implications to using CSS imports, as the browser needs to fetch the external CSS file before it can render the component. In general, it's best to limit the use of CSS imports and instead include as much of the component's styles as possible in the component itself. However, CSS imports can be useful in some cases, such as for including third-party stylesheets or for reusing styles across multiple components.

4. CSS Custom Properties

StencilJS components can also use CSS Custom Properties to define theming variables that can be set externally to customize the component's appearance.

StencilJS supports CSS custom properties (also known as CSS variables), which allow you to define reusable values for your component's styles. This can make it easier to maintain your styles and apply consistent styling across different components.

To use CSS custom properties in StencilJS, you can define them in the :host selector of your component's styles. For example:

@Component({

    tag: 'my-component',
    styles: ` :host {
        --background-color: #f1f1f1;
        --font-family: 'Roboto', sans-serif;
    }

    .container {
        background-color: var(--background-color);
        font-family: var(--font-family);
        padding: 10px;
    }

    `,
    template: ` <div class="container" > <slot></slot> </div> `

}) export class MyComponent {}

In this example, the :host selector is used to define two custom properties: --background-color and --font-family. These properties are then used in the component's styles using the var() function. The values of these properties can be easily changed by overriding them in the component's CSS, or by passing them as properties to the component.

Using CSS custom properties can help make your styles more flexible and reusable, and can make it easier to apply consistent styling across different components. However, it's important to use them judiciously and avoid over-reliance on them, as they can also add complexity to your styles and may have performance implications.

In conclusion, StencilJS provides several ways to apply external theming to components, making it a flexible choice for building themeable web components.