Gatsby Themes introduce a concept called Component Shadowing. This feature allows users to override a component in order to customize its rendering.
With other theming approaches it's impossible to change aspects of a theme if a configuration option hasn't been built in. Component Shadowing provides a powerful escape hatch to let users make quick, one-off changes that might not make sense to support in the theme itself.
For example, imagine you've installed gatsby-theme-blog and want to customize the author Bio component to add a link to their personal website. Before Component Shadowing, this new functionality would require configuration at the theme level or using a fork of the codebase. These approaches result in a difficult upgrade path and a maintenance burden.
Component Shadowing let's you replace the theme's original file, gatsby-theme-blog/src/components/bio.js, with your own to implement any changes you need.
Shadowing example
If you've installed gatsby-theme-blog you'll notice that it renders a Bio component which is used in the BlogPost template. If you'd like to change the Bio component you can do so with Component Shadowing.
Theme File Structure
gatsby-theme-blog has the following file structure (some files have been omitted for brevity):
Customizing the Bio component
Component Shadowing uses a naming convention to determine which component will be rendered. In order to override the Bio component in gatsby-theme-blog, create a file named user-site/src/gatsby-theme-blog/components/bio.js.
Any file that lives in the src/gatsby-theme-blog directory of the user's site will be used instead of a file with the same name in the theme's src directory: gatsby-theme-blog/src.
This means that user-site/src/gatsby-theme-blog/components/bio.js will be rendered in place of gatsby-theme-blog/src/components/bio.js:
With a successful shadow of the Bio component you'll result in the following directory tree:
Leveraging Component Shadowing is a great way to introduce small changes to a theme and even layer in new functionality.
Note: In the future we'll publish a technical post that will dive into the internals of Component Shadowing.
Changing styling
Component Shadowing isn't restricted to React components; you can override any JavaScript, Markdown, MDX, or CSS file. For example, gatsby-theme-blog has a design tokens file which is used to define font sizing, spacing, and colors. If you want to modify these values, you can shadow it.
To do so, create a file named user-site/src/gatsby-theme-blog/components/tokens/index.js.
You'll result in the following directory tree:
Now, blue and red will have your custom values and will be reflected in the theme wherever theme.colors.blue and theme.colors.red are used.
How much shadowing is too much shadowing?
If you've found yourself shadowing a large amount of components in a particular theme it might make sense to use a fork instead.
Roadmap
In the future we'll be introducing new tooling around Component Shadowing that will list when shadowing occurs, what files are available to shadow, and the ability to eject from a theme.
Conclusion
Component Shadowing is a powerful feature for making one-off changes to a theme without the need for complex configuration or maintaining a fork of the code. It's a stable feature in Gatsby Themes and is currently being used in production.