Shadowing is a powerful feature that allows theme users to override components, objects, and anything else in a theme’s src directory.

Note This is a technical deep dive into how Shadowing works. If you’d like to learn about what Shadowing is, see the What is Component Shadowing? blog post.

Shadowing works by using a webpack resolver plugin that maps themes in a gatsby-config.js to possible shadowed files. This gets especially mind melty because themes can add parent themes to a configuration so you need to be able to walk the composition of themes to determine the “last shadow” since the last shadowed theme file wins in the algorithm.

Theme Composition

It’s important to begin discussing how the composition of themes works. An end user of a theme can configure any number of themes. Each of these themes are considered sibling themes. Here is a gatsby-config.js that configures two sibling themes:

Both of the themes above (blog and portfolio) can install and configure any other theme so you end up with a tree of themes which we call a theme composition.

The theme composition itself has a few properties:

  • the last theme wins
  • a theme that uses another theme is the child theme
  • a theme that is used by another theme is the parent theme
  • theme trees are flattened during resolution

These characteristics are used in the component shadowing algorithm to decide which component to render. So, for example, if gatsby-theme-tomato-blog has gatsby-theme-parent as a parent theme it results in the following themes array:

This means that gatsby-theme-tomato-portfolio receives priority for component resolution, because it is last in the array.

Modifying the webpack Config

Component shadowing is a bit meta because it is implemented as an internal Gatsby plugin that applies a webpack plugin which modifies how module resolution happens for files that are shadowed.

The plugin consists of a gatsby-node.js and the webpack plugin code. The gatsby-node file is pretty straightforward:

Structure of a webpack Plugin

The webpack plugin itself has a constructor and an apply function which webpack calls as part of module resolution. We tap into resolve hook and right before the “resolved” hook in the pipeline.

Identify requested theme and component

The request contains the path of the file that was requested. The first step is to extract from that path the theme to which that file belongs, as well as the local path of that file inside its theme’s src directory.

The resolved hook is called after Webpack’s default resolution process has been completed. At that point, the requested path has been resolved to the absolute path on disk of the file that would have been used if no shadowing was being performed. At that point, node_modules, aliases and symlinks have all been resolved. Also, the requested path will contains the file extension that was determined by Webpack.

For example, let’s assume that user code requires a file named gatsby-theme-tomato/src/button/heading. On entering before-resolved hook for GatsbyThemeComponentShadowingResolverPlugin, request.path might looks something like /some/path/my-site/node_modules/gatsby-theme-tomato/src/button/heading.js (that is if gatsby-theme-tomato has been installed from a npm repository; that would be /some/path/my-site/packages/gatsby-theme-tomato/src/button/heading.js if yarn-style workspaces are being used). Now, assuming that theme gatsby-theme-tomato has been properly registered in that site’s gatsby-config.js, then getThemeAndComponent will return:

Extracting the theme and local component’s path so that we can determine the theme that is being required from so we can check for shadowed files in the user’s site or other themes to match against. We also make sure the matched themes are unique because two themes can bring in the same theme to the theme composition. When that’s the case we won’t worry about them being different. Though, it is important to note that when performing resolution to build the site, the last theme added will always win.

Handle too many matches

If there is more than one matching theme there is some sort of ambiguity and we should return an error. This can happen if there’s a path like gatsby-theme-blog/src/components/gatsby-theme-something/src/components in the project.

No matches

If there are no theme matches we return the invoked callback because there’s nothing more to do, time to let webpack continue on it’s way with module resolution.

The component shadow

Once it is determined that a file file being required belongs to theme, we need to figure out if that file is actually shadowed by some other file (and if so, which path should it resolve to instead). We do this by calling resolveComponentPath which uses the theming algorithm to attempt to find a shadowed component. If nothing is found we let Webpack continue with its default resolution algorithm.

If a shadowed component has been found, then we call doResolve on the resolver, telling it to jump back at the describedRelative hook. This is what tells webpack to resolve and bundle that particular file.

If no shadow component has been found, then we call callback instead, which tells Webpack to proceed to the next step in the current pipeline, without any change.

Resolving a shadowed component

When looking for a component we perform a search that occurs in two locations:

  • user’s project
  • themes
User’s project

In order to ensure that the user’s project always takes precedence in terms of shadowing it’s prepended to the theme list when attempting to resolve the component. This ensures that my-site/src/gatsby-theme-tomato/box.js will take priority over any other theme that might want to shadow the same component.

Themes

As discussed before, themes are flattened into a list and then all possible shadow paths are constructed to match against. When concatenating with the user’s project it’s important to note again that the themes array is reversed. This is how we ensure that “the last theme wins” when resolving a shadowed file. We walk the list for matches from start to finish.

Additionally, the original theme is removed because that’s the default behavior of webpack so we don’t need to resolve a theme to itself.

Shadow Extensions

Shadowing algorithm allows to use different extension than a file that is being shadowed. This does rely on theme using import paths that don’t contain extensions (./heading imports would allow to use different extension while ./heading.jsx would require to use exact same extension) and does rely on extension list that webpack can automatically resolve (via resolve.extensions webpack config option).

To get access to original request we tap into resolve hook and store request in _gatsbyThemeShadowingOriginalRequestPath field to have access to it later in before-resolved hook:

Note however that heading.css would not logically be an acceptable shadow for heading.js since they are not providing the same type of content. Because of this we have extension compatibility table:

We currently lack proper public API to extend this table, but it can be extended by passing the extensionsCategory property to the GatsbyThemeComponentShadowingResolverPlugin plugin in the Webpack configuration file. This requires that a Gatsby plugin (or the site itself) intercept the onCreateWebpackConfig event, then grab the existing Webpack configuration, find the existing GatsbyThemeComponentShadowingResolverPlugin entry in resolve.plugins, add or modify the extensionsCategory property, and finally call setWebpackConfig with the modified configuration. Note that it should be considered internal API and it can break at any time so use this method with caution.

Finally, because the table is not exhaustive, we check extensions for the same category as shadowed file first before falling back to rest of extensions defined by resolve.extensions to not introduce breaking changes.

All together

The shadowing algorithm can be boiled down the following function that’s roughly 40 lines of code:

Handling component extending

This is where things begin to get a bit whacky. In addition to overriding a file, we want it to be possible to import the very component you’re shadowing so that you can wrap it or even add props.

Learn more about extending components

This is the first case we’ll handle when attempting to resolve the file.

In order to do this we need to leverage the issuer of the request. This points to the file that the request came from. This means it refers to where the import occurs. The request refers to what the import points to.

This is implemented by another method on the plugin’s class which we call requestPathIsIssuerShadowPath. It checks all possible directories for shadowing of the requested component, and then returns whether the issuer’s path is one of them. Let’s first take a look at the code and then unpack what’s happening here.

In the above code block getBaseShadowDirsForThemes returns:

This constructs the shadowable files for gatsby-theme-tomato’s Box component. Then, we join the component path and end up with:

We then know that if the issuer matches one of these components, then issuer itself is a shadow of the file being required. When this happens, we return the next path, so here the original location of the theme: /Users/johno/c/gatsby-theme-example-component-extending/gatsby-theme-tomato/src/box.

This means that when our shadowed file imports Box from a shadowed file we return the original box component defined in the theme.

As a result, the following will work as we expect:

Now, all usages of the Box in gatsby-theme-tomato will be also wrapped in a purple box.

An edge case

If a theme sets module config the issuer will be null. As such we need to first check that the request.context.issuer is present before we attempt to resolve the shadowed component.

It’s important to note that we don’t recommend appending to the modules list in themes. Though, if you do, we will make sure we don’t arbitrarily error.

Summary

Shadowing uses a predictable algorithm that leverages webpack to dynamically change module resolution based on a gatsby-config and theme composition. The last theme will take precedence in the shadowing algorithm, and the user’s src directory is always take into account first.