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Lazy loading#

Since Volto 5.0.0 you are able to do splitting and lazy loading safely any app component using @loadable/component library. You can also benefit from it in your own project by using it.

Note

Webpack 4 is already lazy load enabled, using import() but @loadable/component makes the process safe since Volto is using Server Side Rendering. The React community is working actively in the React async mode popularly known as Suspense. Suspense will be SSR safe but in the meanwhile it's not ready, @loadable/component is the community accepted replacement.

Lazy load a component#

export const DatetimeWidget = loadable(() =>
  import('@plone/volto/components/manage/Widgets/DatetimeWidget'),
);

Then use DatetimeWidget as you'll do normally using a standard import statement.

Tip

You can find the complete @loadable/component documentation here: https://loadable-components.com

Code splitting bundle analyzer#

You can check the code splitting state by using the included bundle analyzer:

$ yarn analyze

A browser will open with the bundle inspector.

Lazy-loading libraries#

Lazy-loading libraries is not as straight-forward as with the React components. The API offered by @loadable/component is not very ergonomic and importing a library as lazy library introduces a lot of pain points in your code: you have to alway check if the library is loaded, depending on multiple lazy libraries further complicates the code, etc. To aleviate this and to promote the use of lazy libraries everywhere, we have the injectLazyLibs HOC wrapper that can automatically inject lazy-loaded libraries as props to your components. To use it:

import { injectLazyLibs } from '@plone/volto/helpers/Loadable/Loadable';

function MyComponent({toastify}) {
  useEffect(() => {toastify.toast.success('Hello')}};
}

export default injectLazyLibs(['toastify'])(MyComponent);

Wrapping a component in injectLazyLibs makes sure that the component is only rendered once all the libraries are loaded, simplifying the internal component logic.

To define new libraries, use the new settings.loadables entry:

import loadable from '@loadable/component';

...
settings.loadables['reactDnd'] = loadable.lib(() => import('react-dnd'));

Notice that we still use the @loadable/component API to load these libraries. It is not possible to have completely dynamic imports in a webpack-powered system. According to webpack documentation, The import() must contain at least some information about where the module is located.

The useLazyLibs hook#

In functional components you can use the useLazyLibs hook, which allows greater flexibility (the injectLazyLibs hook uses useLazyLibs internally). You can call the hook like:

import { useLazyLibs } from '@plone/volto/helpers/Loadable/Loadable';

useLazyLibs(['toastify', 'reactDnd'])
// or:
useLazyLibs(['toastify', 'reactDnd'], {shouldRerender: false})

Passing the shouldRerender as false as options will cause the component to avoid re-rendering the component once the lazy library has been loaded successfully.

Define bundles of lazy libraries and preload them all at once#

You can define a "bundle" of multiple lazy libraries in the settings lazyBundles key:

settings.lazyBundles = {
  cms: ['prettierStandalone', 'prettierParserHtml', ...]
}

You can quickly load these bundles by wrapping your component in the preloadLazyLibs HOC:

import { preloadLazyLibs } from '@plone/volto/helpers/Loadable/Loadable';

const SomeComponent = (props) => <div>Hello</div>;

export default preloadLazyLibs('cms')(SomeComponent);

Testing with lazy loaded libraries integrated#

Sometimes you'll find that it's difficult to get the lazy loaded libraries properly loaded in your jest tests. In that case, add this to the top of your test:

jest.mock('@plone/volto/helpers/Loadable/Loadable');
beforeAll(
  async () =>
    await require('@plone/volto/helpers/Loadable/Loadable').__setLoadables(),
);

This ensures that all libraries are loaded and injected into a mock before any test is run.