descendants

first_written	2024-02-01
last_updated	2024-06-12

Descendants

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Installation

Yarn

yarn add @leafygreen-ui/descendants

NPM

npm install @leafygreen-ui/descendants

Overview

Descendants is an internal utility that allows components to track all rendered descendants.

This is useful when developing menus in order to track when items are rendered/un-rendered.

Usage

There are 4 steps required to set up a pair of components as Parent/Descendent.

/**
 * 1. Create a new Context
 *
 * We need to create a new Context object
 * in order for the parent & child to know their relationships.
 *
 * Without this, a descendant won't know what parent context it belongs to.
 * This also enables us to nest different descendant contexts
 *
 * e.g.
 * A Menu that a second fly-out menu
 * shouldn't track fly-out menu items as its own descendants)
 *
 */
const MyDescendantsContext = createDescendantsContext('MyDescendantsContext');

export const MyParent = ({ children, ...rest }: ComponentProps<'div'>) => {
  /**
   * 2. Initialize an empty descendants list and setter
   *
   * We call this _outside_ the Provider
   * so we can access the `descendants` object
   * from the Parent level.
   */
  const { descendants, dispatch } = useInitDescendants<HTMLDivElement>();

  /**
   * 3. Pass the context, descendants list and setter into the provider
   *
   * We need to pass the context value into the Provider
   * in order for the Parent to have knowledge
   * of its specific context
   * (see fly-out menu example in step 1.)
   */
  return (
    <DescendantsProvider
      context={MyDescendantsContext}
      descendants={descendants}
      dispatch={dispatch}
    >
      <div {...rest}>{children}</div>
    </DescendantsProvider>
  );
};

export const TestDescendant = ({
  children,
  ...rest
}: ComponentProps<'div'>) => {
  /**
   * 4. Establish a child component as a descendant
   *
   * Pass the context value into the hook
   * in order to establish this element
   */
  const { index, ref } = useDescendant(MyDescendantsContext);

  // This component has access to its index within the Parent context

  return (
    <div ref={ref} {...rest}>
      {children}
    </div>
  );
};

Handling Stale Descendants

Sometimes you'll need to access descendants from within an event handler or useEffect callback, where the descendants may have updated between renders, resulting in a descendants object with references to stale/non-existent DOM nodes. In this case, use the getDescendants() function to access the most up-to-date descendants object.

const { descendants, dispatch, getDescendants } =
  useInitDescendants<HTMLDivElement>();

const handleTransition = () => {
  console.log(descendants); // this list will be empty, or contain references to old DOM nodes
  console.log(getDescendants()); // this call will return a list of the updated descendants list
};

return (
  <DescendantsProvider
    context={MyDescendantsContext}
    descendants={descendants}
    dispatch={dispatch}
  >
    <Transition onEntered={handleTransition}>
      <div {...rest}>{children}</div>
    </Transition>
  </DescendantsProvider>
);

Prior Art

This package heavily references the work of pacocoursey/use-descendants and @reach-ui/descendants. Many thanks to the authors of those packages!

However, in addition to being internal and not available as a v1.0, these packages have a few shortcomings:

• pacocoursey relies on writing to the DOM inside a useEffect, which is highly non-idiomatic for React
• pacocoursey uses two refs to track descendants, which duplicates the space needed, and could be difficult to ensure they stay in sync
• pacocoursey must force-rerender descendant elements in order to have access to their index
• reach-ui does not enable users to props passed into a descendant from the descendants object
• react-ui is not compatible with React 18 and strict-mode
• Both reach-ui and pacocoursey must ignore the linter for useEffect dependencies

The primary architectural difference between this package and those mentioned above is the use of a reducer to avoid unnecessary rerenders.

Architecture

1. We call a context factory function to create a new descendants context

2. A parent component calls useInitDescendants to establish a descendants state and dispatch state setter(s)

   a. useInitDescendants creates a DescendantsReducer with descendants state and dispatch setter. (More on the dispatch function in DescendantsReducer)

3. The descendants state, dispatch and context are passed into DescendantsProvider which establishes a new context provider for the passed-in context

4. A child component calls useDescendant to establish itself as a descendant of the provided context argument

useDescendant

At high level this hook reads descendants and dispatch from the established context, and makes a call to dispatch on initial render to register itself as a descendant. On un-mount it then makes a second call to dispatch to remove itself from the list. A descendant's internal id is a ref object established once on render. Its index is re-calculated each time descendants changes.

The hook can also be called with 2 optional parameters in addition to context. If a 2nd ref argument is provided, this ref object will forwarded and merged into the ref object returned by the hook. It's advised to use the merged ref that's returned from the hook, not the original ref you provide. If a 3rd props argument is provided, these props will be made available on the descendants object.

1. On initial render, we call dispatch with the "register" action type
2. When the component is unmounted, we call dispatch with the "remove" action type
3. If the props object changes, we call dispatch with the "update" action type

DescendantsReducer

The DescendantsReducer holds the list of descendants and a dispatch function to modify the list.

Register

When dispatch is called with the "register" action type, we do the following:

1. Check whether there is a registered descendant with the given id. a. If there is a descendant already registered, we leave the state un-modified

2. If there is no registered descendant with this id, a. Search the DOM with findDOMIndex to find the index of the descendant element in the DOM b. Create a new descendant object c. Duplicate the list of descendants with our new descendant inserted at the given index d. Return the modified list

Remove

When dispatch is called with the "remove" action type, we check if a descendant with provided id exists, and remove it from the list

Update

When dispatch is called with the "update" action type, we set the provided props object onto the relevant descendant, (only if the props have changed to avoid unnecessary re-renders)

Evaluation & Benchmarks

Below is a comparison between this package, pacocoursey and reach-ui, as well as a control test. The control refers to rendering plain div elements, without any descendants tracking.

Overall, this package performed about 2x faster than reach-ui and 60% faster than pacocoursey in most metrics.

Test Format

Each package was tested using Jest with JSDOM and React Testing Library. Each package was tested 100x for each metric. The metrics tested are as follows:

1. Render: Render speed was tested by rendering 500 elements to the DOM

2. Nested: Nested render speed was tested by rendering 100 groups of 5 elements each to the DOM

3. Insert: Insertion was tested by first rendering 500 elements to the DOM, and then inserting an element at the 250th element

4. Remove: Removal speed was tested by first rendering 500 elements to the DOM, and then removing the 250th element

5. Select: Select speed is a proxy for "update" speed. This was tested by adding a click handler to a descendant element that registered its index to an outer context as "selected". The element would then render the attribute data-selected="true" to the DOM. The select speed was measured by first rendering 500 elements to the DOM, clicking the 250th element and measuring the speed to update the DOM with the above data attribute.

Test Tooling

Test tooling can be viewed in commit 525bcdc223a82ee4b2963c499dd458f1bd6051d6

Results

Below are the results of 100 iterations of the above listed tests:

(x100)	Render	Nested	Insert	Remove	Select
control	4.9ms	8.2ms	5.3ms	2.8ms	N/A
leafygreen	27.0ms	32.5ms	14.8ms	14.7ms	10.7ms
pacocoursey	38.5ms	46.1ms	18.6ms	17.6ms	12.7ms
reach-ui	49.8ms	60.5ms	20.3ms	14.9ms	9.2ms