react-transition-group/src/utils/ChildMapping.js

import { Children, cloneElement, isValidElement } from 'react';

/**
 * Given `this.props.children`, return an object mapping key to child.
 *
 * @param {*} children `this.props.children`
 * @return {object} Mapping of key to child
 */
export function getChildMapping(children, mapFn) {
  let mapper = (child) =>
    mapFn && isValidElement(child) ? mapFn(child) : child;

  let result = Object.create(null);
  if (children)
    Children.map(children, (c) => c).forEach((child) => {
      // run the map function here instead so that the key is the computed one
      result[child.key] = mapper(child);
    });
  return result;
}

/**
 * When you're adding or removing children some may be added or removed in the
 * same render pass. We want to show *both* since we want to simultaneously
 * animate elements in and out. This function takes a previous set of keys
 * and a new set of keys and merges them with its best guess of the correct
 * ordering. In the future we may expose some of the utilities in
 * ReactMultiChild to make this easy, but for now React itself does not
 * directly have this concept of the union of prevChildren and nextChildren
 * so we implement it here.
 *
 * @param {object} prev prev children as returned from
 * `ReactTransitionChildMapping.getChildMapping()`.
 * @param {object} next next children as returned from
 * `ReactTransitionChildMapping.getChildMapping()`.
 * @return {object} a key set that contains all keys in `prev` and all keys
 * in `next` in a reasonable order.
 */
export function mergeChildMappings(prev, next) {
  prev = prev || {};
  next = next || {};

  function getValueForKey(key) {
    return key in next ? next[key] : prev[key];
  }

  // For each key of `next`, the list of keys to insert before that key in
  // the combined list
  let nextKeysPending = Object.create(null);

  let pendingKeys = [];
  for (let prevKey in prev) {
    if (prevKey in next) {
      if (pendingKeys.length) {
        nextKeysPending[prevKey] = pendingKeys;
        pendingKeys = [];
      }
    } else {
      pendingKeys.push(prevKey);
    }
  }

  let i;
  let childMapping = {};
  for (let nextKey in next) {
    if (nextKeysPending[nextKey]) {
      for (i = 0; i < nextKeysPending[nextKey].length; i++) {
        let pendingNextKey = nextKeysPending[nextKey][i];
        childMapping[nextKeysPending[nextKey][i]] =
          getValueForKey(pendingNextKey);
      }
    }
    childMapping[nextKey] = getValueForKey(nextKey);
  }

  // Finally, add the keys which didn't appear before any key in `next`
  for (i = 0; i < pendingKeys.length; i++) {
    childMapping[pendingKeys[i]] = getValueForKey(pendingKeys[i]);
  }

  return childMapping;
}

function getProp(child, prop, props) {
  return props[prop] != null ? props[prop] : child.props[prop];
}

export function getInitialChildMapping(props, onExited) {
  return getChildMapping(props.children, (child) => {
    return cloneElement(child, {
      onExited: onExited.bind(null, child),
      in: true,
      appear: getProp(child, 'appear', props),
      enter: getProp(child, 'enter', props),
      exit: getProp(child, 'exit', props),
    });
  });
}

export function getNextChildMapping(nextProps, prevChildMapping, onExited) {
  let nextChildMapping = getChildMapping(nextProps.children);
  let children = mergeChildMappings(prevChildMapping, nextChildMapping);

  Object.keys(children).forEach((key) => {
    let child = children[key];

    if (!isValidElement(child)) return;

    const hasPrev = key in prevChildMapping;
    const hasNext = key in nextChildMapping;

    const prevChild = prevChildMapping[key];
    const isLeaving = isValidElement(prevChild) && !prevChild.props.in;

    // item is new (entering)
    if (hasNext && (!hasPrev || isLeaving)) {
      // console.log('entering', key)
      children[key] = cloneElement(child, {
        onExited: onExited.bind(null, child),
        in: true,
        exit: getProp(child, 'exit', nextProps),
        enter: getProp(child, 'enter', nextProps),
      });
    } else if (!hasNext && hasPrev && !isLeaving) {
      // item is old (exiting)
      // console.log('leaving', key)
      children[key] = cloneElement(child, { in: false });
    } else if (hasNext && hasPrev && isValidElement(prevChild)) {
      // item hasn't changed transition states
      // copy over the last transition props;
      // console.log('unchanged', key)
      children[key] = cloneElement(child, {
        onExited: onExited.bind(null, child),
        in: prevChild.props.in,
        exit: getProp(child, 'exit', nextProps),
        enter: getProp(child, 'enter', nextProps),
      });
    }
  });

  return children;
}