react-native-big-list/lib/BigListItemRecycler.js

High-performance, virtualized list for React Native. Efficiently renders large datasets with recycler API for smooth scrolling and low memory usage. Ideal for fast, scalable, customizable lists on Android, iOS, and web.

import { BigListItemType } from "./BigListItem";

class BigListItemRecycler {
  static lastKey = 0;
  /**
   * Constructor.
   * @param {object[]} items
   */
  constructor(items) {
    this.items = {};
    this.pendingItems = {};
    items.forEach((item) => {
      const { type, section, index } = item;
      const [itemsForType] = this.itemsForType(type);
      itemsForType[`${type}:${section}:${index}`] = item;
    });
  }

  /**
   * Items for type.
   * @param {any} type
   * @returns {(*|{}|*[])[]}
   */
  itemsForType(type) {
    return [
      this.items[type] || (this.items[type] = {}),
      this.pendingItems[type] || (this.pendingItems[type] = []),
    ];
  }

  /**
   * Get item.
   * @param {any} type
   * @param {number} position
   * @param {number} height
   * @param {int} section
   * @param {int} index
   * @returns {{section: int, position: number, index: number, type: any, key: number, height: int}}
   */
  get({ type, position, height, section = 0, index = 0 }) {
    const [items, pendingItems] = this.itemsForType(type);
    const itemKey = `${type}:${section}:${index}`;
    let item = items[itemKey];
    if (item == null) {
      item = { type, key: -1, position, height, section, index };
      pendingItems.push(item);
    } else {
      item.position = position;
      item.height = height;
      delete items[itemKey];
    }
    return item;
  }

  /**
   * Fill.
   */
  fill() {
    Object.values(BigListItemType).forEach((type) => {
      const [items, pendingItems] = this.itemsForType(type);
      let index = 0;
      Object.values(items).forEach(({ key }) => {
        const item = pendingItems[index];
        if (item == null) {
          return false;
        }
        item.key = key;
        index++;
      });

      for (; index < pendingItems.length; index++) {
        pendingItems[index].key = ++BigListItemRecycler.lastKey;
      }
      pendingItems.length = 0;
    });
  }
}
export default BigListItemRecycler;