import React, { useState } from 'react';
import { VirtualList } from 'biz-web-library';
// 定義每一條數(shù)據(jù)顯示的組件
const ItemRender = ({ data }) => {
  let dindex = parseInt(data);
  let lineHeight = dindex % 2 ? '40px' : '80px';
  return (
    <div style={{ lineHeight, background: dindex % 2 ? '#f5f5f5' : '#fff' }}>
      <h3>#{dindex} title name</h3>
      <p>盡情地書寫你想編寫的內(nèi)容，不局限于頁(yè)面高度</p>
    </div>
  );
};
const ItemRenderMemo = React.memo(ItemRender);

// 初始化列表數(shù)據(jù)
const getDatas = () => {
  const datas = [];
  for (let i = 0; i < 100000; i++) {
    datas.push(`${i} Item`);
  }
  return datas;
};

// 使用虛擬列表
export default () => {
  let [resources, setResources] = useState([]);
  const changeResources = () => {
    setResources(getDatas());
  };

  return (
    <div>
      <button onClick={changeResources}>click me </button>

      <div
        style={{
          height: '400px',
          overflow: 'auto',
          border: '1px solid #f5f5f5',
          padding: '0 10px',
        }}
      >
        <VirtualList
          ItemRender={ItemRenderMemo}
          resources={resources}
          estimatedItemSize={60}
        />
      </div>
    </div>
  );
};

// 循環(huán)緩存列表的總體初始化高度
export const initPositinoCache = (
  estimatedItemSize: number = 32,
  length: number = 0,
) => {
  let index = 0,
  positions = Array(length);
  while (index < length) {
    positions[index] = {
      index,
      height: estimatedItemSize,
      top: index * estimatedItemSize,
      bottom: (index++ + 1) * estimatedItemSize,
    };
  }
  return positions;
};

// 緩存所有item的位置
let positions: Array<PositionType>;

class VirtualList extends React.PureComponent{
 
  constructor(props) {
    super(props);
    const { resources } = this.props;

    // 初始化緩存
    positions = initPositinoCache(props.estimatedItemSize, resources.length);
    this.state = {
      resources,
      startOffset: 0,
      listHeight: getListHeight(positions),  // positions最后一條數(shù)據(jù)的bottom屬性

      scrollRef: React.createRef(),  // 虛擬列表容器ref
      items: React.createRef(), // 虛擬列表顯示區(qū)域ref
      visibleCount: 10, // 一頁(yè)可視區(qū)域條數(shù)
      startIndex: 0, // 可視區(qū)域開始索引
      endIndex: 10, // // 可視區(qū)域結(jié)束索引
    };
  }
  // TODO: 隱藏一些其他功能。。。。。


  // 布局
  render() {
  const { ItemRender = ItemRenderComponent, extrea } = this.props;
  const { listHeight, startOffset, resources, startIndex, endIndex, items, scrollRef  } = this.state;
  let visibleData = resources.slice(startIndex, endIndex);

  return (
    <div ref={scrollRef} style={{ height: `${listHeight}px` }}>
      <ul
        ref={items}
        style={{
          transform: `translate3d(0,${startOffset}px,0)`,
        }}
      >
        {visibleData.map((data, index) => {
          return (
            <li key={data.id || data.key || index} data-index={`${startIndex + index}`}>
              <ItemRender data={data} {...extrea}/>
            </li>
          );
        })}
      </ul>
    </div>
  );
  }
}

componentDidMount() {
  events.on(this.getEl(), 'scroll', this.onScroll, false);
  events.on(this.getEl(), 'mousewheel', NOOP, false);

  // 根據(jù)渲染，計(jì)算最新的節(jié)點(diǎn)
  let visibleCount = Math.ceil(this.getEl().offsetHeight / estimatedItemSize);
  if (visibleCount === this.state.visibleCount || visibleCount === 0) {
    return;
  }
  // 因?yàn)?visibleCount變更， 更新endIndex, listHeight/ 偏移量
  this.updateState({ visibleCount, startIndex: this.state.startIndex });
}

getEl = () => {
    let el = this.state.scrollRef || this.state.items;
    let parentEl: any = el.current?.parentElement;
    switch (window.getComputedStyle(parentEl)?.overflowY) {
      case 'auto':
      case 'scroll':
      case 'overlay':
      case 'visible':
        return parentEl;
    }
    return document.body;
};

onScroll = () => {
    requestAnimationFrame(() => {
      let { scrollTop } = this.getEl();
      let startIndex = binarySearch(positions, scrollTop);

      // 因?yàn)?startIndex變更， 更新endIndex, listHeight/ 偏移量
      this.updateState({ visibleCount: this.state.visibleCount, startIndex});
    });
  };

// 工具函數(shù)，放入工具文件
export const binarySearch = (list: Array<PositionType>, value: number = 0) => {
  let start: number = 0;
  let end: number = list.length - 1;
  let tempIndex = null;
  while (start <= end) {
    let midIndex = Math.floor((start + end) / 2);
    let midValue = list[midIndex].bottom;

    // 值相等，則直接返回 查找到的節(jié)點(diǎn)（因?yàn)槭莃ottom, 因此startIndex應(yīng)該是下一個(gè)節(jié)點(diǎn)）
    if (midValue === value) {
      return midIndex + 1;
    }
    // 中間值 小于 傳入值，則說明 value對(duì)應(yīng)的節(jié)點(diǎn) 大于 start, start往后移動(dòng)一位
    else if (midValue < value) {
      start = midIndex + 1;
    }
    // 中間值 大于 傳入值，則說明 value 在 中間值之前，end 節(jié)點(diǎn)移動(dòng)到 mid - 1
    else if (midValue > value) {
      // tempIndex存放最靠近值為value的所有
      if (tempIndex === null || tempIndex > midIndex) {
        tempIndex = midIndex;
      }
      end = midIndex - 1;
    }
  }
  return tempIndex;
};

 updateState = ({ visibleCount, startIndex }) => {
    // 根據(jù)新計(jì)算的節(jié)點(diǎn)，更新data數(shù)據(jù)
    this.setState({
      startOffset: startIndex >= 1 ? positions[startIndex - 1]?.bottom : 0,
      listHeight: getListHeight(positions),
      startIndex,
      visibleCount,
      endIndex: getEndIndex(this.state.resources, startIndex, visibleCount)
    });
  };

// 下面是工具函數(shù)，放在其他文件中的
export const getListHeight = (positions: Array<PositionType>) => {
    let index = positions.length - 1;
    return index < 0 ? 0 : positions[index].bottom;
  };

export const getEndIndex = (
  resources: Array<Data>,
  startIndex: number,
  visibleCount: number,
) => {
  let resourcesLength = resources.length;
  let endIndex = startIndex + visibleCount;
  return resourcesLength > 0 ? Math.min(resourcesLength, endIndex) : endIndex;
}

 componentDidUpdate() {
  this.updateHeight();
}

  
updateHeight = () => {
  let items: HTMLCollection = this.state.items.current?.children;
  if (!items.length) return;

  // 更新緩存
  updateItemSize(positions, items);

  // 更新總高度
  let listHeight = getListHeight(positions);

  // 更新總偏移量
  let startOffset = getStartOffset(this.state.startIndex, positions);

  this.setState({
    listHeight,
    startOffset,
  });
};

// 下面是工具函數(shù)，放在其他文件中的
export const updateItemSize = (
  positions: Array<PositionType>,
  items: HTMLCollection,
) => {
  Array.from(items).forEach(item => {
    let index = Number(item.getAttribute('data-index'));
    let { height } = item.getBoundingClientRect();
    let oldHeight = positions[index].height;

    //存在差值, 更新該節(jié)點(diǎn)以后所有的節(jié)點(diǎn)
    let dValue = oldHeight - height;
    if (dValue) {
      positions[index].bottom = positions[index].bottom - dValue;
      positions[index].height = height;

      for (let k = index + 1; k < positions.length; k++) {
        positions[k].top = positions[k - 1].bottom;
        positions[k].bottom = positions[k].bottom - dValue;
      }
    }
  });
};

//獲取當(dāng)前的偏移量
export const getStartOffset = (
  startIndex: number,
  positions: Array<PositionType> = [],
) => {
  return startIndex >= 1 ? positions[startIndex - 1]?.bottom : 0;
};

export const getListHeight = (positions: Array<PositionType>) => {
  let index = positions.length - 1;
  return index < 0 ? 0 : positions[index].bottom;
};

 static getDerivedStateFromProps(
    nextProps: VirtualListProps,
    prevState: VirtualListState,
  ) {
    const { resources, estimatedItemSize } = nextProps;
    if (resources !== prevState.resources) {
      positions = initPositinoCache(estimatedItemSize, resources.length);

      // 更新高度
      let listHeight = getListHeight(positions);

      // 更新總偏移量
      let startOffset = getStartOffset(prevState.startIndex, positions);

     
      let endIndex = getEndIndex(resources, prevState.startIndex, prevState.visibleCount);
     
      return {
        resources,
        listHeight,
        startOffset,
        endIndex,
      };
    }
    return null;
  }