Method getSpaceBefore

src/react-list.js:393–416

(index, cache = {})

  }

  getSpaceBefore(index, cache = {}) {
    if (cache[index] != null) return cache[index];

    // Try the static itemSize.
    const { itemSize, itemsPerRow } = this.state;
    if (itemSize) {
      return (cache[index] = Math.floor(index / itemsPerRow) * itemSize);
    }

    // Find the closest space to index there is a cached value for.
    let from = index;
    while (from > 0 && cache[--from] == null);

    // Finally, accumulate sizes of items from - index.
    let space = cache[from] || 0;
    for (let i = from; i < index; ++i) {
      cache[i] = space;
      const itemSize = this.getSizeOfItem(i);
      if (itemSize == null) break;
      space += itemSize;
    }

    return (cache[index] = space);
  }

  cacheSizes() {
    const { cache } = this;