assetGridItemBuilder method

@override Widget assetGridItemBuilder(BuildContext context, int index, List<AssetEntity> currentAssets)

There are several conditions within this builder:

• Return specialItemBuilder while the current path is all and specialItemPosition is not equal to SpecialItemPosition.none.
• Return item builder according to the asset's type.
  • AssetType.audio -> audioItemBuilder
  • AssetType.image, AssetType.video -> imageAndVideoItemBuilder
• Load more assets when the index reached at third line counting backwards.

资源构建有几个条件：

• 当前路径是全部资源且 specialItemPosition 不等于 SpecialItemPosition.none 时，将会通过 specialItemBuilder 构建内容。
• 根据资源类型返回对应类型的构建：
  • AssetType.audio -> audioItemBuilder 音频类型
  • AssetType.image, AssetType.video -> imageAndVideoItemBuilder 图片和视频类型
• 在索引到达倒数第三列的时候加载更多资源。

Implementation

@override
Widget assetGridItemBuilder(
  BuildContext context,
  int index,
  List<AssetEntity> currentAssets,
) {
  final AssetPathEntity? currentPathEntity =
      context.select<DefaultAssetPickerProvider, AssetPathEntity?>(
    (DefaultAssetPickerProvider p) => p.currentPathEntity,
  );

  int currentIndex;
  switch (specialItemPosition) {
    case SpecialItemPosition.none:
    case SpecialItemPosition.append:
      currentIndex = index;
      break;
    case SpecialItemPosition.prepend:
      currentIndex = index - 1;
      break;
  }

  // Directly return the special item when it's empty.
  if (currentPathEntity == null) {
    if (allowSpecialItemWhenEmpty &&
        specialItemPosition != SpecialItemPosition.none) {
      return specialItemBuilder!(context);
    }
    return const SizedBox.shrink();
  }

  final int _length = currentAssets.length;
  if (currentPathEntity.isAll &&
      specialItemPosition != SpecialItemPosition.none) {
    if ((index == 0 && specialItemPosition == SpecialItemPosition.prepend) ||
        (index == _length &&
            specialItemPosition == SpecialItemPosition.append)) {
      return specialItemBuilder!(context);
    }
  }

  if (!currentPathEntity.isAll) {
    currentIndex = index;
  }

  if (index == _length - gridCount * 3 &&
      context.select<DefaultAssetPickerProvider, bool>(
        (DefaultAssetPickerProvider p) => p.hasMoreToLoad,
      )) {
    provider.loadMoreAssets();
  }

  final AssetEntity asset = currentAssets.elementAt(currentIndex);
  Widget builder;
  switch (asset.type) {
    case AssetType.audio:
      builder = audioItemBuilder(context, currentIndex, asset);
      break;
    case AssetType.image:
    case AssetType.video:
      builder = imageAndVideoItemBuilder(context, currentIndex, asset);
      break;
    case AssetType.other:
      builder = const SizedBox.shrink();
      break;
  }
  return Stack(
    key: ValueKey<String>(asset.id),
    children: <Widget>[
      builder,
      selectedBackdrop(context, currentIndex, asset),
      if (!isWeChatMoment || asset.type != AssetType.video)
        selectIndicator(context, asset),
      itemBannedIndicator(context, asset),
    ],
  );
}