🐛 Fix refresh issue on switching folders

[yanivshaked]

@AlexV525
This fixes the following issue on iOS. It is possible this is an overkill, and issue can be solved in a softer manner.

[AlexV525]

We need to replace

flutter_wechat_assets_picker/lib/src/delegates/asset_picker_builder_delegate.dart

Lines 805 to 808 in 07e22d6

	@override
	Widget assetsGridBuilder(BuildContext context) {
	// First, we need the count of the assets.
	int totalCount = provider.currentPathEntity!.assetCount;

with below snippet (wrap it with a Selector, along with some format changes), instead of changing this flag:

  @override
  Widget assetsGridBuilder(BuildContext context) {
    return Selector<DefaultAssetPickerProvider, AssetPathEntity?>(
      selector: (_, DefaultAssetPickerProvider p) => p.currentPathEntity,
      builder: (_, AssetPathEntity? path, __) {
        // First, we need the count of the assets.
        int totalCount = path!.assetCount;
        // If user chose a special item's position, add 1 count.
        if (specialItemPosition != SpecialItemPosition.none) {
          totalCount += 1;
        }
        // Then use the [totalCount] to calculate how many placeholders we need.
        final int placeholderCount;
        if (isAppleOS && totalCount % gridCount != 0) {
          // When there are left items that not filled into one row,
          // filled the row with placeholders.
          placeholderCount = gridCount - totalCount % gridCount;
        } else {
          // Otherwise, we don't need placeholders.
          placeholderCount = 0;
        }
        // Calculate rows count.
        final int row = (totalCount + placeholderCount) ~/ gridCount;
        // Here we got a magic calculation. [itemSpacing] needs to be divided by
        // [gridCount] since every grid item is squeezed by the [itemSpacing],
        // and it's actual size is reduced with [itemSpacing / gridCount].
        final double dividedSpacing = itemSpacing / gridCount;
        final double topPadding = context.topPadding + kToolbarHeight;

        Widget _sliverGrid(BuildContext ctx, List<AssetEntity> assets) {
          return SliverGrid(
            delegate: SliverChildBuilderDelegate(
              (_, int index) => Builder(
                builder: (BuildContext c) {
                  if (isAppleOS) {
                    if (index < placeholderCount) {
                      return const SizedBox.shrink();
                    }
                    index -= placeholderCount;
                  }
                  return Directionality(
                    textDirection: Directionality.of(context),
                    child: assetGridItemBuilder(c, index, assets),
                  );
                },
              ),
              childCount: assetsGridItemCount(
                context: ctx,
                assets: assets,
                placeholderCount: placeholderCount,
              ),
              findChildIndexCallback: (Key? key) {
                if (key is ValueKey<String>) {
                  return findChildIndexBuilder(
                    id: key.value,
                    assets: assets,
                    placeholderCount: placeholderCount,
                  );
                }
                return null;
              },
            ),
            gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
              crossAxisCount: gridCount,
              mainAxisSpacing: itemSpacing,
              crossAxisSpacing: itemSpacing,
            ),
          );
        }

        return LayoutBuilder(
          builder: (BuildContext c, BoxConstraints constraints) {
            final double itemSize = constraints.maxWidth / gridCount;
            // Check whether all rows can be placed at the same time.
            final bool onlyOneScreen = row * itemSize <=
                constraints.maxHeight -
                    context.bottomPadding -
                    topPadding -
                    permissionLimitedBarHeight;
            final double height;
            if (onlyOneScreen) {
              height = constraints.maxHeight;
            } else {
              // Reduce [permissionLimitedBarHeight] for the final height.
              height = constraints.maxHeight - permissionLimitedBarHeight;
            }
            // Use [ScrollView.anchor] to determine where is the first place of
            // the [SliverGrid]. Each row needs [dividedSpacing] to calculate,
            // then minus one times of [itemSpacing] because spacing's count
            // in the cross axis is always less than the rows.
            final double anchor = math.min(
              (row * (itemSize + dividedSpacing) + topPadding - itemSpacing) /
                  height,
              1,
            );

            return Directionality(
              textDirection: effectiveGridDirection(context),
              child: ColoredBox(
                color: theme.canvasColor,
                child: Selector<DefaultAssetPickerProvider, List<AssetEntity>>(
                  selector: (_, DefaultAssetPickerProvider provider) =>
                      provider.currentAssets,
                  builder: (_, List<AssetEntity> assets, __) =>
                      CustomScrollView(
                    physics: const AlwaysScrollableScrollPhysics(),
                    controller: gridScrollController,
                    anchor: isAppleOS ? anchor : 0,
                    center: isAppleOS ? gridRevertKey : null,
                    slivers: <Widget>[
                      if (isAppleOS)
                        SliverGap.v(context.topPadding + kToolbarHeight),
                      _sliverGrid(_, assets),
                      // Ignore the gap when the [anchor] is not equal to 1.
                      if (isAppleOS && anchor == 1)
                        SliverGap.v(
                          context.bottomPadding + bottomSectionHeight,
                        ),
                      if (isAppleOS)
                        SliverToBoxAdapter(
                          key: gridRevertKey,
                          child: const SizedBox.shrink(),
                        ),
                    ],
                  ),
                ),
              ),
            );
          },
        );
      },
    );
  }

[AlexV525]

LGTM. Thanks for taking my suggestion. 👍