ente/lib/ui/huge_listview/huge_listview.dart

import 'dart:math' show max;

import 'package:flutter/material.dart';
import 'package:photos/ui/huge_listview/draggable_scrollbar.dart';
import 'package:scrollable_positioned_list/scrollable_positioned_list.dart';

typedef HugeListViewItemBuilder<T> = Widget Function(
  BuildContext context,
  int index,
);
typedef HugeListViewErrorBuilder = Widget Function(
  BuildContext context,
  dynamic error,
);

class HugeListView<T> extends StatefulWidget {
  /// A [ScrollablePositionedList] controller for jumping or scrolling to an item.
  final ItemScrollController? controller;

  /// Index of an item to initially align within the viewport.
  final int startIndex;

  /// Total number of items in the list.
  final int totalCount;

  /// Called to build the thumb. One of [DraggableScrollbarThumbs.RoundedRectThumb], [DraggableScrollbarThumbs.ArrowThumb]
  /// or [DraggableScrollbarThumbs.SemicircleThumb], or build your own.
  final String Function(int) labelTextBuilder;

  /// Background color of scroll thumb, defaults to white.
  final Color thumbBackgroundColor;

  /// Drawing color of scroll thumb, defaults to gray.
  final Color thumbDrawColor;

  /// Height of scroll thumb, defaults to 48.
  final double thumbHeight;

  /// Height of bottomSafeArea so that scroll thumb does not become hidden
  /// or un-clickable due to footer elements. Default value is 120
  final double bottomSafeArea;

  /// Called to build an individual item with the specified [index].
  final HugeListViewItemBuilder<T> itemBuilder;

  /// Called to build a progress widget while the whole list is initialized.
  final WidgetBuilder? waitBuilder;

  /// Called to build a widget when the list is empty.
  final WidgetBuilder? emptyResultBuilder;

  /// Called to build a widget when there is an error.
  final HugeListViewErrorBuilder? errorBuilder;

  /// Event to call with the index of the topmost visible item in the viewport while scrolling.
  /// Can be used to display the current letter of an alphabetically sorted list, for instance.
  final ValueChanged<int>? firstShown;

  final bool isDraggableScrollbarEnabled;

  final EdgeInsetsGeometry? thumbPadding;

  final bool disableScroll;

  final bool isScrollablePositionedList;

  const HugeListView({
    Key? key,
    this.controller,
    required this.startIndex,
    required this.totalCount,
    required this.labelTextBuilder,
    required this.itemBuilder,
    this.waitBuilder,
    this.emptyResultBuilder,
    this.errorBuilder,
    this.firstShown,
    this.thumbBackgroundColor = Colors.red, // Colors.white,
    this.thumbDrawColor = Colors.yellow, //Colors.grey,
    this.thumbHeight = 48.0,
    this.bottomSafeArea = 120.0,
    this.isDraggableScrollbarEnabled = true,
    this.thumbPadding,
    this.disableScroll = false,
    this.isScrollablePositionedList = true,
  }) : super(key: key);

  @override
  HugeListViewState<T> createState() => HugeListViewState<T>();
}

class HugeListViewState<T> extends State<HugeListView<T>> {
  final scrollKey = GlobalKey<DraggableScrollbarState>();
  final listener = ItemPositionsListener.create();
  int lastIndexJump = -1;
  dynamic error;

  @override
  void initState() {
    super.initState();

    widget.isScrollablePositionedList
        ? listener.itemPositions.addListener(_sendScroll)
        : null;
  }

  @override
  void dispose() {
    listener.itemPositions.removeListener(_sendScroll);
    super.dispose();
  }

  void _sendScroll() {
    final int current = _currentFirst();
    widget.firstShown?.call(current);
    scrollKey.currentState?.setPosition(current / widget.totalCount, current);
  }

  int _currentFirst() {
    try {
      return listener.itemPositions.value.first.index;
    } catch (e) {
      return 0;
    }
  }

  @override
  Widget build(BuildContext context) {
    if (error != null && widget.errorBuilder != null) {
      return widget.errorBuilder!(context, error);
    }
    if (widget.totalCount == -1 && widget.waitBuilder != null) {
      return widget.waitBuilder!(context);
    }
    if (widget.totalCount == 0 && widget.emptyResultBuilder != null) {
      return widget.emptyResultBuilder!(context);
    }

    return widget.isScrollablePositionedList
        ? DraggableScrollbar(
            key: scrollKey,
            totalCount: widget.totalCount,
            initialScrollIndex: widget.startIndex,
            onChange: (position) {
              final int currentIndex = _currentFirst();
              final int floorIndex = (position * widget.totalCount).floor();
              final int cielIndex = (position * widget.totalCount).ceil();
              int nextIndexToJump;
              if (floorIndex != currentIndex && floorIndex > currentIndex) {
                nextIndexToJump = floorIndex;
              } else if (cielIndex != currentIndex &&
                  cielIndex < currentIndex) {
                nextIndexToJump = floorIndex;
              } else {
                return;
              }
              if (lastIndexJump != nextIndexToJump) {
                lastIndexJump = nextIndexToJump;
                widget.controller?.jumpTo(index: nextIndexToJump);
              }
            },
            labelTextBuilder: widget.labelTextBuilder,
            backgroundColor: widget.thumbBackgroundColor,
            drawColor: widget.thumbDrawColor,
            heightScrollThumb: widget.thumbHeight,
            bottomSafeArea: widget.bottomSafeArea,
            currentFirstIndex: _currentFirst(),
            isEnabled: widget.isDraggableScrollbarEnabled,
            padding: widget.thumbPadding,
            child: ScrollablePositionedList.builder(
              physics: widget.disableScroll
                  ? const NeverScrollableScrollPhysics()
                  : null,
              itemScrollController: widget.controller,
              itemPositionsListener: listener,
              initialScrollIndex: widget.startIndex,
              itemCount: max(widget.totalCount, 0),
              itemBuilder: (context, index) {
                return ExcludeSemantics(
                  child: widget.itemBuilder(context, index),
                );
              },
            ),
          )
        : ListView.builder(
            itemCount: max(widget.totalCount, 0),
            itemBuilder: (context, index) {
              return ExcludeSemantics(
                child: widget.itemBuilder(context, index),
              );
            },
          );
  }

  /// Jump to the [position] in the list. [position] is between 0.0 (first item) and 1.0 (last item), practically currentIndex / totalCount.
  /// To jump to a specific item, use [ItemScrollController.jumpTo] or [ItemScrollController.scrollTo].
  void setPosition(double position) {
    scrollKey.currentState?.setPosition(position, _currentFirst());
  }
}