Move Flutter widgets to packages directory

packages/flutter_widgets/lib/src/lazy_load_scroll_view.dart

@@ -0,0 +1,148 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+enum _LoadingStatus { LOADING, STABLE }
+
+/// A widget that wraps a [Widget] and will trigger [onEndOfPage]/[onStartOfPage] when it
+/// reaches the bottom/start of the list
+class LazyLoadScrollView extends StatefulWidget {
+  /// The [Widget] that this widget watches for changes on
+  final Widget child;
+
+  /// Called when the [child] reaches the start of the list
+  final AsyncCallback onStartOfPage;
+
+  /// Called when the [child] reaches the end of the list
+  final AsyncCallback onEndOfPage;
+
+  /// Called when the list scrolling starts
+  final VoidCallback onPageScrollStart;
+
+  /// Called when the list scrolling ends
+  final VoidCallback onPageScrollEnd;
+
+  /// Called every time the [child] is in-between the list
+  final VoidCallback onInBetweenOfPage;
+
+  /// The offset to take into account when triggering [onEndOfPage]/[onStartOfPage] in pixels
+  final double scrollOffset;
+
+  /// Used to determine if loading of new data has finished. You should use set this if you aren't using a FutureBuilder or StreamBuilder
+  final bool isLoading;
+
+  /// Initiates a LazyLoadScrollView widget
+  const LazyLoadScrollView({
+    Key key,
+    @required this.child,
+    this.onStartOfPage,
+    this.onEndOfPage,
+    this.onPageScrollStart,
+    this.onPageScrollEnd,
+    this.onInBetweenOfPage,
+    this.isLoading = false,
+    this.scrollOffset = 100,
+  })  : assert(child != null),
+        super(key: key);
+
+  @override
+  State<StatefulWidget> createState() => _LazyLoadScrollViewState();
+}
+
+class _LazyLoadScrollViewState extends State<LazyLoadScrollView> {
+  _LoadingStatus _loadMoreStatus = _LoadingStatus.STABLE;
+  double _scrollPosition = 0.0;
+
+  @override
+  Widget build(BuildContext context) {
+    return NotificationListener(
+      child: widget.child,
+      onNotification: _onNotification,
+    );
+  }
+
+  bool _onNotification(Notification notification) {
+    if (notification is ScrollStartNotification) {
+      if (widget.onPageScrollStart != null) {
+        widget.onPageScrollStart();
+        return true;
+      }
+    }
+    if (notification is ScrollEndNotification) {
+      if (widget.onPageScrollEnd != null) {
+        widget.onPageScrollEnd();
+        return true;
+      }
+    }
+    if (notification is ScrollUpdateNotification) {
+      final pixels = notification.metrics.pixels;
+      final maxScrollExtent = notification.metrics.maxScrollExtent;
+      final minScrollExtent = notification.metrics.minScrollExtent;
+      final scrollOffset = widget.scrollOffset;
+
+      if (pixels > (minScrollExtent + scrollOffset) &&
+          pixels < (maxScrollExtent - scrollOffset)) {
+        if (widget.onInBetweenOfPage != null) {
+          widget.onInBetweenOfPage();
+          return true;
+        }
+      }
+
+      final extentBefore = notification.metrics.extentBefore;
+      final extentAfter = notification.metrics.extentAfter;
+      final scrollingDown = _scrollPosition < pixels;
+
+      if (scrollOffset == null || scrollOffset == 0) {
+        if (extentAfter == 0) {
+          _onEndOfPage();
+        }
+        if (extentBefore == 0) {
+          _onStartOfPage();
+        }
+      } else {
+        if (scrollingDown) {
+          if (extentAfter <= scrollOffset) {
+            _onEndOfPage();
+          }
+        } else {
+          if (extentBefore <= scrollOffset) {
+            _onStartOfPage();
+          }
+        }
+      }
+      _scrollPosition = pixels;
+      return true;
+    }
+    if (notification is OverscrollNotification) {
+      if (notification.overscroll > 0) {
+        _onEndOfPage();
+      }
+      if (notification.overscroll < 0) {
+        _onStartOfPage();
+      }
+      return true;
+    }
+    return false;
+  }
+
+  void _onEndOfPage() {
+    if (_loadMoreStatus != null && _loadMoreStatus == _LoadingStatus.STABLE) {
+      _loadMoreStatus = _LoadingStatus.LOADING;
+      if (widget.onEndOfPage != null) {
+        widget.onEndOfPage().whenComplete(() {
+          _loadMoreStatus = _LoadingStatus.STABLE;
+        });
+      }
+    }
+  }
+
+  void _onStartOfPage() {
+    if (_loadMoreStatus != null && _loadMoreStatus == _LoadingStatus.STABLE) {
+      _loadMoreStatus = _LoadingStatus.LOADING;
+      if (widget.onStartOfPage != null) {
+        widget.onStartOfPage().whenComplete(() {
+          _loadMoreStatus = _LoadingStatus.STABLE;
+        });
+      }
+    }
+  }
+}