Flutter ListView獲取頁面顯示的第一個item

1.listview 中只用利用custom才能監聽到第一個條目
new ListView.custom(
  controller: controller,
  cacheExtent: 1.0, // 只有設置了1.0 才能夠準確的標記position 位置
  childrenDelegate:  MyChildrenDelegate(
        (BuildContext context, int index) {
      return new Dismissible(
          key: new Key(list[index]),
          onDismissed: (direction) {
            //被移除回掉
            list.removeAt(index);
            var item = list[index];
            Scaffold.of(context).showSnackBar(
                new SnackBar(content: new Text("$item")));
          },
          child: new ListTile(
            title: new Text(list[index]),
          ));
    },
    childCount: list.length,
  ),
)

2. SliverChildBuilderDelegate

class _SaltedValueKey extends ValueKey<Key>{
  const _SaltedValueKey(Key key): assert(key != null), super(key);
}
class MyChildrenDelegate extends SliverChildBuilderDelegate {
  MyChildrenDelegate(
      Widget Function(BuildContext, int) builder, {
        int childCount,
        bool addAutomaticKeepAlive = true,
        bool addRepaintBoundaries = true,
      }) : super(builder,
      childCount: childCount,
      addAutomaticKeepAlives: addAutomaticKeepAlive,
      addRepaintBoundaries: addRepaintBoundaries);
  // Return a Widget for the given Exception
  Widget _createErrorWidget(dynamic exception, StackTrace stackTrace) {
    final FlutterErrorDetails details = FlutterErrorDetails(
      exception: exception,
      stack: stackTrace,
      library: 'widgets library',
      context: ErrorDescription('building'),
    );
    FlutterError.reportError(details);
    return ErrorWidget.builder(details);
  }
  @override
  Widget build(BuildContext context, int index) {
    assert(builder != null);
    if (index < 0 || (childCount != null && index >= childCount))
      return null;
    Widget child;
    try {
      child = builder(context, index);
    } catch (exception, stackTrace) {
      child = _createErrorWidget(exception, stackTrace);
    }
    if (child == null)
      return null;
    final Key key = child.key != null ? _SaltedValueKey(child.key) : null;
    if (addRepaintBoundaries)
      child = RepaintBoundary(child: child);
    if (addSemanticIndexes) {
      final int semanticIndex = semanticIndexCallback(child, index);
      if (semanticIndex != null)
        child = IndexedSemantics(index: semanticIndex + semanticIndexOffset, child: child);
    }
    if (addAutomaticKeepAlives)
      child = AutomaticKeepAlive(child: child);
    return KeyedSubtree(child: child, key: key);
  }

  @override
  void didFinishLayout(int firstIndex, int lastIndex) {
    // TODO: implement didFinishLayout
    super.didFinishLayout(firstIndex, lastIndex);
  }
 ///監聽 在可見的列表中 顯示的第一個位置和最後一個位置
@override
  double estimateMaxScrollOffset(int firstIndex, int lastIndex, double leadingScrollOffset, double trailingScrollOffset) {
  print('firstIndex sss : $firstIndex, lastIndex ssss : $lastIndex, leadingScrollOffset ssss : $leadingScrollOffset,'
      'trailingScrollOffset ssss : $trailingScrollOffset  ' );
    return super.estimateMaxScrollOffset(firstIndex, lastIndex, leadingScrollOffset, trailingScrollOffset);
  }
}