如何知道用户是否看到了抖动的图像

Question

我Image在可滚动屏幕中有一个组件。当屏幕打开时，开始时看不到图像，但是您需要向下滚动以查看它。

用户滚动到图像后，如何确保它们完全可见？我想计算用户的图像印象。

您如何在颤抖中实现这一目标？

Answer 1

I didn't have much information about your code, so this is how I solved it. The impression is only counted when the image is completely visible on the screen, you can change that using _count = expression. And I used simple Container for Image.

Take a look at this screenshot first.

Code

void main() => runApp(MaterialApp(home: HomePage()),);

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  ScrollController _scrollController;
  double _heightListTile = 56, _heightContainer = 200, _oldOffset = 0, _heightBox, _initialAdd;
  int _initialCount, _count, _previousCount = 0, _itemsInList = 4;

  @override
  void initState() {
    super.initState();
    _heightBox = ((_itemsInList) * _heightListTile) + _heightContainer;
    _scrollController = ScrollController();
    _scrollController.addListener(() {
      double offset = _scrollController.offset;
      if (offset >= _oldOffset) {
        _oldOffset = offset;
        _count = _initialCount + (offset + _initialAdd) ~/ _heightBox;
        if (_count != _previousCount) setState(() {});
        _previousCount = _count;
      }
    });

    Timer.run(() {
      bool isIos = Theme.of(context).platform == TargetPlatform.iOS;
      var screenHeight = MediaQuery.of(context).size.height - (isIos ? 100 : 80); // for non notches phone use 76 instead of 100 (it's the height of status and navigation bar)
      _initialCount = screenHeight ~/ _heightBox;
      _initialAdd = screenHeight % _heightBox;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(_count == null ? "Let's count" : "Images shown = ${_count}")),
      body: ListView.builder(
        itemCount: 100,
        controller: _scrollController,
        itemBuilder: (context, index) {
          if (index == 0) return Container();

          if (index != 0 && index % (_itemsInList + 1) == 0) {
            return Container(
              height: _heightContainer,
              alignment: Alignment.center,
              color: Colors.blue[(index * 20) % 1000],
              child: Text("Image #${(index + 1) ~/ 5}"),
            );
          }

          return SizedBox(height: _heightListTile, child: ListTile(title: Text("Item ${index}")));
        },
      ),
    );
  }
}