StreamBuilder 在构建时被多次调用

Question

我有一个导航视图上有多个索引的应用程序，当我第一次加载应用程序时，正如预期的那样，StreamBuilder 被调用了两次。但是，当我导航到另一个索引并使用流构建器返回构建时，似乎正在调用流构建器以重新下载其中的所有项目。如果此循环运行时间过长，我担心这会消耗我所有的 firebase 数据。切换页面时如何重组以阻止 StreamBuilder 被多次调用。请注意，我什至会在切换并返回时保存此索引的状态。

我的构建方法：

@override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
        stream: Firestore.instance
            .collection("posts/player/post")
            .orderBy("time", descending: true)
            .snapshots()
            .take(2),
        builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
          if (!snapshot.hasData) return const Text('Loading...');
          final int highLightCount = snapshot.data.documents.length;
          return new StreamBuilder<QuerySnapshot>(
              stream: Firestore.instance
                  .collection("posts/player/post")
                  .snapshots(),
              builder: (BuildContext context,
                  AsyncSnapshot<QuerySnapshot> snapshot) {
                if (!snapshot.hasData)
                  return new Text("There are no current posts");
                return new ListView(
                  physics: const AlwaysScrollableScrollPhysics(),
                  scrollDirection: Axis.vertical,
                  shrinkWrap: true,
                  children: getPostItems(snapshot),
                );
              });
        });
  }

当我切换它时，它正在调用几乎不间断地调用的 getPostItem(snaphot)，我如何防止它被调用并消耗我的数据，或者，除非实际更新，否则防止 StreamBuilder 发生变化？

其余重要代码：

getPostItems(AsyncSnapshot<QuerySnapshot> snapshot) {
    return snapshot.data.documents.map((doc) => getListItem(doc)).toList();
  }

  Widget getListItem(var doc) {
    getProfUrl(doc);
    getDownUrl(doc);

    print("item hit");

    print("user: " + doc["user"] + "- current: " + widget.auth.getUserId());
    if (doc["user"] != widget.auth.getUserId()) {
      print("will show");
      if (doc["type"] == "image") {
        return new Column(
          children: <Widget>[
            new ListTile(
                title: new Text(doc["title"]),
                subtitle: new Text(doc["description"].toString()),
                leading: new Container(
                    width: 44.0,
                    height: 44.0,
                    decoration: new BoxDecoration(
                      shape: BoxShape.circle,
                      image: new DecorationImage(
                          fit: BoxFit.fill, image: NetworkImage(profUrl)),
                    ))),
            new Padding(
              padding: EdgeInsets.fromLTRB(4, 4, 4, 4),
              child: new Center(
                child: new AspectRatio(
                  aspectRatio: 1 / 1,
                  child: new Container(
                    decoration: new BoxDecoration(
                        image: new DecorationImage(
                      fit: BoxFit.fill,
                      alignment: FractionalOffset.topCenter,
                      image: new NetworkImage(downUrl),
                    )),
                  ),
                ),
              ),
            ),
          ],
        );
      } else {
        VideoPlayerController _controller = VideoPlayerController.network(
            'http://www.sample-videos.com/video123/mp4/720/big_buck_bunny_720p_20mb.mp4')
          ..initialize().then((_) {
            // Ensure the first frame is shown after the video is initialized, even before the play button has been pressed.
            setState(() {});
          });

        return new Column(children: <Widget>[
          new ListTile(
              title: new Text(doc["title"]),
              subtitle: new Text(doc["description"].toString()),
              leading: new Container(
                  width: 44.0,
                  height: 44.0,
                  decoration: new BoxDecoration(
                    shape: BoxShape.circle,
                    image: new DecorationImage(
                        fit: BoxFit.fill, image: NetworkImage(profUrl)),
                  ))),
          new Padding(
            padding: EdgeInsets.fromLTRB(4, 4, 4, 4),
            child: new Center(
              child: new AspectRatio(
                aspectRatio: 500 / 500,
                child: VideoPlayer(_controller),
              ),
            ),
          ),
        ]);
      }
    }
  }

Answer 1

为了减少创建Stream 的次数，您可以将其初始化移到State 类的initState 方法中。

class MyPage extends StatefulWidget {
  @override
  _MyPageState createState() => _MyPageState();
}

class _MyPageState extends State<MyPage> {
  Stream<QuerySnapshot> _outerStream;
  Stream<QuerySnapshot> _innerStream;

  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
      stream: _outerStream,
      builder: (context, snapshot) {
        if (!snapshot.hasData) return const Text('Loading...');
        final int highLightCount = snapshot.data.documents.length;
        return StreamBuilder<QuerySnapshot>(
          stream: _innerStream,
          builder: (context, snapshot) {
            if (!snapshot.hasData) return Text('There are no current posts');
            return ListView(
              physics: const AlwaysScrollableScrollPhysics(),
              scrollDirection: Axis.vertical,
              shrinkWrap: true,
              children: getPostItems(snapshot),
            );
          },
        );
      },
    );
  }

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

    _outerStream = Firestore
        .instance
        .collection('posts/player/post')
        .orderBy('time', descending: true)
        .snapshots()
        .take(2);

    _innerStream = Firestore
        .instance
        .collection('posts/player/post')
        .snapshots();
  }
}

或者，如果您想在应用程序的生命周期内创建一次Stream，那么您可以使用StreamProvider 来提供Stream。

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        StreamProvider<QuerySnapshot>(
          create: (context) {
            return Firestore.instance.collection('posts/player/post').snapshots();
          },
        ),
      ],
      child: MaterialApp(
        home: MyPage(),
      ),
    );
  }
}

class MyPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final snapshot = context.watch<QuerySnapshot>();

    return Scaffold(
      body: snapshot == null
          ? Text('There are no current posts')
          : ListView(
              physics: const AlwaysScrollableScrollPhysics(),
              scrollDirection: Axis.vertical,
              shrinkWrap: true,
              children: getPostItems(snapshot),
            ),
    );
  }
}