如何防止 FutureBuilder 重建，即使它的数据没有改变？

Question

I am currently developing a Flutter App which uses a lots of REST APIs to communicate with server side. I am facing some performance issues related to FutureBuilder widget implemented as below:

RefreshIndicator(
  displacement: 100.0,
  onRefresh: _refreshLastEntries,
  child: FutureBuilder<List<Entry>> (
    future: lastEntriesList,
    builder: (context, snapshot) {
      if(snapshot.hasError) print(snapshot.error);
      if(snapshot.data == null) return Container();
      else return ListView.builder(
        controller: _controller,
        itemCount: snapshot.data.length,
        itemBuilder: (context, index) {
          return Padding(
            padding: const EdgeInsets.all(10.0),
            child: EntryCard(entry: snapshot.data[index])
          );
        },
      );
    },
  ),
),

By the way, there are 2 more FutureBuilder implementations with RefreshIndicator in the same page build, and they are wrapped with a PageView. The Flutter Performance tab in Android Studio showing avg. fps of 30 frames/second (sometimes < 30) in debug mode. It is better in release mode but still below 60 fps that I can notice a laggy animation.

Well I know the problem (not sure though) is FutureBuilder trying to rebuild as long as the AsyncSnapshot has data. But I couldn't figure out a better implementation of this. The refresh function _refreshLastEntries() is implemented as:

Future<Null> _refreshLastEntries() async {
  setState(() {
     lastEntriesList = fetchLastEntries(2);
  });
  return Future.delayed(Duration(milliseconds: 600));
}

And the fetchLastEntries(int) is just for the getting the data and map it into some EntryModel class. I have tried a setting a global value after successfully getting the data and then use it for building the ListView but it did not work.

I am testing this app with a Xiaomi MI6 with Snapdragon 835 and Adreno 540 if that matters.

I will not attach any other code as the whole PageView build code since it is hard to reproduce the same build, thanks!

Answer 1

The FutureBuilder widget is not supposed to rebuild if the future status does not change. What probably happens in your case is that

setState(() {
   lastEntriesList = fetchLastEntries(2);
});

inside _refreshLastEntries() causes a rebuild with the uncompleted future which, upon completion, causes another rebuild through the FutureBuilder.

Since you're using a RefreshIndicator, my opinion is that you don't need a FutureBuilder. Do the asynchronous work inside _refreshLastEntries() and when complete, update a ListView.builder widget. To be clearer:

//Declare lastEntriesList as a list
List<Entry> lastEntriesList;

Future<void> _refreshLastEntries() async {
  return fetchLastEntries(2).then((lastEntries) {
    setState(() {
      lastEntriesList = lastEntries;
    });
  });
}

And modify your subtree as follows:

RefreshIndicator(
  displacement: 100.0,
  onRefresh: _refreshLastEntries,
  child: ListView.builder(
    controller: _controller,
    itemCount: lastEntriesList.length,
    itemBuilder: (context, index) {
      return Padding(
        padding: const EdgeInsets.all(10.0),
        child: EntryCard(entry: lastEntrieslist[index])
      );
    },
  );,
)