Flutter StreamBuilder在初始化时称为两次

StreamBuilder是否总是被调用两次？一次用于初始数据，然后一次用于输入流？

初始化以下StreamBuilder，显示build方法被调用两次。第二个通话是在第一个通话后的0.4秒。

流:内部版本1566239814897

流:内部版本1566239815284

import 'dart:async';
import 'dart:ui';

import 'package:flutter/material.dart';
import 'package:nocd/utils/bloc_provider.dart';

void main() =>
    runApp(BlocProvider<MyAppBloc>(bloc: MyAppBloc(), child: MyApp()));

class MyAppBloc extends BlocBase {
  String _page = window.defaultRouteName ?? "";

  /// Stream for [getPage].
  StreamController<String> pageController = StreamController<String>();

  /// Observable navigation route value.
  Stream get getPage => pageController.stream;

  MyAppBloc() {}

  @override
  void dispose() {
    pageController.close();
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final MyAppBloc myAppBloc = BlocProvider.of<MyAppBloc>(context);
    return StreamBuilder(
      stream: myAppBloc.getPage,
      initialData: "Build",
      builder: (context, snapshot) {
        print("Stream: " +
            snapshot.data +
            DateTime.now().millisecondsSinceEpoch.toString());
        return Container();
      },
    );
  }
}

为什么StreamBuilder被调用两次？

最佳答案

Streambuilder将被调用2次，第一次是Initial，第二次是Stream。并且仅当状态为 ConnectionState.active时才更改数据。
请参见官方文档示例。

    StreamBuilder<int>(
  //stream:fire, // a Stream<int> or null
  builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
    if (snapshot.hasError) return Text('Error: ${snapshot.error}');
    switch (snapshot.connectionState) {
      case ConnectionState.none:
        return Text('Select lot');
      case ConnectionState.waiting:
        return Text('Awaiting bids...');
      case ConnectionState.active:
        return Text('\$${snapshot.data}');
      case ConnectionState.done:
        return Text('\$${snapshot.data} (closed)');
    }
    return null; // unreachable
  },
);

StreamBuilder documentation



initialData

关于Flutter StreamBuilder在初始化时称为两次，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/57562407/