性能优化demo

Future<String> fetchData2(String input) async {
  await Future.delayed(const Duration(seconds: 5));
  return 'Hello, $input!';
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int index = 0;
  StreamController<int> _counterController = StreamController<int>();
  int _counter = 0;

  void _incrementCounter() {
    if (index == 0) {
      index = 1;
    } else {
      index = 0;
    }

    _counter++;
    _counterController.sink.add(_counter);
    setState(() {});
  }

  @override
  void dispose() {
    _counterController.close();
    super.dispose();
  }

  Future<String> fetchData() async {
    return Future.delayed(const Duration(seconds: 3), () => 'Hello, world!');
  }

  Future<List<String>> fetchData3() async {
    List<Future<String>> futures = [
      Future.delayed(const Duration(seconds: 1), () => 'Hello'),
      Future.delayed(const Duration(seconds: 2), () => 'World!!!'),
    ];
    List<String> results = await Future.wait(futures);
    return results;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          children: [
            IndexedStackWidget(
              index: index,
            ),
            // 异步
            FutureBuilder(
              future: fetchData(),
              builder: (context, snapshot) {
                if (snapshot.connectionState == ConnectionState.done) {
                  return Text(snapshot.data ?? '');
                } else {
                  return const CircularProgressIndicator();
                }
              },
            ),
            // isolate获取数据
            FutureBuilder(
              future: compute(fetchData2, 'world'),
              builder: (context, snapshot) {
                if (snapshot.connectionState == ConnectionState.done) {
                  return Text(snapshot.data ?? '');
                } else {
                  return const CircularProgressIndicator();
                }
              },
            ),
            // stream 流显示
            StreamBuilder<int>(
              stream: _counterController.stream,
              builder: (context, snapshot) {
                if (snapshot.hasData) {
                  return Text('Counter: ${snapshot.data}');
                } else {
                  return const CircularProgressIndicator();
                }
              },
            ),

            /// 多异步操作
            FutureBuilder(
              future: fetchData3(),
              builder: (context, snapshot) {
                if (snapshot.connectionState == ConnectionState.done) {
                  if (snapshot.hasData) {
                    return Text(snapshot.data?.join(' ') ?? '');
                  } else if (snapshot.hasError) {
                    return Text('Error: ${snapshot.error}');
                  } else {
                    return const Text('No data');
                  }
                } else {
                  return const CircularProgressIndicator();
                }
              },
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

class FlexDemo extends StatelessWidget {
  const FlexDemo({super.key});

  @override
  Widget build(BuildContext context) {
    return Flex(
      direction: Axis.horizontal,
      children: [
        Expanded(
          child: Container(
            height: 100,
            color: Colors.red,
          ),
        ),
        Expanded(
          child: Container(
            height: 100,
            color: Colors.blue,
          ),
        ),
      ],
    );
  }
}

class MyLayoutDelegate extends MultiChildLayoutDelegate {
  MyLayoutDelegate({this.position});

  final Offset? position;

  @override
  void performLayout(Size size) {
    // `size` is the size of the `CustomMultiChildLayout` itself.

    Size leadingSize = Size
        .zero; // If there is no widget with id `1`, the size will remain at zero.
    // Remember that `1` here can be any **id** - you specify them using LayoutId.
    if (hasChild(1)) {
      leadingSize = layoutChild(
        1, // The id once again.
        BoxConstraints.tightFor(
            height: size
                .height), // This just says that the child cannot be bigger than the whole layout.
      );
      // No need to position this child if we want to have it at Offset(0, 0).
    }

    if (hasChild(2)) {
      final secondSize = layoutChild(
        2,
        BoxConstraints.tightFor(width: size.width / 2),
      );

      positionChild(
        2,
        Offset(
          leadingSize.width, // This will place child 2 to the right of child 1.
          size.height / 2 -
              secondSize.height / 2, // Centers the second child vertically.
        ),
      );
    }
  }

  @override
  bool shouldRelayout(MyLayoutDelegate oldDelegate) =>
      oldDelegate.position != position;
}

class MultiChild extends StatelessWidget {
  const MultiChild({super.key});

  @override
  Widget build(BuildContext context) {
    return CustomMultiChildLayout(
      delegate: MyLayoutDelegate(),
      children: [
        LayoutId(
          id: 1, // The id can be anything, i.e. any Object, also an enum value.
          child: const Text(
              'Widget one'), // This is the widget you actually want to show.
        ),
        LayoutId(
          id: 2, // You will need to refer to that id when laying out your children.
          child: const Card(
            child: Padding(
              padding: EdgeInsets.all(8.0),
              child: Text('Widget two'),
            ),
          ),
        ),
      ],
    );
  }
}

class IndexedStackWidget extends StatelessWidget {
  final int index;
  const IndexedStackWidget({super.key, required this.index});

  @override
  Widget build(BuildContext context) {
    return IndexedStack(
      index: index,
      children: [
        Container(
          width: 100,
          height: 100,
          color: Colors.red,
        ),
        Container(
          width: 100,
          height: 100,
          color: Colors.blue,
        ),
      ],
    );
  }
}