Cómo mejorar el rendimiento de su aplicación Flutter

Hay muchas preguntas, soluciones y conceptos erróneos sobre cómo podemos mejorar el rendimiento de nuestra aplicación Flutter. Debe aclarar de inmediato que Flutter es potente de forma predeterminada, pero debemos evitar algunos errores al escribir código para que la aplicación siempre funcione bien y rápidamente. A continuación, he preparado una serie de consejos y trucos sobre cómo escribir para que no tenga que recurrir constantemente a las herramientas de creación de perfiles.

Contenido:

• No coloque widgets en métodos de clase.
• Evite reconstruir todos los widgets
• Use const
• Use itemExtent en ListView para listas grandes.
• Evite reconstrucciones innecesarias de widgets dentro de AnimatedBuilder
• No use el widget de opacidad en animaciones
• Conclusión

No coloque widgets en métodos de clase.

, , , .

, , "" (footer).

class MyHomePage extends StatelessWidget {
  Widget _buildHeaderWidget() {
    final size = 40.0;
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: CircleAvatar(
        backgroundColor: Colors.grey[700],
        child: FlutterLogo(
          size: size,
        ),
        radius: size,
      ),
    );
  }

  Widget _buildMainWidget(BuildContext context) {
    return Expanded(
      child: Container(
        color: Colors.grey[700],
        child: Center(
          child: Text(
            'Hello Flutter',
            style: Theme.of(context).textTheme.display1,
          ),
        ),
      ),
    );
  }

  Widget _buildFooterWidget() {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: Text('This is the footer '),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Padding(
        padding: const EdgeInsets.all(15.0),
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            _buildHeaderWidget(),
            _buildMainWidget(context),
            _buildFooterWidget(),
          ],
        ),
      ),
    );
  }
}

, , – . ? , , MyHomePage , , , , .

, StatelessWidget .

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Padding(
        padding: const EdgeInsets.all(15.0),
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            HeaderWidget(),
            MainWidget(),
            FooterWidget(),
          ],
        ),
      ),
    );
  }
}

class HeaderWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final size = 40.0;
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: CircleAvatar(
        backgroundColor: Colors.grey[700],
        child: FlutterLogo(
          size: size,
        ),
        radius: size,
      ),
    );
  }
}

class MainWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Expanded(
      child: Container(
        color: Colors.grey[700],
        child: Center(
          child: Text(
            'Hello Flutter',
            style: Theme.of(context).textTheme.display1,
          ),
        ),
      ),
    );
  }
}

class FooterWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: Text('This is the footer '),
    );
  }
}

Stateful/Stateless "", , , . , . ( )

const . , .

, , Flutter StatefulWidget setState.

— , FloatingActionButton , . , . , print build , , .

class _MyHomePageState extends State<MyHomePage> {
  Color _currentColor = Colors.grey;

  Random _random = new Random();

  void _onPressed() {
    int randomNumber = _random.nextInt(30);
    setState(() {
      _currentColor = Colors.primaries[randomNumber % Colors.primaries.length];
    });
  }

  @override
  Widget build(BuildContext context) {
    print('building `MyHomePage`');
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: BackgroundWidget(),
          ),
          Center(
            child: Container(
              height: 150,
              width: 150,
              color: _currentColor,
            ),
          ),
        ],
      ),
    );
  }
}

class BackgroundWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('building `BackgroundWidget`');
    return Image.network(
      'https://cdn.pixabay.com/photo/2017/08/30/01/05/milky-way-2695569_960_720.jpg',
      fit: BoxFit.cover,
    );
  }
}

flutter: building `MyHomePage`
flutter: building `BackgroundWidget`

, , : Scaffold, BackgroundWidget , , , , – -Container.

, – . , . , : flutter_bloc, mobx, provider . . , , Flutter , - .

, , ValueNotifier.

class _MyHomePageState extends State<MyHomePage> {
  final _colorNotifier = ValueNotifier<Color>(Colors.grey);
  Random _random = new Random();

  void _onPressed() {
    int randomNumber = _random.nextInt(30);
    _colorNotifier.value =
        Colors.primaries[randomNumber % Colors.primaries.length];
  }

  @override
  void dispose() {
    _colorNotifier.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    print('building `MyHomePage`');
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: BackgroundWidget(),
          ),
          Center(
            child: ValueListenableBuilder(
              valueListenable: _colorNotifier,
              builder: (_, value, __) => Container(
                height: 150,
                width: 150,
                color: value,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

… . , , , (. ValueListenableBuilder ValueNotifier).

, , - (, , ) ( ).

, ChangeNotifier.

//------ ChangeNotifier class ----//
class MyColorNotifier extends ChangeNotifier {
  Color myColor = Colors.grey;
  Random _random = new Random();

  void changeColor() {
    int randomNumber = _random.nextInt(30);
    myColor = Colors.primaries[randomNumber % Colors.primaries.length];
    notifyListeners();
  }
}
//------ State class ----//

class _MyHomePageState extends State<MyHomePage> {
  final _colorNotifier = MyColorNotifier();

  void _onPressed() {
    _colorNotifier.changeColor();
  }

  @override
  void dispose() {
    _colorNotifier.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    print('building `MyHomePage`');
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: BackgroundWidget(),
          ),
          Center(
            child: AnimatedBuilder(
              animation: _colorNotifier,
              builder: (_, __) => Container(
                height: 150,
                width: 150,
                color: _colorNotifier.myColor,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

, .

const

const , , ( const, ), , (. const).

setState, , 1 .

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _onPressed() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    print('building `MyHomePage`');
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: BackgroundWidget(),
          ),
          Center(
              child: Text(
            _counter.toString(),
            style: Theme.of(context).textTheme.display4.apply(
                  color: Colors.white,
                  fontWeightDelta: 2,
                ),
          )),
        ],
      ),
    );
  }
}

class BackgroundWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('building `BackgroundWidget`');
    return Image.network(
      'https://cdn.pixabay.com/photo/2017/08/30/01/05/milky-way-2695569_960_720.jpg',
      fit: BoxFit.cover,
    );
  }
}

2 , , – BackgroundWidget. , , , , .

flutter: building `MyHomePage`
flutter: building `BackgroundWidget`

const BackgroundWidget:

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _onPressed() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    print('building `MyHomePage`');
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        child: Icon(Icons.colorize),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: const BackgroundWidget(),
          ),
          Center(
              child: Text(
            _counter.toString(),
            style: Theme.of(context).textTheme.display4.apply(
                  color: Colors.white,
                  fontWeightDelta: 2,
                ),
          )),
        ],
      ),
    );
  }
}

class BackgroundWidget extends StatelessWidget {
  const BackgroundWidget();

  @override
  Widget build(BuildContext context) {
    print('building `BackgroundWidget`');
    return Image.network(
      'https://cdn.pixabay.com/photo/2017/08/30/01/05/milky-way-2695569_960_720.jpg',
      fit: BoxFit.cover,
    );
  }
}

(, , , ) , const.

itemExtent ListView

, , , , , itemExtent.

. 10 . . itemExtent .

class MyHomePage extends StatelessWidget {
  final widgets = List.generate(
    10000,
    (index) => Container(
      height: 200.0,
      color: Colors.primaries[index % Colors.primaries.length],
      child: ListTile(
        title: Text('Index: $index'),
      ),
    ),
  );

  final _scrollController = ScrollController();

  void _onPressed() async {
    _scrollController.jumpTo(
      _scrollController.position.maxScrollExtent,
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: ListView(
        controller: _scrollController,
        children: widgets,
      ),
    );
  }
}

, (~10 ). - , . UI!

, itemExtent, , .

class MyHomePage extends StatelessWidget {
  final widgets = List.generate(
    10000,
    (index) => Container(
      color: Colors.primaries[index % Colors.primaries.length],
      child: ListTile(
        title: Text('Index: $index'),
      ),
    ),
  );

  final _scrollController = ScrollController();

  void _onPressed() async {
    _scrollController.jumpTo(
      _scrollController.position.maxScrollExtent,
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: ListView(
        controller: _scrollController,
        children: widgets,
        itemExtent: 200,
      ),
    );
  }
}

- .

AnimatedBuilder

, . (addListener) AnimationController setState. , , . AnimatedBuilder , .

, , 360 . CounterWidget, , , .

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  int counter = 0;

  void _onPressed() {
    setState(() {
      counter++;
    });
    _controller.forward(from: 0.0);
  }

  @override
  void initState() {
    _controller = AnimationController(
        vsync: this, duration: const Duration(milliseconds: 600));
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: AnimatedBuilder(
        animation: _controller,
        builder: (_, child) => Transform(
          alignment: Alignment.center,
          transform: Matrix4.identity()
            ..setEntry(3, 2, 0.001)
            ..rotateY(360 * _controller.value * (pi / 180.0)),
          child: CounterWidget(
            counter: counter,
          ),
        ),
      ),
    );
  }
}

class CounterWidget extends StatelessWidget {
  final int counter;

  const CounterWidget({Key key, this.counter}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    print('building `CounterWidget`');
    return Center(
      child: Text(
        counter.toString(),
        style: Theme.of(context).textTheme.display4.apply(fontWeightDelta: 3),
      ),
    );
  }
}

.

flutter: building `CounterWidget`
flutter: building `CounterWidget`
flutter: building `CounterWidget`
flutter: building `CounterWidget`
flutter: building `CounterWidget`
...
flutter: building `CounterWidget`

, 1, , , , setState. CounterWidget?

! AnimatedBuilder child, . , , , , , Transform .

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: AnimatedBuilder(
        animation: _controller,
        child: CounterWidget(
          counter: counter,
        ),
        builder: (_, child) => Transform(
          alignment: Alignment.center,
          transform: Matrix4.identity()
            ..setEntry(3, 2, 0.001)
            ..rotateY(360 * _controller.value * (pi / 180.0)),
          child: child,
        ),
      ),
    );
  }

, , , , . .

Opacity

.

- , Opacity. , Transform, Opacity, .

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  int counter = 0;

  void _onPressed() {
    setState(() {
      counter++;
    });
    _controller.forward(from: 0.0);
  }

  @override
  void initState() {
    _controller = AnimationController(
        vsync: this, duration: const Duration(milliseconds: 600));
    _controller.addStatusListener((status) {
      if (status == AnimationStatus.completed) {
        _controller.reverse();
      }
    });
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: AnimatedBuilder(
        animation: _controller,
        child: CounterWidget(
          counter: counter,
        ),
        builder: (_, child) => Opacity(
          opacity: (1 - _controller.value),
          child: child,
        ),
      ),
    );
  }
}

, , , Opacity (, , ) , .

:

1 – FadeTransition

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  int counter = 0;

  void _onPressed() {
    setState(() {
      counter++;
    });
    _controller.forward(from: 0.0);
  }

  @override
  void initState() {
    _controller = AnimationController(
        vsync: this, duration: const Duration(milliseconds: 600));
    _controller.addStatusListener((status) {
      if (status == AnimationStatus.completed) {
        _controller.reverse();
      }
    });
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: FadeTransition(
        opacity: Tween(begin: 1.0, end: 0.0).animate(_controller),
        child: CounterWidget(
          counter: counter,
        ),
      ),
    );
  }
}

2 – AnimatedOpacity

const duration = const Duration(milliseconds: 600);

class _MyHomePageState extends State<MyHomePage> {
  int counter = 0;
  double opacity = 1.0;

  void _onPressed() async {
    counter++;
    setState(() {
      opacity = 0.0;
    });
    await Future.delayed(duration);
    setState(() {
      opacity = 1.0;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _onPressed,
        splashColor: Colors.red,
        child: Icon(Icons.slow_motion_video),
      ),
      body: AnimatedOpacity(
        opacity: opacity,
        duration: duration,
        child: CounterWidget(
          counter: counter,
        ),
      ),
    );
  }
}

, Opacity.

, Flutter , . , .

, - Filip Hráček Flutter Europe, Flutter .

:

• https://flutter.dev/docs/perf/rendering/best-practices
• https://api.flutter.dev/flutter/widgets/StatefulWidget-class.html#performance-considerations
• https://api.flutter.dev/flutter/widgets/Opacity-class.html#transparent-image

:

Diego Velásquez –  Flutter Dart GDE , . Flutter. Diego . Twitter.

PD Una vez más, un enlace al artículo original Cómo mejorar el rendimiento de su aplicación Flutter por Diego Velásquez