从StatefulWidget对象更改状态

Question

像标题状态一样，如何从StatefulWidget访问StatefulWidget的状态。

背景： 我有一个星级评分小部件，该部件连续包含5个“ StarWidget”。StarWidget类只是一个包裹有检测器的Icon（不使用IconButton，因为它的尺寸很大）。StarWidget将其是否被选中存储在相应的State对象中，并相应地显示一个实心或轮廓图标。

在我的主窗口小部件中，我可以访问StatefulWidget对象，并希望配置它们的状态。

import 'package:flutter/material.dart';

import 'package:font_awesome_flutter/font_awesome_flutter.dart';

class StarRatingWidget extends StatefulWidget {
  @override
  _StarRatingWidgetState createState() {
    return _StarRatingWidgetState();
  }
}

class _StarRatingWidgetState extends State<StarRatingWidget>
    implements StarSelectionInterface {
  //Properties
  int _currentRating = 0;
  List<RatingStarWidget> starWidgets = [];

  //Methods
  @override
  void initState() {
    super.initState();
    starWidgets.add(
      RatingStarWidget(
        starSelectionInterface: this,
        starPosition: 0,
      ),
    );
    starWidgets.add(
      RatingStarWidget(
        starSelectionInterface: this,
        starPosition: 1,
      ),
    );
    starWidgets.add(
      RatingStarWidget(
        starSelectionInterface: this,
        starPosition: 2,
      ),
    );
    starWidgets.add(
      RatingStarWidget(
        starSelectionInterface: this,
        starPosition: 3,
      ),
    );
    starWidgets.add(
      RatingStarWidget(
        starSelectionInterface: this,
        starPosition: 4,
      ),
    );
  }

  @override
  Widget build(BuildContext buildContext) {
    return Row(
      children: starWidgets,
    );
  }

  //Star Selection Interface Methods
  void onStarSelected(_RatingStarWidgetState starWidgetState) {
    print("listener: star selected ${starWidgetState._starPosition}");

    //a new, rating has been selected, update rating
    if (_currentRating != starWidgetState._starPosition) {
      _currentRating = (starWidgetState._starPosition + 1);
    }

    //same star as rating has been selected, set rating to 0
    else {
      _currentRating = 0;
    }

    //update stars according to rating
    for(int i = 1; i <= 5; i++) {
      //what should I do here?!
    }
  }
}

class RatingStarWidget extends StatefulWidget {
  //Properties
  final int starPosition;
  final StarSelectionInterface starSelectionInterface;

  //Constructors
  RatingStarWidget({this.starSelectionInterface, this.starPosition});

  //Methods
  @override
  _RatingStarWidgetState createState() {
    return _RatingStarWidgetState(starSelectionInterface, starPosition);
  }
}

class _RatingStarWidgetState extends State<RatingStarWidget> {
  //Properties
  int _starPosition;
  bool _isSelected = false;
  StarSelectionInterface selectionListener;

  //Constructors
  _RatingStarWidgetState(this.selectionListener, this._starPosition);

  //Methods
  @override
  Widget build(BuildContext buildContext) {
    return AnimatedCrossFade(
      firstChild: GestureDetector(
        child: Icon(
          FontAwesomeIcons.star,
          size: 14,
        ),
        onTap: () {
          print("star: selected");
          selectionListener.onStarSelected(this);
        },
      ),
      secondChild: GestureDetector(
        child: Icon(
          FontAwesomeIcons.solidStar,
          size: 14,
        ),
        onTap: () {
          selectionListener.onStarSelected(this);
        },
      ),
      duration: Duration(milliseconds: 300),
      crossFadeState:
          _isSelected ? CrossFadeState.showSecond : CrossFadeState.showFirst,
    );
  }
}

class StarSelectionInterface {
  void onStarSelected(_RatingStarWidgetState starWidgetState) {}
}

Answer 1

Flutter 的方法是在需要时重建小部件。不要害怕构建小部件，它们对于 SDK 来说很便宜，特别是在这种情况下对于简单的星星。

访问另一个小部件状态需要的工作不仅仅是重建它。要访问状态，您应该使用键，或者您应该在小部件本身中添加特殊方法。

在这种情况下，无论如何重建星形，使用普通的无状态小部件会更好更简单，因为在重建的时候父级可以提供选定的状态。

而且由于状态存储在父小部件中，我认为最好不要将其存储为每个星星中的墙。

接下来是一个遵循该想法的非常简单的解决方案。但是，是的，它仍然重建星星。

import 'package:flutter/material.dart';

import 'package:font_awesome_flutter/font_awesome_flutter.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(body: Center(child: StarRatingWidget())),
    );
  }
}

class StarRatingWidget extends StatefulWidget {
  @override
  _StarRatingWidgetState createState() {
    return _StarRatingWidgetState();
  }
}

class _StarRatingWidgetState extends State<StarRatingWidget> {
  int _currentRating = 0;

  List<Widget> buildStars() {
    List<RatingStarWidget> starWidgets = [];
    for (int i = 0; i < 5; i++) {
      starWidgets.add(
        RatingStarWidget(
          clickCallback: () => setState(() {
                _currentRating = i + 1;
              }),
          highlighted: _currentRating > i,
        ),
      );
    }
    return starWidgets;
  }

  @override
  Widget build(BuildContext buildContext) {
    return Row(
      children: buildStars(),
    );
  }
}

class RatingStarWidget extends StatelessWidget {
  //Properties
  final VoidCallback clickCallback;
  final bool highlighted;

  //Constructors
  RatingStarWidget({this.clickCallback, this.highlighted});

  @override
  StatelessElement createElement() {
    print("Element created");
    return super.createElement();
  }

  //Methods
  @override
  Widget build(BuildContext buildContext) {
    return GestureDetector(
      onTap: () {
        clickCallback();
      },
      child: AnimatedCrossFade(
        firstChild: Icon(
          FontAwesomeIcons.star,
          size: 14,
        ),
        secondChild: Icon(
          FontAwesomeIcons.solidStar,
          size: 14,
        ),
        duration: Duration(milliseconds: 300),
        crossFadeState:
            highlighted ? CrossFadeState.showSecond : CrossFadeState.showFirst,
      ),
    );
  }
}