Dart 编码规范：`多态` 和 `控制反转`最佳实践

前言

我们通常都知道程序设计要依赖抽象，提高复用性，做到对扩展开放，对修改关闭。贯彻SOLID五大原则的最重要法宝就是抽象和继承。多态是一种手段，下面，通过简单 demo 介绍 flutter 开发中常用的最佳实践。

举栗子🌰

/// 不推荐，避免把逻辑放在公共底层处理
class TestWidget extends StatefulWidget {
  const TestWidget({Key? key}) : super(key: key);

  @override
  TestWidgetState createState() => TestWidgetState();
}

class TestWidgetState extends State<TestWidget> {
  dynamic param;
  Widget childWidget = Container();

  ///提供给外部调用
  void update(dynamic value) {
    setState(() {
      param = value;
      _updateBody();
    });
  }
  
  void _updateBody(){
    if(param == null){
      childWidget = Container();
      return;
    }
    switch(param.runtimeType){
      case A:
        childWidget = AWidget(a: param as A,);
        break;
      case B:
        childWidget = BWidget(b: param as B,);
        break;
      default:
        childWidget = Container();
        break;
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: childWidget,
    );
  }
}

如上的写法是不推荐的，应该进行依赖倒置，将可变的部分放上层处理，保证底层干净，如下：

• 方式一：通过 传参构造器 进行控制反转

typedef ChildBuilder = Widget Function(dynamic param);
class TestWidget extends StatefulWidget {
  final ChildBuilder builder;

  const TestWidget({Key? key, required this.builder}) : super(key: key);

  @override
  TestWidgetState createState() => TestWidgetState();
}

class TestWidgetState extends State<TestWidget> {
  dynamic param;

  void update(dynamic value) {
    setState(() {
      param = value;
    });
  }
  
  @override
  Widget build(BuildContext context) {
    return Container(
      child: widget.builder(param),
    );
  }
}


///外部使用 TestWidget
Widget _builder(dynamic param){
  if (param == null) {
    return Container();
  }
  switch (param.runtimeType) {
    case A:
      return AWidget(
        a: param as A,
      );
    case B:
      return BWidget(
        b: param as B,
      );
    default:
      return Container();
  }
}

return TestWidget(builder: _builder);

• 方式二：通过 继承 + 泛型 进行解耦

/// 将 TestWidget 改成抽象类并指定泛型
abstract class TestWidget<T> extends StatefulWidget {
  const TestWidget({Key? key}) : super(key: key);

  Widget childBuilder(T param);

  @override
  TestWidgetState<T> createState() => TestWidgetState<T>();
}

class TestWidgetState<T> extends State<TestWidget> {
  T? param;
  Widget childWidget = Container();

  void update(T value) {
    setState(() {
      param = value;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: widget.childBuilder(param),
    );
  }
}

/// 实例A
class ATestWidget extends TestWidget<A> {
  const ATestWidget({Key? key}) : super(key: key);

  @override
  Widget childBuilder(A param) {
    return AWidget(a: param);
  }
}

/// 实例B
class BTestWidget extends TestWidget<B> {
  const BTestWidget({Key? key}) : super(key: key);

  @override
  Widget childBuilder(B param) {
    return BWidget(b: param);
  }
}