inheritance - 如何在 Dart 中扩展 Rectangle 类？

Question

我想创建我的自定义矩形类，它是从 Rectangle 类扩展而来的。 我有一个错误，即类 Rectangle 没有声明性构造函数“Rectangle”，我已经看到 Rectangle 类的源代码并且有常量构造函数。 有可能做到吗？我认为可能的解决方案是使用组合而不是继承。感谢您的回答。

    part of Plot;

    class PlotRectangle extends Rectangle{

      // Rectangle<int> rectangle; // as variant use composition
      String color = "red"; 

      PlotRectangle(int left, int top, int width, int height): super.Rectangle(left, top, width, height){

      }

      PlotRectangle.withColor(int left, int top, int width, int height, this.color): super.Rectangle(left, top, width, height){
        this.color = color;
      }

      void setColor(String color){
        this.color = color;
      }  
    }

Answer 1

尝试了一下，成功了

library x;

import 'dart:math';

class PlotRectangle<T extends num> extends Rectangle<T> {
  PlotRectangle(T left, T top, T width, T height) : super(left, top, width, height);

  String color = 'red';

  factory PlotRectangle.fromPoints(Point<T> a, Point<T> b) {
    T left = min(a.x, b.x);
    T width = max(a.x, b.x) - left;
    T top = min(a.y, b.y);
    T height = max(a.y, b.y) - top;
    return new PlotRectangle<T>(left, top, width, height);
  }

  PlotRectangle.withColor(T left, T top, T width, T height, this.color) : super(left, top, width, height);

  @override
  String toString() => '${super.toString()}, $color';
}

void main(List<String> args) {
  var r = new PlotRectangle(17, 50, 30, 28);
  print(r);
  var rc = new PlotRectangle.withColor(17, 50, 30, 28, 'blue');
  print(rc);
}