Dart 2类的构造函数与其他语言的多态性非常相似 [英] Dart 2 class with a constructor very similar to the polymorphism of other languages

问题描述

我需要用 Dart 2 类来代表一张照片.照片可以是矩形或圆形.因此，通过多态性，我可以编写:

I need to represent a photo with a Dart 2 class. The photo can be rectangular or circular. So, with a polymorphism I could write:

import 'dart:math';

class Photo {
  double width;
  double height;
  double radius;
  double area;

  Photo(double width, double height) {
    this.width = width;
    this.height = height;
    this.area = width * height;
  }

  Photo(double radius) {
    this.radius = radius;
    this.area = pi * pow(radius, 2);
  }
}

因此，我可以创建具有半径的照片或具有宽度和高度的照片； 没有其他选择 .

So I could allow to create a Photo with radius or a Photo with width and height; no other option.

我该如何使用 Dart 2 ?

谢谢！

推荐答案

尝试一下

import 'dart:math';

class Photo {
  final double area;

  // This constructor is library-private. So no other code can extend
  // from this class.
  Photo._(this.area);

  // These factories aren't needed – but might be nice
  factory Photo.rect(double width, double height) => new RectPhoto(width, height);
  factory Photo.circle(double radius) => new CirclePhoto(radius);
}

class CirclePhoto extends Photo {
  final double radius;

  CirclePhoto(this.radius) : super._(pi * pow(radius, 2));
}

class RectPhoto extends Photo {
  final double width, height;

  RectPhoto(this.width, this.height): super._(width * height);
}

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