Dart 2.0.0访问扩展了另一个的类的变量

Question

I have these classes (as @KevinMoore showed here ):

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);
}

My question is: if I create a Photo object in this way: Photo photo = new CirclePhoto(15.0, 10.0); , how can I get the radius from photo object? Can I make the radius variable private and get it with a getter?

Thank you.

Answer 1

You need a get method :

class Rectangle {
  num left, top, width, height;

  Rectangle(this.left, this.top, this.width, this.height);

  // Define two calculated properties: right and bottom.
  num get right => left + width;
  set right(num value) => left = value - width;
  num get bottom => top + height;
  set bottom(num value) => top = value - height;
}

void main() {
  var rect = Rectangle(3, 4, 20, 15);
  assert(rect.left == 3);
  rect.right = 12;
  assert(rect.left == -8);
}

Doc: https://www.dartlang.org/guides/language/language-tour

Answer 2

You just need to cast the value to a CirclePhoto to access the radius value. A Photo does not have a radius, so if you do:

Photo photo = new CirclePhoto(15.0);
print(photo.radius); // Compile-time error, Photo has no "radius"

you get an error, but if you do:

Photo photo = new CirclePhoto(15.0);
print((photo as CirclePhoto).radius);

it works.

This performs a down-cast from Photo to CirclePhoto . The static type system cannot tell that this is safe (some photos are not circle-photos), so it checks at run-time. If the photo isn't actually a CirclePhoto , you get a run-time error.

Another option is to use type-check-based type promotion:

Photo photo = new CirclePhoto(15.0);
if (photo is CirclePhoto) print(photo.radius);

This promotes the photo variable to be a CirclePhoto in the code guarded by the is -check. (Type promotion is fairly primitive, it basically needs to be a local variable that you are not assigning to, and the type you check must be a sub-type of the current type of the variable).

Making radius private and adding a getter makes no difference . You already have a getter names radius on CirclePhoto , the one introduced by your final field. There is no advantage in renaming the field to be private and adding another getter, it's pure overhead.