目錄
description bug_report

Methods

目錄

Methods are functions that provide behavior for an object.

Instance methods

Instance methods on objects can access instance variables and this. The distanceTo() method in the following sample is an example of an instance method:

import 'dart:math';

class Point {
  final double x;
  final double y;

  Point(this.x, this.y);

  double distanceTo(Point other) {
    var dx = x - other.x;
    var dy = y - other.y;
    return sqrt(dx * dx + dy * dy);
  }
}

Operators

Operators are instance methods with special names. Dart allows you to define operators with the following names:

< + | >>>
> / ^ []
<= ~/ & []=
>= * << ~
- % >> ==

An operator declaration is identified using the built-in identifier operator. The following example defines vector addition (+), subtraction (-), and equality (==):

class Vector {
  final int x, y;

  Vector(this.x, this.y);

  Vector operator +(Vector v) => Vector(x + v.x, y + v.y);
  Vector operator -(Vector v) => Vector(x - v.x, y - v.y);

  @override
  bool operator ==(Object other) =>
      other is Vector && x == other.x && y == other.y;

  @override
  int get hashCode => Object.hash(x, y);
}

void main() {
  final v = Vector(2, 3);
  final w = Vector(2, 2);

  assert(v + w == Vector(4, 5));
  assert(v - w == Vector(0, 1));
}

Getters and setters

Getters and setters are special methods that provide read and write access to an object’s properties. Recall that each instance variable has an implicit getter, plus a setter if appropriate. You can create additional properties by implementing getters and setters, using the get and set keywords:

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

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

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

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

With getters and setters, you can start with instance variables, later wrapping them with methods, all without changing client code.

Abstract methods

Instance, getter, and setter methods can be abstract, defining an interface but leaving its implementation up to other classes. Abstract methods can only exist in abstract classes or mixins.

To make a method abstract, use a semicolon (;) instead of a method body:

abstract class Doer {
  // Define instance variables and methods...

  void doSomething(); // Define an abstract method.
}

class EffectiveDoer extends Doer {
  void doSomething() {
    // Provide an implementation, so the method is not abstract here...
  }
}