dart - 如何在 Dart 构造函数中设置 final/const 属性

Question

我有一个具有不可变属性 int id 的类。
如何将 id 的值传递给构造函数？

class Hey
{
  var val;
  final int id;
  Hey(int id,var val)
  {
    this.id=id;
    this.val=val;
  }
}

void main()
{
  Hey hey=new Hey(0,1);
}

hey.dart:10:10: Error: Setter not found: 'id'. this.id=id; ^^ hey.dart:10:10: Error: The setter 'id' isn't defined for the class 'Hey'. - 'Hey' is from 'hey.dart'. Try correcting the name to the name of an existing setter, or defining a setter or field named 'id'. this.id=id; ^^

我认为 const 或 final 字段属性不需要 setter 。
API 不清楚如何处理这个问题。

Answer 1

来自 Dart language tour :

Note: Instance variables can be final but not const. Final instance variables must be initialized before the constructor body starts — at the variable declaration, by a constructor parameter, or in the constructor’s initializer list.

初始化列表部分说:

Besides invoking a superclass constructor, you can also initialize instance variables before the constructor body runs. Separate initializers with commas.

// Initializer list sets instance variables before
// the constructor body runs.
Point.fromJson(Map<String, num> json)
    : x = json['x'],
      y = json['y'] {
  print('In Point.fromJson(): ($x, $y)');
}

所以一般的方法是通过初始化列表。

如上所述，您还可以在变量声明时进行初始化:

class Foo {
  final x = 42;
}

或者可以通过构造函数参数初始化它们:

class Foo {
  final x;

  Foo(this.x);
}

尽管这些其他方法可能并不总是适用。