构造函数如何使用复杂算法初始化最终字段？ [英] How constructors can initialize final fields with complex algorithm?

问题描述

我写了一个不可变的类Vector3。

  class Vector3 
 {
 final num x， y，z; 
 Vector3（this.x，this.y，this.z）; 
 num get length => ...一些重的代码
} 
  

现在，我想写一个构造函数，来自另一向量的单位向量。我想根据Dart的建议（避免在我们创建一个新的对象时写静态方法）。

我的问题是，我不能写这个构造函数因为最终字段必须在构造函数的body之前初始化，并且不能在里面初始化。我可以写如下：

  Vector3.unit（Vector3 vector）
：x = vector.x / vector。 length，
y = vector.y / vector.length，
z = vector.z / vector.length; 
  

但是这很难过，我必须计算三倍的矢量长度...

我应该怎么做呢？我应该最终写一个静态方法来计算单位向量？ （这将是一个耻辱）

最后，是不是可能在最终字段的初始化的构造函数体中写任何东西？

谢谢！

解决方案

使用工厂构造函数：

  class Vector3 
 {
 final num x，y，z; 
 Vector3（this.x，this.y，this.z）; 
 
 factory Vector3.unit（Vector3 vector）{
 var length = vector.length; 
 return new Vector3（vector.x / length，vector.y / length，vector.z / length）; 
} 
 
 num get length => ...一些重代码
} 
  

I am writing an immutable class Vector3.

class Vector3
{
  final num x, y, z;
  Vector3(this.x, this.y, this.z);
  num get length => ...some heavy code
}

Now, I want to write a constructor that computes the unit vector from another vector. I want to do that according to the Dart's recommendations (avoid writing a static method when we create a new object).

My problem is that I can't write this constructor because the final fields must be initialized before the constructor's body and cannot be initialized inside. I can write something like :

Vector3.unit(Vector3 vector)
  : x = vector.x / vector.length,
    y = vector.y / vector.length,
    z = vector.z / vector.length;

But this is so sad I have to compute three times the length of the vector...

How am I supposed to do that? Should I finally write a static method to compute the unit vector? (It would be a shame)

And, at last, isn't it possible to write anything in the constructor's body related to the initialization of the final fields? And why?

Thanks!

解决方案

Use a factory constructor:

class Vector3
{
  final num x, y, z;
  Vector3(this.x, this.y, this.z);

  factory Vector3.unit(Vector3 vector) {
    var length = vector.length;
    return new Vector3(vector.x/length, vector.y/length, vector.z/length);
  }

  num get length => ...some heavy code
}

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