dart 中 没有 struct, 如何解决引用问题...

在 c# 中, struct 是值类型. 但是在 dart 中, 没有东西对标 struct.

以下片断是C# 代码, 可以看到 directionVector 这个变量被初始为 Vector2.zero , 然后又在 while 循环里对这个变量进行修改, 很正常的操作, 因为 Vector2 是 struct , 所以 directionVector 的初始值永远都是 (0,0)

internal bool TryConvertNamedDirectionToAngle(string token, out double angle)
{
    var reader = new CssReader(token, ' ');

    if (reader.CanRead && reader.Read() == "to")
    {
        var defaultVector = Vector2.Down;   // By default gradient is drawn top-down
        var directionVector = Vector2.Zero;

        reader.MoveNext();

        while (reader.CanRead)
        {
            directionVector.SetNamedDirection(reader.Read());
            reader.MoveNext();
        }

...
...
internal struct Vector2
{
    public static Vector2 Zero { get; } = new Vector2(0d, 0d);
    public static Vector2 Left { get; } = new Vector2(-1d, 0d);
    public static Vector2 Right { get; } = new Vector2(1d, 0d);
    public static Vector2 Up { get; } = new Vector2(0d, -1d);
    public static Vector2 Down { get; } = new Vector2(0d, 1d);

    public double X { get; private set; }
    public double Y { get; private set; }

    private Vector2(double x, double y)
    {
        X = x;
        Y = y;
    }

    public void SetNamedDirection(string direction)
    {
        switch (direction)
        {
            case "left":
                X = -1;
                break;
            case "right":
                X = 1;
                break;
            case "top":
                Y = -1;
                break;
            case "bottom":
                Y = 1;
                break;
            case "center":
                X = 0;
                Y = 0;
                break;
            default:
                throw new ArgumentOutOfRangeException($"Unrecognized direction: '{direction}'");
        }
    }

...

但是 dart 中没有 struct 对标的东西, 所以只用 class 来写上面的代码:

class Vector {
  ///
  static Vector zero = Vector(0, 0);

  ///
  static Vector left = Vector(-1, 0);

  ///
  static Vector right = Vector(1, 0);

  ///
  static Vector up = Vector(0, -1);

  ///
  static Vector down = Vector(0, 1);

  ///
  double x;

  ///
  double y;

  ///
  Vector(this.x, this.y);

  ///
  void setNamedDirection(String direction) {
    switch (direction) {
      case "left":
        this.x = -1;
        break;
      case "right":
        this.x = 1;
        break;
      case "top":
        this.y = -1;
        break;
      case "bottom":
        this.y = 1;
        break;
      case "center":
        this.x = 0;
        this.y = 0;
        break;
      default:
        throw new Exception("Unrecognized direction: '$direction'");
    }
  }

这样写的问题很明显, 因为 class 是地址引用

final directionVector = Vector.zero;

对 directionVector 的任何修改, 都会直接修改 Vector.zero 的值 .

如果尝试用 const 构造函数来修改这个 Vector 类, 就会发现那根本就行不通, 因为 const 构造函数需要用 final 来修饰属性. final 的意思就是: 值不能被修改. 但是这里的场景需要对属性做更改, 所以不能使用 const .

我也尝试添加一个 copy 方法:

final directionVector = Vector.zero.copy();
...
  Vector copy(){
    return new Vector(this.x, this.y);
  }

但是这种写法治标不治本, 保不准哪里无意就把 Vector.zero 给修改了.

貌似无解, 那只能祭出 get 了:

class Vector {
  static Vector get zero => Vector(0, 0);
  static Vector get left => Vector(-1, 0);
  static Vector get right => Vector(1, 0);
  static Vector get up => Vector(0, -1);
  static Vector get down => Vector(0, 1);
...

虽然每次请求 Vector.zero 都会重新实例一个 Vector 对象, 但是解决了问题...