设计模式二十桥接模式：边学边做个生成电子书的工具（python+dart）

意图：将抽象部分与实现部分分离，使它们都可以独立的变化。

主要解决：在有多种可能会变化的情况下，用继承会造成类爆炸问题，扩展起来不灵活。

何时使用：实现系统可能有多个角度分类，每一种角度都可能变化。

如何解决：把这种多角度分类分离出来，让它们独立变化，减少它们之间耦合。

python实现

from abc import  ABCMeta,abstractmethod

class Shape(metaclass=ABCMeta):
    def __init__(self,color):
        self.color = color
    @abstractmethod
    def draw(self):
        pass

class Color(metaclass=ABCMeta):
    @abstractmethod
    def paint(self,shape):
        pass


class Rectangle(Shape):
    name = "长方形"
    def draw(self):
        self.color.paint(self)

class Circle(Shape):
    name = "圆形"
    def draw(self):
        self.color.paint(self)

class Red(Color):
    def paint(self,shape):
        print(f"红色的{shape.name}")


class Green(Color):
    def paint(self,shape):
        print(f"绿色的{shape.name}")


obj = Rectangle(Green())
obj.draw()

dart 实现

abstract class DrawAPI {
  void drawCircle(int radius, int x, int y);
}

class RedCircle implements DrawAPI {
  @override
  void drawCircle(int radius, int x, int y) {
    print("Drawing Circle[ color: red, radius: $radius"
        ", x: $x, y:$y]");
  }
}

class GreenCircle implements DrawAPI {
  @override
  void drawCircle(int radius, int x, int y) {
    print("Drawing Circle[ color: green, radius: $radius"
        ", x: $x, y:$y]");
  }
}

abstract class Shape {
  DrawAPI _drawAPI;
  Shape(this._drawAPI);
  void draw();
}

class Circle extends Shape {
  late int _x, _y, _radius;

  Circle(this._x, this._y, this._radius, DrawAPI drawAPI) : super(drawAPI);

  @override
  void draw() {
    _drawAPI.drawCircle(_radius, _x, _y);
  }
}

void main(List<String> args) {
  Shape redCircle = Circle(100, 100, 10, RedCircle());
  Shape greenCircle = Circle(100, 100, 10, GreenCircle());
  redCircle.draw();
  greenCircle.draw();
}