flyweight pattern

实例的属性可分为两种：

  • 内在状态intrinsic state：可以被共享的状态
  • 外在状态extrinsic state：不可被共享的状态，经常被外部实例更改

享元模式的本质是，抽取公共属性作为单独的类，以静态形式存储。每个实例都拥有这个公共属性的指针。也即“共享”一部分“元数据”。

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class TreeType {
  std::string name_;
  std::string color_;
  std::string texture_;
  public:
    TreeType(std::string name, std::string color, Texture texture): name_(name), color_(color), texture_(texture) {}
    void draw(const Canvas& c, int x, int y) {
      // draw with the intrinsic color & texture + extrinsic xy coord
    }
}

class TreeFactory {
  public:
    static std::unordered_map<std::string, std::shared_ptr<TreeType>> treeTypes_;
    std::shared_ptr<TreeType> getTreeType(const std::string name, static const std::string color, const std::string texture) {
      auto it = treeTypes_.find(name);
      if (it != treeTypes_.end()) return it->second;
      auto t = std::make_shared<TreeType>(name, color, texture);
      treeTypes_[name] = t;
      return t;
    }
}

std::unordered_map<std::string, std::shared_ptr<TreeType>> TreeFactory::treeTypes_;

class Tree {
  int x_;
  int y_;
  std::shared_ptr<TreeType> type_; // 享元模式的核心。type_在相同种类的树之间是共享的。此处必须使用指针，而不是实例！否则存储的将是副本，而不是共享对象。
  public:
    Tree(int x, int y, std::shared_ptr<TreeType> type): x_(x), y_(y), type_(type) {}
    void draw(const Canvas& c) {
      type_->draw(c, x_, y_);
    }
}

class Forest {
  std::vector<Tree> trees_;
  void plantTree(int x, int y, std::string name, std::string color, std::string texture) {
    auto type = TreeFactory::getTreeType(name, color, texture);
    Tree tree = Tree(x, y, type);
    trees_.push_back(tree);
  }
  void draw(const Canvas& c) {
    for (int i = 0; i < trees_.size(); i++) trees_[i].draw(c);
  }
}