2_flyweight, 轻量化模式

### instanced rendering.
send shared data to gpu just once
    mesh, texture, leaves
push every instance’s unique data
    position, color, scale

With a single draw call, an entire forest grows.

unity3d
    Using instances of a prefab automatically are using the same mesh and material.


### the flyweight pattern

当拥有大量同样的obj时，则需要更多的轻量化

把一个ojb分成两类
    1: intrinsic state, 共享的数据, content—free stuff
    2: extrinsic state, 独有的数据,

example

```
intrinsic state: 世界包含若干种地形
extrinsic state: 不同的位置具有不同的地形,指向相应的地形

class Terrain
{
public:
  Terrain(int movementCost,
          bool isWater,
          Texture texture)
  : movementCost_(movementCost),
    isWater_(isWater),
    texture_(texture)
  {}

  int getMovementCost() const { return movementCost_; }
  bool isWater() const { return isWater_; }
  const Texture& getTexture() const { return texture_; }

private:
  int movementCost_;
  bool isWater_;
  Texture texture_;
};

class World
{
public:
  World()
  : grassTerrain_(1, false, GRASS_TEXTURE),
    hillTerrain_(3, false, HILL_TEXTURE),
    riverTerrain_(2, true, RIVER_TEXTURE)
  {}

private:
  Terrain grassTerrain_;
  Terrain hillTerrain_;
  Terrain riverTerrain_;

  // Other stuff...
};

void World::generateTerrain()
{
  // Fill the ground with grass.
  for (int x = 0; x < WIDTH; x++)
  {
    for (int y = 0; y < HEIGHT; y++)
    {
      // Sprinkle some hills.
      if (random(10) == 0)
      {
        tiles_[x][y] = &hillTerrain_;
      }
      else
      {
        tiles_[x][y] = &grassTerrain_;
      }
    }
  }

  // Lay a river.
  int x = random(WIDTH);
  for (int y = 0; y < HEIGHT; y++) {
    tiles_[x][y] = &riverTerrain_;
  }
}

const Terrain& World::getTile(int x, int y) const
{
  return *tiles_[x][y];
}

int cost = world.getTile(2, 3).getMovementCost();


```
###其他应用
    工厂方法
    对象池
    状态机