Intent

Simulate a collection of independent objects by telling each to process one frame of behavior at a time.

Let’s create some skeletons that guard an entrance:

while(true) {
  for (double x = 0; x < 100; x++) skeleton.setX(x);
  for (double x = 100; x > 0; x--) skeleton.setX(x);
}

Of course, the player doesn’t see this. It’s locked in an infinite loop.

Instead, we’ll rely on the outer loop for iteration.

Entity skeleton;
bool patrollingLeft = false;
double x = 0;

while (true) {
  if (patrollingLeft) {
    x--;
    if (x == 0) patrollingLeft = false;
  } else {
    x++;
    if (x == 100) patrollingLeft = true;
  }
  skeleton.setX(x);
}

Then we’ll have to add some more enchanted statues…

Entity leftStatue;
Entity rightStatue;
int leftStatueFrames = 0;
int rightStatueFrames = 0;

// Main game loop:
while (true) {
  if (++leftStatueFrames == 90) {
    leftStatueFrames = 0;
    leftStatue.shootLightning();
  }

  if (++rightStatueFrames == 80) {
    rightStatueFrames = 0;
    rightStatue.shootLightning();
  }

  // Handle user input and render game...
}

This is maintainable. We have lots of state stuffed in the game loop.

Instead, Each entity in the game should encapsulate its own behavior.

We need an abstraction layer: we create that by defining an abstract update() method. The game loop maintains a collection of objects, and all it does is call update() on them once, every frame.

The Pattern

The game world maintains a collection of objects. Each object implements an update method that simulates one frame of the object’s behavior. Each frame, the game updates every object in the collection.

When to Use It

This pattern works well when:

1. Your game has a number of objects or systems that need to run simultaneously.
2. Each object’s behavior is mostly independent of the others.
3. The objects need to be simulated over time.

Keep in Mind

• Splitting code into single frame slices makes it more complex
• You have to store state to resume where you left off each frame
• Objects are updated sequentially. If objects rely on each other’s state, this can end up in edge cases if both are updated at the same time.
• Be careful modifying the object list while updating.
• You can easily handle the case where objects are added to the end of the list, but not being removed. If you don’t want to update dead objects, allow them to be labeled as dead. The update loop can skip over dead objects, and at the end of the loop, remove them.

Sample Code

class Entity {
  public:
    Entity() : x_(0), y_(0) {}
    virtual ~Entity() {}
    virtual void update = 0;

    double x() const { return x_; }
    double y() const { return y_; }

    void setX(double x) { x_ = x; }
    void setY(double y) { y_ = y; }

  private:
    double x_;
    double y_;
};

Then we need a world to hold all of these objects.

class World {
  public:
    World() : numEntities_(0) {}
    void gameLoop();
  private:
    Entity* entities_[MAX_ENTITIES];
    int numEntities_;
};

And of course, implementing the gameLoop method:

void World::gameLoop() {
  while (true) {
    for (const auto entity : entities_) {
      entities_.update();
      // code to handle if entities are dead
    }
  }
}

Defining Entities

class Skeleton : public Entity {
public:
  Skeleton() : patrollingLeft_(false) {}

  virtual void update() {
    if (patrollingLeft_) {
      setX(x() - 1);
      if (x() == 0) patrollingLeft_ = false;
    }
    else {
      setX(x() + 1);
      if (x() == 100) patrollingLeft_ = true;
    }
  }

private:
  bool patrollingLeft_;
};

And the statue:

class Statue : public Entity {
public:
  Statue(int delay) : frames_(0), delay_(delay) {}

  virtual void update() {
    if (++frames_ == delay_) {
      shootLightning();

      // Reset the timer.
      frames_ = 0;
    }
  }

private:
  int frames_;
  int delay_;

  void shootLightning() { /* Shoot the lightning... */ }
};

If we allow our game to have variable time pass in the game loop, we can pass that in:

void Skeleton::update(double elapsed) {
  if (patrollingLeft_) {
    x -= elapsed;
    if (x <= 0) {
      patrollingLeft_ = false;
      x = -x;
    }
  }
  else {
    x += elapsed;
    if (x >= 100) {
      patrollingLeft_ = true;
      x = 100 - (x - 100);
    }
  }
}

What class does the update method live on?

1. The entity class:

• This is the simplest option, but it uses inheritance, and can be brittle.

2. The component class:

• If you’re using the Component pattern, this makes sense. Each component can update itself independently.

3. A delegate class:

• The state pattern or Type object lets you delegate behavior to another object.

How are dormant objects handled?

Dormant objects can be handled by setting a flag that says whether or not they’re alive (simpler) or you can separate entities into a collection of alive or not.

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