State Patterns in Practice (C++)

Recently I posted a workflow here. I want to express the workflow in terms of state patterns. There are only two steps. Step 1: Prepare the framework.

---

// state/state.h
#ifndef STATE_STATE_H_
#define STATE_STATE_H_
#pragma once

namespace state {

class State {
 public:
  virtual ~State() {}

  virtual void update() = 0;
  virtual void draw() const = 0;
  virtual void onEnter() {}
  virtual void onLeave() {}
};

}  // namespace state

#endif // STATE_STATE_H_

---

// state/state_factory.h
#ifndef STATE_STATE_FACTORY_H_
#define STATE_STATE_FACTORY_H_
#pragma once

#include <string>

namespace state {

class State;

class StateFactory {
 public:
  virtual ~StateFactory() {}
  virtual State* createState(const std::string& name) = 0;
};

}  // namespace state 

#endif // STATE_STATE_FACTORY_H_

---

// state/state_manager.h
#ifndef STATE_STATE_MANAGER_H_
#define STATE_STATE_MANAGER_H_
#pragma once

namespace state {

class State;

class StateManager {
 public:
  StateManager();
  virtual ~StateManager() {}

  inline State* current_state() { return current_state_; }
  virtual void switchToState(State* state);

 private:
  State* current_state_;
};

}  // namespace state 

#endif // STATE_STATE_MANAGER_H_

---

// state/state_manager.cc
#include "state_manager.h"
#include "state.h"
#include "base/basictypes.h"

namespace state {

StateManager::StateManager() 
    : current_state_(NULL) {
}

void StateManager::switchToState(State* state) {
  if (!!current_state_) {
    current_state_->onLeave();
  }

  if (!!state) {
    current_state_ = state;
    current_state_->onEnter(); 
  }
}

}  // namespace state

---

Step 2: Combine them. I will not post all details but post important files instead.

---

// app_state_factory.h (not completed)
#include "state/state_factory.h"

class AppController;

class AppStateFactory : public state::StateFactory {
 public:
  explicit AppStateFactory(AppController* controller);
  virtual state::State* createState(const std::string& name);

 private:
  AppController* controller_;
};

---

// app_controller.h
namespace state {
class State;
}

class AppStateManager;
class AppStateFactory;

class AppController {
 public:
  AppController();
  ~AppController();

  void init();
  void deinit();

  void update();
  void draw() const;

  void switchToState(const std::string& name);

 private:
  void initAllStates();

 private:
  AppStateManager* state_manager_;
  AppStateFactory* state_factory_;
  std::map<std::string, state::State*> states_;
};

---

// app_state_manager.h
#include "state/state_manager.h"

class AppStateManager : public state::StateManager {
 public:
  AppStateManager();
  virtual ~AppStateManager();
};

---

// state/initial_state.h
#include "state/state.h"

class AppController;

class InitialState : public state::State {
 public:
  explicit InitialState(AppController* controller);
  virtual ~InitialState();

  virtual void update();
  virtual void draw() const;
  virtual void onEnter();
  virtual void onLeave();

 private:
  AppController* controller_;
};

---

// state/initial_state.cc
#include "initial_state.h"
#include "../app_controller.h"

InitialState::InitialState(AppController* controller)
    : controller_(controller) {
}

InitialState::~InitialState() {
}

void InitialState::update() {
}

void InitialState::draw() const {
}

void InitialState::onEnter() {
  // TODO: Load necessary resources.
  if (!!controller_) {
    controller_->switchToState("idle");
  }
}

void InitialState::onLeave() {
}

---

Keep working!