1.观察者模式的类图

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1.1 发布者

发布者提供了观察者订阅(注册）以及取消订阅(取消注册）的接口，当事件来临时，发布者会遍历每个订阅者对象，并以此调用每一个订阅者的update通知方法。

1.2 订阅者

该接口通常仅包含一个update方法

2. 示例代码

#include <iostream>
#include <list>
#include <string>
/**
 * 定义抽象观察者类，仅包含一个update方法
 */
class IObserver {
 public:
  virtual ~IObserver(){};
  virtual void Update(const std::string &message_from_subject) = 0;
};
/**
 * 定义抽象subject基类
 */
class ISubject {
 public:
  virtual ~ISubject(){};
  virtual void Attach(IObserver *observer) = 0;
  virtual void Detach(IObserver *observer) = 0;
  virtual void Notify() = 0;
};

/**
 * The Subject owns some important state and notifies observers when the state
 * changes.
 * 此类为Subject类，当状态改变时，通知每一个观察者
 */

class Subject : public ISubject {
 public:
  virtual ~Subject() {
    std::cout << "Goodbye, I was the Subject.\n";
  }

  /**
   * The subscription management methods.
   * 注册观察者
   */
  void Attach(IObserver *observer) override {
    list_observer_.push_back(observer);
  }
  /**
   * 删除观察者
   */
  void Detach(IObserver *observer) override {
    list_observer_.remove(observer);
  }
  /**
   * 遍历观察者，通知每一个观察者，并将信息传递给每一个observer
   */
  void Notify() override {
    std::list<IObserver *>::iterator iterator = list_observer_.begin();
    HowManyObserver();
    while (iterator != list_observer_.end()) {
      (*iterator)->Update(message_);
      ++iterator;
    }
  }
 /**
   * 发布者的状态改变，即创建了一条信息，通知每一个观察者
   */
  void CreateMessage(std::string message = "Empty") {
    this->message_ = message;
    Notify();
  }
  /**
   * 统计观察者的数量
   */
  void HowManyObserver() {
    std::cout << "There are " << list_observer_.size() << " observers in the list.\n";
  }

  /**
   * Usually, the subscription logic is only a fraction of what a Subject can
   * really do. Subjects commonly hold some important business logic, that
   * triggers a notification method whenever something important is about to
   * happen (or after it).
   */
  void SomeBusinessLogic() {
    this->message_ = "change message message";
    Notify();
    std::cout << "I'm about to do some thing important\n";
  }

 private:
  std::list<IObserver *> list_observer_;
  std::string message_;
};
/**
   * 构造函数将发布者注册到观察者内部
   */
class Observer : public IObserver {
 public:
  Observer(Subject &subject) : subject_(subject) {
    this->subject_.Attach(this);
    std::cout << "Hi, I'm the Observer \"" << ++Observer::static_number_ << "\".\n";
    this->number_ = Observer::static_number_;
  }
  virtual ~Observer() {
    std::cout << "Goodbye, I was the Observer \"" << this->number_ << "\".\n";
  }

  void Update(const std::string &message_from_subject) override {
    message_from_subject_ = message_from_subject;
    PrintInfo();
  }
  /**
   * 观察者也有权限从发布者中取消订阅，同时更新发布者的观察者数目
   */
  void RemoveMeFromTheList() {
    subject_.Detach(this);
    std::cout << "Observer \"" << number_ << "\" removed from the list.\n";
  }
  void PrintInfo() {
    std::cout << "Observer \"" << this->number_ << "\": a new message is available --> " << this->message_from_subject_ << "\n";
  }

 private:
  std::string message_from_subject_;
  Subject &subject_;
  static int static_number_;
  int number_;
};

int Observer::static_number_ = 0;
/**
   * 客户端创建了一个发布者，5个观察者
   */
void ClientCode() {
  Subject *subject = new Subject;
  Observer *observer1 = new Observer(*subject);
  Observer *observer2 = new Observer(*subject);
  Observer *observer3 = new Observer(*subject);
  Observer *observer4;
  Observer *observer5;

  subject->CreateMessage("Hello World! :D");
  observer3->RemoveMeFromTheList();

  subject->CreateMessage("The weather is hot today! :p");
  observer4 = new Observer(*subject);

  observer2->RemoveMeFromTheList();
  observer5 = new Observer(*subject);

  subject->CreateMessage("My new car is great! ;)");
  observer5->RemoveMeFromTheList();

  observer4->RemoveMeFromTheList();
  observer1->RemoveMeFromTheList();

  delete observer5;
  delete observer4;
  delete observer3;
  delete observer2;
  delete observer1;
  delete subject;
}

int main() {
  ClientCode();
  return 0;
}

3. 项目源代码剖析

3.1 观察者抽象基类

该抽象基类仅有一个OnValueChanged的接口

class ValueChangeObserver : public virtual AceType {
    DECLARE_ACE_TYPE(ValueChangeObserver, AceType);

public:
    virtual void OnValueChanged(bool needFireChangeEvent = true, bool needFireSelectChangeEvent = true) = 0;

protected:
    ValueChangeObserver() = default;
    ~ValueChangeObserver() override = default;

    ACE_DISALLOW_COPY_AND_MOVE(ValueChangeObserver);
};

3.2 发布者类基类：模板类

该类提供了AddObserver，RemoveObserver的接口；SetValue用于判断传入的值是否发生改变，如果改变调用Notify接口：即通知每一个观察者调用OnValueChanged接口。
观察者用链表observers_进行存储。

template<class U>
class ValueChangeNotifier : public virtual AceType {
    DECLARE_ACE_TYPE(ValueChangeNotifier<U>, AceType);

public:
    void AddObserver(const WeakPtr<ValueChangeObserver>& observer)
    {
        observers_.emplace_back(observer);
    }

    void RemoveObserver(const WeakPtr<ValueChangeObserver>& observer)
    {
        observers_.remove(observer);
    }

    void SetValue(U&& newValue, bool needFireChangeEvent = true)
    {
        if (newValue == value_) {
            return;
        }

        preValue_ = value_;
        value_ = std::move(newValue);
        Notify(needFireChangeEvent);
    }

    void SetValue(const U& newValue, bool needFireChangeEvent = true)
    {
        if (newValue == value_) {
            return;
        }

        preValue_ = value_;
        value_ = newValue;
        Notify(needFireChangeEvent);
    }

    const U& GetValue() const
    {
        return value_;
    }

    const U& GetPreValue() const
    {
        return preValue_;
    }

private:
    void Notify(bool needFireChangeEvent = true)
    {
        auto iter = observers_.begin();
        while (iter != observers_.end()) {
            const auto& observer = iter->Upgrade();
            if (observer) {
                observer->OnValueChanged(needFireChangeEvent);
                iter++;
            } else {
                iter = observers_.erase(iter);
            }
        }
    }

    std::list<WeakPtr<ValueChangeObserver>> observers_;

    U value_;
    U preValue_;
};

3.3 ConCrete观察者类：RenderTextField

该类为具体的观察者类，继承了观察者基类。

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拥有成员TextEditController ，通过该具体发布者类，注册观察者类以及当数据更新时通知注册的观察者类。

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判断组件的值是否变化时IsValueUpdated，调用发布者的SetText,通知观察者进行更新数据。

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观察者具体需要更新的事情是：
1.对输入框中的文本进行格式化
2.更新光标所选的文本区域
3.触发相应的回调事件

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3.4 ConCrete发布者类：TextEditController

该类指定了发布者类的模板T为：TextEditingValue

class TextEditController : public ValueChangeNotifier<TextEditingValue> {
public:
    void SetText(const std::string& newText, bool needFireChangeEvent = true)
    {
        auto value = GetValue();
        value.text = newText;
        // Default set selection to the end of text is more consistent with the intuition of user.
        value.selection.Update(value.GetWideText().length());
        SetValue(std::move(value), needFireChangeEvent);
    }

    void SetHint(const std::string& hint)
    {
        auto value = GetValue();
        value.hint = hint;
        SetValue(std::move(value));
    }

    void SetSelection(const TextSelection& selection)
    {
        auto value = GetValue();
        value.selection = selection;
        SetValue(std::move(value));
    }

    void Clear()
    {
        SetValue(TextEditingValue());
    }

    const std::string& GetText() const
    {
        return GetValue().text;
    }

    const TextSelection& GetSelection() const
    {
        return GetValue().selection;
    }
};