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【设计模式】【行为型模式（Behavioral Patterns）】之观察者模式（Observer Pattern）

news 2025/6/30 11:26:35

1. 设计模式原理说明

观察者模式（Observer Pattern） 是一种行为设计模式，它定义了一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都会得到通知并自动更新。这种模式非常适合处理事件驱动系统，其中对象之间需要保持同步但又不想紧密耦合。

主要角色

Subject（主题/被观察者）：它知道它的观察者是谁。任何数量的观察者都可以观察一个主题。提供注册和删除观察者对象的方法。
Observer（观察者）：为那些在主题发生改变时需获得通知的对象定义一个更新接口。
ConcreteSubject（具体主题）：将有关状态存储于具体观察者对象；当它的状态发生改变时，向它的各个观察者发出通知。
ConcreteObserver（具体观察者）：实现Observer的更新接口以使自身状态与主题的状态保持一致。

2. UML 类图及解释

UML 类图

+------------------+                +-----------------------+
|   Subject        |                |     Observer          |
|------------------|                |-----------------------|
| - observers: List<Observer> |     | - update(message: String): void |
| - register(observer: Observer) |   +-----------------------+
| - unregister(observer: Observer) |   |
| - notifyObservers(message: String) |   |
+------------------+                +-----------------------+||v+---------------------------+| ConcreteObserverA         ||--------------------------|| - update(message: String)  |+---------------------------+^|+---------------------------+| ConcreteObserverB         ||--------------------------|| - update(message: String)  |+---------------------------+

类图解释

Subject：定义了添加、删除和通知观察者的接口。具体主题会实现这些方法，并维护一个观察者列表。
Observer：定义了一个更新接口，当主题发生变化时，观察者会被调用。
ConcreteSubject：实现了Subject接口，当其状态变化时，会通知所有已注册的观察者。
ConcreteObserverA 和 ConcreteObserverB：实现了Observer接口，当收到主题的通知时，会更新自己的状态。

3. 代码案例及逻辑详解

Java 代码案例

// 观察者接口
interface Observer {void update(String message);
}// 具体观察者A
class ConcreteObserverA implements Observer {@Overridepublic void update(String message) {System.out.println("ConcreteObserverA received: " + message);}
}// 具体观察者B
class ConcreteObserverB implements Observer {@Overridepublic void update(String message) {System.out.println("ConcreteObserverB received: " + message);}
}// 被观察者接口
interface Subject {void register(Observer observer);void unregister(Observer observer);void notifyObservers(String message);
}// 具体被观察者
class ConcreteSubject implements Subject {private List<Observer> observers = new ArrayList<>();@Overridepublic void register(Observer observer) {observers.add(observer);}@Overridepublic void unregister(Observer observer) {observers.remove(observer);}@Overridepublic void notifyObservers(String message) {for (Observer observer : observers) {observer.update(message);}}public void changeState() {// 模拟状态变化System.out.println("State changed, notifying observers...");notifyObservers("State has been changed!");}
}// 客户端
public class Client {public static void main(String[] args) {ConcreteSubject subject = new ConcreteSubject();Observer observerA = new ConcreteObserverA();Observer observerB = new ConcreteObserverB();subject.register(observerA);subject.register(observerB);subject.changeState();subject.unregister(observerA);subject.changeState();}
}

C++ 代码案例

#include <iostream>
#include <vector>
#include <algorithm>// 观察者接口
class Observer {
public:virtual void update(const std::string& message) = 0;virtual ~Observer() {}
};// 具体观察者A
class ConcreteObserverA : public Observer {
public:void update(const std::string& message) override {std::cout << "ConcreteObserverA received: " << message << std::endl;}
};// 具体观察者B
class ConcreteObserverB : public Observer {
public:void update(const std::string& message) override {std::cout << "ConcreteObserverB received: " << message << std::endl;}
};// 被观察者接口
class Subject {
private:std::vector<Observer*> observers;
public:void registerObserver(Observer* observer) {observers.push_back(observer);}void unregisterObserver(Observer* observer) {observers.erase(std::remove(observers.begin(), observers.end(), observer), observers.end());}void notifyObservers(const std::string& message) {for (Observer* observer : observers) {observer->update(message);}}void changeState() {// 模拟状态变化std::cout << "State changed, notifying observers..." << std::endl;notifyObservers("State has been changed!");}
};// 客户端
int main() {Subject subject;Observer* observerA = new ConcreteObserverA();Observer* observerB = new ConcreteObserverB();subject.registerObserver(observerA);subject.registerObserver(observerB);subject.changeState();subject.unregisterObserver(observerA);subject.changeState();delete observerA;delete observerB;return 0;
}

Python 代码案例

# 观察者接口
class Observer:def update(self, message):raise NotImplementedError# 具体观察者A
class ConcreteObserverA(Observer):def update(self, message):print(f"ConcreteObserverA received: {message}")# 具体观察者B
class ConcreteObserverB(Observer):def update(self, message):print(f"ConcreteObserverB received: {message}")# 被观察者接口
class Subject:def __init__(self):self._observers = []def register(self, observer):self._observers.append(observer)def unregister(self, observer):self._observers.remove(observer)def notify_observers(self, message):for observer in self._observers:observer.update(message)def change_state(self):# 模拟状态变化print("State changed, notifying observers...")self.notify_observers("State has been changed!")# 客户端
if __name__ == "__main__":subject = Subject()observerA = ConcreteObserverA()observerB = ConcreteObserverB()subject.register(observerA)subject.register(observerB)subject.change_state()subject.unregister(observerA)subject.change_state()

Go 代码案例

package mainimport ("fmt"
)// 观察者接口
type Observer interface {update(message string)
}// 具体观察者A
type ConcreteObserverA struct{}func (c *ConcreteObserverA) update(message string) {fmt.Printf("ConcreteObserverA received: %s\n", message)
}// 具体观察者B
type ConcreteObserverB struct{}func (c *ConcreteObserverB) update(message string) {fmt.Printf("ConcreteObserverB received: %s\n", message)
}// 被观察者接口
type Subject struct {observers []Observer
}func (s *Subject) register(observer Observer) {s.observers = append(s.observers, observer)
}func (s *Subject) unregister(observer Observer) {for i, obs := range s.observers {if obs == observer {s.observers = append(s.observers[:i], s.observers[i+1:]...)break}}
}func (s *Subject) notifyObservers(message string) {for _, observer := range s.observers {observer.update(message)}
}func (s *Subject) changeState() {// 模拟状态变化fmt.Println("State changed, notifying observers...")s.notifyObservers("State has been changed!")
}// 客户端
func main() {subject := &Subject{}observerA := &ConcreteObserverA{}observerB := &ConcreteObserverB{}subject.register(observerA)subject.register(observerB)subject.changeState()subject.unregister(observerA)subject.changeState()
}