观察者模式
一、定义:
观察者模式定义了一种一对多的依赖关系,让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时,会通知所有观察者对象,使它们能够自动更新自己。此种模式通常被用来实时事件处理系统。其中两个重要对象是观察者和主题,要想主题对象发生改变时,能通知到所有观察者角色,则自然主题角色必须引用观察者,从而完成观察模式。
特点:
1、定义了一对多的关系。
2、主题用一个共同的接口来更新观察者。
3、观察者和主题用松耦合的方式结合,不需要知道实现细节,只需要实现接口就可以。
二、UML类图:
三、基本代码:
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class Program { static void Main(string[] args) { ConcreteSubject cs = new ConcreteSubject(); cs.Attach(new ConcreteObserver(cs, "x")); cs.Attach(new ConcreteObserver(cs, "y")); cs.SubjectState = "abc"; cs.Notify(); Console.Read(); } } abstract class Subject { private IList<Observer> observers = new List<Observer>(); public void Attach(Observer observer) { observers.Add(observer); } public void Detach(Observer observer) { observers.Remove(observer); } public void Notify() { foreach (Observer ob in observers) { ob.Update(); } } } class ConcreteSubject : Subject { private string subjectState; public string SubjectState { get { return subjectState; } set { subjectState = value; } } } abstract class Observer { public abstract void Update(); } class ConcreteObserver : Observer { private string name; private string observerState; private ConcreteSubject subject; public ConcreteSubject Subject { get { return subject; } set { subject = value; } } public ConcreteObserver(ConcreteSubject subject, string name) { this.subject = subject; this.name = name; } public override void Update() { observerState = subject.SubjectState; Console.WriteLine("观察者{0}的新状态是{1}", name, observerState); } }
class Program { static void Main(string[] args) { ConcreteSubject cs = new ConcreteSubject(); cs.Attach(new ConcreteObserver(cs, "x")); cs.Attach(new ConcreteObserver(cs, "y")); cs.SubjectState = "abc"; cs.Notify(); Console.Read(); } } abstract class Subject { private IList<Observer> observers = new List<Observer>(); public void Attach(Observer observer) { observers.Add(observer); } public void Detach(Observer observer) { observers.Remove(observer); } public void Notify() { foreach (Observer ob in observers) { ob.Update(); } } } class ConcreteSubject : Subject { private string subjectState; public string SubjectState { get { return subjectState; } set { subjectState = value; } } } abstract class Observer { public abstract void Update(); } class ConcreteObserver : Observer { private string name; private string observerState; private ConcreteSubject subject; public ConcreteSubject Subject { get { return subject; } set { subject = value; } } public ConcreteObserver(ConcreteSubject subject, string name) { this.subject = subject; this.name = name; } public override void Update() { observerState = subject.SubjectState; Console.WriteLine("观察者{0}的新状态是{1}", name, observerState); } }
四、适用场景:
当一个对象的改变需要同时改变其他对象时,而且不知道具体有多少对象有待改变时,应该考虑使用观察者模式。
举例说明:
实例代码:
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interface ISubject { void Attach(Observer observer); void Detach(Observer observer); void Notify(); string SubjectState { get; set; } } class Boss : ISubject { private IList<Observer> observers = new List<Observer>(); private string bossAction; public string SubjectState { get { return bossAction; } set { bossAction = value; } } public void Attach(Observer observer) { observers.Add(observer); } public void Detach(Observer observer) { observers.Remove(observer); } public void Notify() { foreach (Observer ob in observers) { ob.Update(); } } } abstract class Observer { protected string name; protected ISubject subject; public Observer(string name, ISubject subject) { this.name = name; this.subject = subject; } public abstract void Update(); } class StockObserver : Observer { public StockObserver (string name,ISubject subject):base(name,subject ) {} public override void Update() { Console.WriteLine("{0},{1} 关闭股票,继续工作", subject.SubjectState, name); } } class NBAObserver : Observer { public NBAObserver (string name,ISubject subject):base(name,subject) {} public override void Update() { Console.WriteLine("{0},{1} 关闭NBA,继续工作", subject.SubjectState, name); } }
Boss boss = new Boss(); Observer s1 = new StockObserver("张三", boss); Observer s2 = new NBAObserver("李四", boss); boss.Attach(s1); boss.Attach(s2); boss.SubjectState = "老板回来了"; boss.Notify();
五、观察者模式的缺点及解决方法:
在具体前台、同事和老板的实例中,使用依赖倒转原则,但是“抽象通知者”还是依赖“抽象观察者”,即如果没有抽象观察者这样的接口,通知功能就完不成。另外就是每个具体观察者,它不一定是“更新”的方法要调用,可能是其他的方法。解决方法:通知者和观察者之间根本互相不知道,由客户端决定通知谁。对于前台的实例,决定使用委托的方式来实现由客户端来决定具体通知谁的功能。
实例说明:
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interface ISubject { void Attach(Observer observer); void Detach(Observer observer); void Notify(); string SubjectState { get; set; } } class Boss : ISubject { private IList<Observer> observers = new List<Observer>(); private string bossAction; public string SubjectState { get { return bossAction; } set { bossAction = value; } } public void Attach(Observer observer) { observers.Add(observer); } public void Detach(Observer observer) { observers.Remove(observer); } public void Notify() { foreach (Observer ob in observers) { ob.Update(); } } } abstract class Observer { protected string name; protected ISubject subject; public Observer(string name, ISubject subject) { this.name = name; this.subject = subject; } public abstract void Update(); } class StockObserver : Observer { public StockObserver (string name,ISubject subject):base(name,subject ) {} public override void Update() { Console.WriteLine("{0},{1} 关闭股票,继续工作", subject.SubjectState, name); } } class NBAObserver : Observer { public NBAObserver (string name,ISubject subject):base(name,subject) {} public override void Update() { Console.WriteLine("{0},{1} 关闭NBA,继续工作", subject.SubjectState, name); } }
interface ISubject { void Notify(); string SubjectState { get; set; } } public delegate void DelegateHandler(); class Boss : ISubject { public event DelegateHandler Update; public string SubjectState { get; set; } public void Notify() { Update(); } } abstract class Observer { protected string name; protected ISubject subject; public Observer(string name, ISubject subject) { this.name = name; this.subject = subject; } public abstract void Update(); } class StockObserver : Observer { public StockObserver(string name, ISubject subject) : base(name, subject) { } public override void Update() { Console.WriteLine("{0},{1} 关闭股票,继续工作", subject.SubjectState, name); } } class NBAObserver : Observer { public NBAObserver(string name, ISubject subject) : base(name, subject) { } public override void Update() { Console.WriteLine("{0},{1} 关闭NBA,继续工作", subject.SubjectState, name); } }
Boss boss = new Boss(); StockObserver s1 = new StockObserver("张三", boss); NBAObserver s2 = new NBAObserver("李四", boss); boss.Update += new DelegateHandler(s1.Update); boss.Update += new DelegateHandler(s2.Update); boss.SubjectState = "老板回来了"; boss.Notify();
六、总结:
观察者模式所做的工作其实就是在解除耦合。让耦合的双方都依赖于抽象,而不是依赖于具体,从而使各自的变化都不会影响到另一边的变化。
分类:
设计模式
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