抽象编程怎么说呢，以观察者模式为例：
观察者模式有两个对象，一个是观察者，一个是可观察者(字面翻译很别扭observable),消息发布者(提供者)。
第一层如下,三个对象A、B、C分别有一个接收消息的方法，还有一个存储数据的字段，X就是发布消息的对象，它通过setdata方法设置自己的字段data,然后通知abc，abc如愿以偿地拿到了通知，完美!
internal class A { public int Data; public void Update(int data) { this.Data = data; } } internal class B { public int Count; public void Notify(int data) { this.Count = data; } } internal class C { public int N; public void Set(int data) { this.N = data; } } internal class X { private int data; public A instanceA; public B instanceB; public C instanceC; public void SetData(int data) { this.data = data; instanceA.Update(data); instanceB.Notify(data); instanceC.Set(data); } } using ObserverOne; A a = new A(); B b = new B(); C c = new C(); Console.WriteLine("订阅前................."); Console.WriteLine($"a.Data = {a.Data}"); Console.WriteLine($"b.Count = {b.Count}"); Console.WriteLine($"c.N = {c.N}"); X x =new X(); x.instanceA = a; x.instanceB = b; x.instanceC = c; x.SetData(10); Console.WriteLine("X发布data=10, 订阅后................."); Console.WriteLine($"a.Data = {a.Data}"); Console.WriteLine($"b.Count = {b.Count}"); Console.WriteLine($"c.N = {c.N}");
再想一想，这好像不够灵活，订阅者是死的，那改进一下：
internal interface IUpdatebleObject { int Data { get; } void Update(int newData); } internal class A : IUpdatebleObject { public int Data => data; private int data; public void Update(int newData) { this.data = newData; } } internal class B : IUpdatebleObject { public int Data => data; private int data; public void Update(int newData) { this.data = newData; } } internal class C : IUpdatebleObject { public int Data => data; private int data; public void Update(int newData) { this.data = newData; } } internal class X { private IUpdatebleObject[] updates=new IUpdatebleObject[3]; public IUpdatebleObject this[int index] { set { updates[index] = value; } } private int data; public void Update(int newData) { this.data = newData; foreach (var update in updates) { update.Update(newData); } } } using ObserverTwo; X x = new X(); IUpdatebleObject a = new A(); IUpdatebleObject b = new B(); IUpdatebleObject c = new C(); Console.WriteLine("订阅前................."); Console.WriteLine($"a.Data = {a.Data}"); Console.WriteLine($"b.Data = {b.Data}"); Console.WriteLine($"c.Data = {c.Data}"); x[0] = a; x[1] = b; x[2] = c; x.Update(10); Console.WriteLine("X发布data=10, 订阅后................."); Console.WriteLine($"a.Data = {a.Data}"); Console.WriteLine($"b.Data = {b.Data}"); Console.WriteLine($"c.Data = {c.Data}");
虽然写到这个例子已经很了不起了，但是对于有想法的来说还是可以继续改进，要不然怎么常挂嘴边说面对抽象编程呢，那就继续改进了：
/// <summary> /// 观察者 /// </summary> /// <typeparam name="T"></typeparam> internal interface IObserver<T> { void Update(SubjectBase<T> subject); }
/// <summary> /// 可观察者(发出通知的对象) /// </summary> /// <typeparam name="T"></typeparam> internal abstract class SubjectBase<T> { protected IList<IObserver<T>> observers = new List<IObserver<T>>(); protected T state; public virtual T State => state; public static SubjectBase<T> operator +(SubjectBase<T> subject,IObserver<T> observer) { subject.observers.Add(observer); return subject; } public static SubjectBase<T> operator -(SubjectBase<T> subject,IObserver<T> observer) { subject.observers.Remove(observer); return subject; } public virtual void Notify() { foreach (var observer in observers) { observer.Update(this); } } public virtual void Update(T state) { this.state = state; Notify(); } }
internal class Observer<T> : IObserver<T> { public T State; public void Update(SubjectBase<T> subject) { this.State = subject.State; } }
internal class Subject<T>:SubjectBase<T> { }
到这里基本上可以说是把骨架搭起来了，这些可以称之为底层的代码。实现代码如下：
internal class TestObserver { public void TestMulticst() { SubjectBase<int> subject = new Subject<int>(); Observer<int> observer1 = new Observer<int>(); observer1.State = 10; Observer<int> observer2 = new Observer<int>(); observer2.State = 20; subject += observer1; subject += observer2; subject.Update(1); Console.WriteLine($"observer1.State={observer1.State} observer2.State={observer2.State}"); subject -= observer1; subject.Update(100); Console.WriteLine($"update state = 100, observer1.State={observer1.State} observer2.State={observer2.State}"); } public void TestMultiSubject() { SubjectBase<string> subject1 = new Subject<string>(); SubjectBase<string> subject2 = new Subject<string>(); Observer<string> observer1 = new Observer<string>(); observer1.State = "运动"; Console.WriteLine($"observer1.State={observer1.State}"); subject1 += observer1; subject2 += observer1; subject1.Update("看电影"); Console.WriteLine($"observer1.State={observer1.State}"); subject2.Update("喝茶"); Console.WriteLine($"observer1.State={observer1.State}"); subject1 -= observer1; subject2 -= observer1; observer1.State = "休息"; subject1 -= observer1; subject2 -= observer1; Console.WriteLine($"observer1.State={observer1.State}"); } }
using ObserverThree; //new TestObserver().TestMulticst(); new TestObserver().TestMultiSubject();
到这里基本上就完成了任务，也就可以结束了。但是，学习需要深度也需要宽度，所以观察者模式在C#可以通过事件来实现一样的效果。下面就看下上面写这么多的代码用事件怎么写呢,这里的实例稍作变化,实现改变名字通知观察者，这里观察者就是控制台了，打印通知：
internal class UserEventArgs:EventArgs { private string name; public string Name => name; public UserEventArgs(string name) { this.name = name; } }
internal class User { public event EventHandler<UserEventArgs> NameChanged; private string name; public string Name { get { return name; } set { name = value; NameChanged?.Invoke(this, new UserEventArgs(value)); } } }
using ObserverFour; User user = new User(); user.NameChanged += OnNameChanged; user.Name = "joe"; void OnNameChanged(object sender, UserEventArgs args) { Console.WriteLine($"{args.Name} Changed "); }
再放一个麻烦一点的例子，字典新增的通知(监听)事件：
internal class DictionaryEventArgs<TKey,TValue> : EventArgs { private TKey key; private TValue value; public DictionaryEventArgs(TKey key,TValue value) { this.key = key; this.value = value; } public TKey Key => key; public TValue Value => value; }
internal interface IObserverableDictionary<TKey,TValue>:IDictionary<TKey, TValue> { EventHandler<DictionaryEventArgs<TKey,TValue>> NewItemAdded { get; set; } }
internal class ObserverableDictionary<TKey, TValue> : Dictionary<TKey, TValue>, IObserverableDictionary<TKey, TValue> { protected EventHandler<DictionaryEventArgs<TKey, TValue>> newItemAdded; public EventHandler<DictionaryEventArgs<TKey, TValue>> NewItemAdded { get => newItemAdded;set=> newItemAdded = value;} public new void Add(TKey key,TValue value) { base.Add(key, value); if(NewItemAdded != null) NewItemAdded(this, new DictionaryEventArgs<TKey, TValue>(key, value)); } }
using ObserverFive; string key = "hello"; string value = "world"; IObserverableDictionary<string,string> dictionary = new ObserverableDictionary<string,string>(); dictionary.NewItemAdded += Validate; dictionary.Add(key, value); void Validate(object sender, DictionaryEventArgs<string,string> args) { Console.WriteLine($"{args.Key} {args.Value}"); }
事件说完了！再回头看看观察者设计模式。
微软已经很重视观察者模式这个设计，把IObserver、IObservable集成到runtime里面去了，也就是基类库里面。aspnetcore框架也有用到这个，比如日志模块。所以感觉有必要了解一下，放个小例子作为结束：
internal class Message { public string Notify { get; set; } }
internal class Teacher : IObservable<Message> { private readonly List<IObserver<Message>> _observers; public Teacher() { _observers = new List<IObserver<Message>>(); } public IDisposable Subscribe(IObserver<Message> observer) { _observers.Add(observer); return new Unsubscribe(observer, _observers); } public void SendMessage(string message) { foreach (var observer in _observers) { observer.OnNext(new Message() { Notify = "message" }); } } public void OnCompleted() { foreach (var observer in _observers) { observer.OnCompleted(); } _observers.Clear(); } } internal class Unsubscribe:IDisposable { private readonly IObserver<Message> _observer; private readonly List<IObserver<Message>> _observers; public Unsubscribe(IObserver<Message> observer, List<IObserver<Message>> observers) { this._observers = observers; this._observer = observer; } public void Dispose() { if(_observers.Contains(_observer)) _observers.Remove(_observer); } }
internal abstract class Student : IObserver<Message> { private string name; public Student(string name) { this.name = name; } private IDisposable _unsubscribe; public virtual void OnCompleted() { Console.WriteLine("放学了..."); } public virtual void OnError(Exception error) { Console.WriteLine("生病了..."); } public virtual void OnNext(Message value) { Console.WriteLine($"大家好: 我是 {name} -_- "); Console.WriteLine($"老师说:{value.Notify}"); } public virtual void Subscribe(IObservable<Message> obserable) { if (obserable != null) _unsubscribe = obserable.Subscribe(this); } }
internal class StudentZhang : Student { public StudentZhang(string name) : base(name) { } } internal class StudentLi : Student { public StudentLi(string name) : base(name) { } }
using ObserverSeven; Teacher teacher = new Teacher(); teacher.Subscribe(new StudentLi("李逵")); teacher.Subscribe(new StudentZhang("张麻子")); teacher.SendMessage("明天放假"); teacher.OnCompleted(); //这里学生是多个，也定义可以多个老师，实现多对多关系
示例代码：
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