委托与事件

委托

class Program
{
    static void Main(string[] args)
    {
        Publisher publisher = new Publisher("篮球先锋报");

        Observer observerA = new Observer("老A");
        publisher.Magazine += observerA.RecvMagazine;

        Observer observerB = new Observer("老B");
        publisher.Magazine += observerB.RecvMagazine;

        publisher.PublishMagezine();
        //或者使用下面的方式  区别就是一个在定义的内部触发，一个在外部触发
        publisher.Magazine?.Invoke(publisher.magazineName);
        Console.ReadKey();
    }
}

public class Observer
{
    private string name;
    public Observer(string name)
    {
        this.name = name;
    }

    public void RecvMagazine(string message)
    {
        Console.WriteLine($"{this.name} recv {message}, 仔细读了一番");
    }
}

public class Publisher
{
    public string magazineName;
    public Publisher(string magazineName)
    {
        this.magazineName = magazineName;
    }

    public delegate void MagazineDelegate(string message);
    public MagazineDelegate Magazine;

    public void PublishMagezine()
    {
        Magazine?.Invoke(this.magazineName);
    }
}

事件

class Program
{
    static void Main(string[] args)
    {
        Publisher publisher = new Publisher("篮球先锋报");

        Observer observerA = new Observer("老A");
        publisher.Magazine += observerA.RecvMagazine;

        Observer observerB = new Observer("老B");
        publisher.Magazine += observerB.RecvMagazine;

        publisher.PublishMagezine();
        //下面的方式会出现编译错误  只允许在定义的内部触发，不允许在外部触发
        publisher.Magazine?.Invoke(publisher.magazineName);
        Console.ReadKey();
    }
}

public class Observer
{
    private string name;
    public Observer(string name)
    {
        this.name = name;
    }

    public void RecvMagazine(string message)
    {
        Console.WriteLine($"{this.name} recv {message}, 仔细读了一番");
    }
}

public class Publisher
{
    public string magazineName;
    public Publisher(string magazineName)
    {
        this.magazineName = magazineName;
    }

    public delegate void MagazineDelegate(string message);
    public event MagazineDelegate Magazine;

    public void PublishMagezine()
    {
        Magazine?.Invoke(this.magazineName);
    }
}

经典面试题

猫叫、老鼠跑了，主人醒来了

class Program
{
    static void Main(string[] args)
    {
        Cat cat = new Cat();
        Mouse m = new Mouse(cat);
        People p = new People(cat);
        cat.Scream();
    }
}

public class Cat
{
    public delegate void ScreamHandler();
    public event ScreamHandler OnScream;

    public void Scream()
    {
        Console.WriteLine("猫叫了一声");
        OnScream?.Invoke();
    }

}

public class Mouse
{
    public Mouse(Cat c)
    {
        c.OnScream += () => { Console.WriteLine("老鼠跑了"); };
    }
}

public class People
{
    public People(Cat c)
    {
        c.OnScream += () => { Console.WriteLine("主人醒来了"); };
    }
}

//输出：
//猫叫了一声
//老鼠跑了
//主人醒来了

委托与事件的区别

1. 事件基于委托，但并非委托 可以把事件看成委托的代理。在使用者看来，只有事件，而没有委托。
2. 事件是对委托的包装 保护委托字段，对外不开放。所以外部对象没法直接操作委托。提供了Add和Remove方法，供外部对象订阅事件和取消事件。事件的处理方法在对象外部定义，而事件的执行是在对象的内部。至于事件的触发，何时何地无所谓。

什么时候使用委托与事件？

如果一个委托不需要再其定义的类之外被触发，那么就可以将其转化为事件，这样可以保证它不会在外部被随意触发。