Asynchronious call Delegate

参见 http://blogs.yodersolutions.com/net/?p=17
当我们声明了Delegate 变量(比如FooDelegate), 我们可以使用下面两种方法,来触发这个Delegate,
1. FooDelegate.Invoke(), 或者直接是FooDelegate().
2. FooDelegate.BeginInvoke().
第一种调用方法是同步执行这个delegate.
而第二种方式是异步执行这个delegate, 这个delegate会在另一个线程中被执行, 当前线程将继续执行其它代码. 需要说明的是, 一定要确保这个delegate有机会调用了EndInvoke(), 否则会有资源和内存泄漏.
异步调用的一个简单示例是:
subscriber.BeginInvoke(this, new EventArgs(), Callback, subscriber);
其中第3个参数是回调函数, 它的签名是AsynCallback delegate.当subscriber被执行完后, Callback会被调用.
其中第4个参数一定要是subscriber本身,或者是subscriber本身的一个wrapper, 这样在callback函数体中, 就能访问到subscriber本身来EndInvoke.

    class Program
    {
        static void Main(string[] args)
        {
            EventPublisher eventPublisher = new EventPublisher();
            eventPublisher.FooEvent += new EventHandler(eventPublisher_FooEvent1);
            eventPublisher.FooEvent += new EventHandler(eventPublisher_FooEvent2);
            Stopwatch sw = new Stopwatch();
            sw.Start();
            eventPublisher.AsynFireFooEvent();
            Console.WriteLine(sw.ElapsedMilliseconds.ToString());
            Console.ReadKey();
        }

        static void eventPublisher_FooEvent1(object sender, EventArgs e)
        {
            Thread.Sleep(100);
        }
        static void eventPublisher_FooEvent2(object sender, EventArgs e)
        {
            Thread.Sleep(200);
        }
    }


    
    class EventPublisher
    {
        public event EventHandler FooEvent; 


        /// <summary>
        /// synchronized invoke the all the subscriber services. 
        /// The process will return until all delegates executions are over.
        /// </summary>
        public void FireFooEvent()
        {
            if (FooEvent != null)
                FooEvent(this, new EventArgs());
        }


        /// <summary> 
        /// Raising events asynchronously lets the event subscribers do their processing in another thread 
        /// and allows the thread that raised the event to continue processing.
        /// </summary>
        public void AsynFireFooEvent()
        {
            if (FooEvent != null)
            {
                Delegate[] subscribers =  FooEvent.GetInvocationList();
                foreach (EventHandler subscriber in subscribers)
                {
                    //the 3rd arg should be a AsyncCallback type delegate. When one subscriber service will completed, the AsyncCallback method will be raised. 

                    //the 4th arg should be the exact subscriber or subscriber wrapper, so that in callback function, 
                    // we can access the subcriber to call EndInvoke()
                    subscriber.BeginInvoke(this, new EventArgs(), Callback, subscriber);
                }
            }
        }



        void Callback(IAsyncResult ar)
        {
            EventHandler raisedEvent = (EventHandler)ar.AsyncState;
            raisedEvent.EndInvoke(ar);
        }
    }