Prism for WPF 第一讲 Event机制

在本篇文章中主要讲解在Prism中模块与模块之间事件关联的机制。在这里牵涉到三个名词：事件定义，事件发布，事件订阅。

第一：事件定义

在公共类库中定义事件。

①没有参数事件

public class NullClass { }

public class NullableEvent : CompositePresentationEvent<NullClass> { }

②简单类型参数事件

public class MessageAddedEvent : CompositePresentationEvent<string> { }

③复合类型参数事件

　 public class News
    {
        /// <summary>
        /// 标题。
        /// </summary>
        public string Title { get; set; }
        /// <summary>
        /// 内容。
        /// </summary>
        public string Content { get; set; }
    }

public class NewsAddedEvent : CompositePresentationEvent<News> { }

第二：事件发布(以下为示例代码)

using Microsoft.Practices.Composite.Events;
using PrismEvent.Infrastructure;
using System.Windows;
using System.Windows.Controls;

namespace PrismEvent.Publisher
{
    /// <summary>
    /// PublisherView.xaml 的交互逻辑
    /// </summary>
    public partial class PublisherView : UserControl
    {
        IEventAggregator eventAggregator;
        /// <summary>
        /// 构造函数。
        /// </summary>
        public PublisherView()
        {
            InitializeComponent();
        }
        /// <summary>
        /// 构造函数。
        /// </summary>
        /// <param name="eventAggregator">事件聚合。</param>
        public PublisherView(IEventAggregator eventAggregator)
            : this()
        {
            this.eventAggregator = eventAggregator;
        }
        /// <summary>
        /// 简单参数。
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnSimpleParam_Click(object sender, RoutedEventArgs e)
        {
            string message = "Hellow world";
            eventAggregator.GetEvent<MessageAddedEvent>().Publish(message);
        }
        /// <summary>
        /// 复合参数。
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnCompositeParam_Click(object sender, RoutedEventArgs e)
        {
            News news = new News()
            {
                Title = "Bao's demo was published.",
                Content = "This message is wonderful."
            };
            eventAggregator.GetEvent<NewsAddedEvent>().Publish(news);
        }

        private void btnWithoutParam_Click(object sender, RoutedEventArgs e)
        {
            eventAggregator.GetEvent<NullableEvent>().Publish(null);
        }

        private void btnStandard_Click(object sender, RoutedEventArgs e)
        {
            string message = "Open seasame!";
            eventAggregator.GetEvent<StandardMessageAddedEvent>().Publish(message);
        }
    }
}

第三：事件订阅

using Microsoft.Practices.Composite.Events;
using Microsoft.Practices.Composite.Presentation.Events;
using PrismEvent.Infrastructure;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace PrismEvent.Subscriber
{
    /// <summary>
    /// Subscriberview.xaml 的交互逻辑
    /// </summary>
    public partial class Subscriberview : UserControl
    {
        IEventAggregator eventAggregator;
        private SubscriptionToken subscriptionToken;
        public Subscriberview()
        {
            InitializeComponent();
        }

        public Subscriberview(IEventAggregator eventAggregator)
            : this()
        {
            this.eventAggregator = eventAggregator;
            this.eventAggregator.GetEvent<MessageAddedEvent>().Subscribe(ShowMessage);
            this.eventAggregator.GetEvent<NewsAddedEvent>().Subscribe(ShowMessage);
            this.eventAggregator.GetEvent<NullableEvent>().Subscribe(DoSomething);
            StandardMessageAddedEvent messageAddedEvent = eventAggregator.GetEvent<StandardMessageAddedEvent>();
            if (subscriptionToken != null)
            {
                messageAddedEvent.Unsubscribe(subscriptionToken);
            }
            subscriptionToken = messageAddedEvent.Subscribe(ShowMessage2, ThreadOption.UIThread, true, Filter);
        }

        private bool Filter(string message)
        {
            return true;
        }

        private void ShowMessage2(string message)
        {
            this.tbSimpleParam.Text = message;
            StandardMessageAddedEvent messageAddedEvent = eventAggregator.GetEvent<StandardMessageAddedEvent>();
            if (subscriptionToken != null)
            {
                messageAddedEvent.Unsubscribe(subscriptionToken);
            }
        }

        private void DoSomething(NullClass obj)
        {

        }
        /// <summary>
        /// 显示新闻消息。
        /// </summary>
        /// <param name="news">新闻对象。</param>
        private void ShowMessage(News news)
        {
            this.tbSimpleParam.Text = news.Title;
        }
        /// <summary>
        /// 显示消息。
        /// </summary>
        /// <param name="message">消息内容。</param>
        private void ShowMessage(string message)
        {
            this.tbSimpleParam.Text = message;
        }

    }
}

对于事件的订阅，正规的写法如下所示：

StandardMessageAddedEvent messageAddedEvent = eventAggregator.GetEvent<StandardMessageAddedEvent>();

            if (subscriptionToken != null)

            {

                messageAddedEvent.Unsubscribe(subscriptionToken);

            }

            subscriptionToken = messageAddedEvent.Subscribe(ShowMessage2, ThreadOption.UIThread, true, Filter);

在上面的代码中，我们看到这样的顺序：

1.先从IEventAggregator中获取注册到其中的事件对象，也就是messageAddedEvent;

StandardMessageAddedEvent messageAddedEvent=eventAggregator.GetEvent<StandardMessageAddedEvent>();

2.这个messageAddedEvent对象具有Subscribe方法，它返回一个SubscriptionToken类型的对象，用来标志事件触发后，在订阅一方所调用的方法:

subscriptionToken=messageAddedEvent.Subscribe(ShowMessage2,ThreadOption.UIThread,true,Filter);

3.在第2步之前，我们需要检查SubscriptionToken对象是否为空，否则就要注销之前的messageAddedEventHandler方法。

if(subscriptionToken!=null)

{

messageAddedEvnet.Unsubscribe(subscriptionToken);

}

之所以这么写，是因为系统弱引用后的垃圾自动回收时间，于是，将Subscribe方法的第三个参数设置为true(默认为false,弱引用，垃圾自动回收)。这样，订阅的就是强引用了，Prism就会要求我们必须手动取消事件的订阅。

在Subscribe方法中，第二个参数是ThreadOption枚举。值有：

publisherThread     //在publisherThread所在线程上执行，默认值。

UIThread               //在UI线程上触发。

BackgroundThread //在后台线程上异步调用。

第四个参数是一个bool类型的委托，它以事件传递的消息类型作为参数。

subscriptionToken=messageAddedEvent.Subscribe(ShowMessage2,ThreadOption.UIThread,true,Filter);

public bool Filter(string text){

return true;

}

注意：Filter方法总会在ShowMessage2方法前执行。如果返回true，则执行ShowMessage2。若为false则不执行。

在此，Prism的事件机制告一段落。