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。
