Learning design patterns from Prism: Event Aggregator Mode

Event Aggregator mode definition: Channel Events simplify the registration of clients from multiple objects through a single object.

Structure:

Event Aggregator mode of Prism: Event Aggregator allows multiple objects to locate, publish, and subscribe to events.

We can learn how to manage and transmit event messages from EventAggregator and CompositePresentationEvent In the Prism source code.

  1     public interface IEventAggregator  2     {  3         TEventType GetEvent<TEventType>() where TEventType : EventBase, new();  4     }  5   6     public class EventAggregator : IEventAggregator  7     {  8         private readonly Dictionary<Type, EventBase> events = new Dictionary<Type, EventBase>();  9  10         public TEventType GetEvent<TEventType>() where TEventType : EventBase, new() 11         { 12             EventBase existingEvent = null; 13  14             if (!this.events.TryGetValue(typeof(TEventType), out existingEvent)) 15             { 16                 TEventType newEvent = new TEventType(); 17                 this.events[typeof(TEventType)] = newEvent; 18  19                 return newEvent; 20             } 21             else 22             { 23                 return (TEventType)existingEvent; 24             } 25         } 26     } 27  28   public class CompositePresentationEvent<TPayload> : EventBase 29     { 30         private IDispatcherFacade uiDispatcher; 31  32         private IDispatcherFacade UIDispatcher 33         { 34             get 35             { 36                 if (uiDispatcher == null) 37                 { 38                     this.uiDispatcher = new DefaultDispatcher(); 39                 } 40  41                 return uiDispatcher; 42             } 43         } 44  45         public SubscriptionToken Subscribe(Action<TPayload> action) 46         { 47             return Subscribe(action, ThreadOption.PublisherThread); 48         } 49  50         public SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption) 51         { 52             return Subscribe(action, threadOption, false); 53         } 54  55         public SubscriptionToken Subscribe(Action<TPayload> action, bool keepSubscriberReferenceAlive) 56         { 57             return Subscribe(action, ThreadOption.PublisherThread, keepSubscriberReferenceAlive); 58         } 59  60         public SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption, bool keepSubscriberReferenceAlive) 61         { 62             return Subscribe(action, threadOption, keepSubscriberReferenceAlive, null); 63         } 64  65         public virtual SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption, bool keepSubscriberReferenceAlive, Predicate<TPayload> filter) 66         { 67             IDelegateReference actionReference = new DelegateReference(action, keepSubscriberReferenceAlive); 68             IDelegateReference filterReference; 69             if (filter != null) 70             { 71                 filterReference = new DelegateReference(filter, keepSubscriberReferenceAlive); 72             } 73             else 74             { 75                 filterReference = new DelegateReference(new Predicate<TPayload>(delegate { return true; }), true); 76             } 77             EventSubscription<TPayload> subscription; 78             switch (threadOption) 79             { 80                 case ThreadOption.PublisherThread: 81                     subscription = new EventSubscription<TPayload>(actionReference, filterReference); 82                     break; 83                 case ThreadOption.BackgroundThread: 84                     subscription = new BackgroundEventSubscription<TPayload>(actionReference, filterReference); 85                     break; 86                 case ThreadOption.UIThread: 87                     subscription = new DispatcherEventSubscription<TPayload>(actionReference, filterReference, UIDispatcher); 88                     break; 89                 default: 90                     subscription = new EventSubscription<TPayload>(actionReference, filterReference); 91                     break; 92             } 93  94  95             return base.InternalSubscribe(subscription); 96         } 97  98         public virtual void Publish(TPayload payload) 99         {100             base.InternalPublish(payload);101         }102 103         public virtual void Unsubscribe(Action<TPayload> subscriber)104         {105             lock (Subscriptions)106             {107                 IEventSubscription eventSubscription = Subscriptions.Cast<EventSubscription<TPayload>>().FirstOrDefault(evt => evt.Action == subscriber);108                 if (eventSubscription != null)109                 {110                     Subscriptions.Remove(eventSubscription);111                 }112             }113         }114 115         public virtual bool Contains(Action<TPayload> subscriber)116         {117             IEventSubscription eventSubscription;118             lock (Subscriptions)119             {120                 eventSubscription = Subscriptions.Cast<EventSubscription<TPayload>>().FirstOrDefault(evt => evt.Action == subscriber);121             }122             return eventSubscription != null;123         }124 125     }

• Subscription
• Cancel subscription
• Release
• Comparison and positioning

Simple usage:

 1 public class Model{ 2     private readonly IEventAggregator eventAggregator; 3  4     this.eventAggregator = eventAggregator; 5     this.eventAggregator.GetEvent<UpdatedEvent>().Subscribe(this.Updated, ThreadOption.UIThread); 6  7         private void Updated() 8         { 9             //Todo Something10         }11 12 }13 14     public class UpdatedEvent: CompositePresentationEvent<IDictionary<string, decimal>>15     {16     }            

 

References:

Http://www.martinfowler.com/eaaDev/EventAggregator.html