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The Prism Library provides an event mechanism that enables communication between loosely coupled components in the application. This mechanism, based on the event aggregator service, allows publishers and subscribers to communicate through events without having a direct reference to each other. The EventAggregator provides multicast publish/subscribe functionality. This means there can be multiple publishers that raise the same event and there can be multiple subscribers listening to the same event. Consider using the EventAggregator to publish an event across modules and when sending a message between business logic code, such as controllers and presenters. One example of this, from the Stock Trader RI, is when the Process Order button is clicked and the order successfully processes. In this case, other modules need to know that the order is successfully processed so that they can update their views. Events created with the Prism Library are typed events. This means that you can take advantage of compile-time type checking to detect errors before you run the application. In the Prism Library, the EventAggregator allows subscribers or publishers to locate a specific EventBase. The event aggregator service also allows for multiple publishers and multiple subscribers, as shown in the following illustration.
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CompositePresentationEvent CompositePresentationEvent
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Subscriber
About .NET Framework Events Using .NET Framework events is the most simple and straightforward approach for communication between components if loose coupling is not a requirement. Events in the .NET Framework implement the Publish-Subscribe pattern. However, to subscribe to an object, you need a direct reference to that object, which, in composite applications, typically resides in another module. This results in a tightly coupled design. Therefore, in a loosely coupled application, .NET Framework events are used for communication within modules instead of between modules. If you use .NET Framework events, you have to be very careful of memory leaks, especially if you have a non-static or short-lived component that subscribes to an event in a static or longer-lived component. If you do not unsubscribe the subscriber, it will be kept alive by the publisher, and this will prevent it from being garbage-collected.
communication bet ween loosely coupled components
The EventAggregator class is offered as a service in the container and can be retrieved through the IEventAggregator interface. The event aggregator is responsible for locating or building events and for keeping a collection of the events in the system.
C# public interface IEventAggregator { TEventType GetEvent<TEventType>() where TEventType : EventBase; }
IEventAggregator
The EventAggregator constructs the event during its first access if it has not already been constructed. This relieves the publisher or subscriber from needing to determine whether the event is available. The real work of connecting publishers and subscribers is done by the CompositePresentationEvent class. This is the only implementation of the EventBase class that is included in the Prism Library. This class maintains the list of subscribers and handles event dispatching to the subscribers. The CompositePresentationEvent class is a generic class that requires the payload type to be defined as the generic type. This helps enforce, at compile time, that publishers and subscribers provide the correct methods for successful event connection. The following code shows a partial definition of the CompositePresentationEvent class.
C# CompositePresentationEvent.cs public class CompositePresentationEvent<TPayload> : EventBase { ... public SubscriptionToken Subscribe(Action<TPayload> action); public SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption); public SubscriptionToken Subscribe(Action<TPayload> action, bool keepSubscriberReferenceAlive) public virtual SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption, bool keepSubscriberReferenceAlive); public virtual SubscriptionToken Subscribe(Action<TPayload> action, ThreadOption threadOption, bool keepSubscriberReferenceAlive, Predicate<TPayload> filter); public virtual void Publish(TPayload payload); public virtual void Unsubscribe(Action<TPayload> subscriber); public virtual bool Contains(Action<TPayload> subscriber) ... }
CompositePresentationEvent
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The following sections describe how to use the IEventAggregator interface to create, publish, and subscribe to CompositePresentationEvent. Creating an Event The CompositePresentationEvent<TPayload> is intended to be the base class for application- or module-specific events. TPayLoad is the type of the event s payload. The payload is the argument that will be passed to subscribers when the event is published. For example, the following code shows the TickerSymbolSelectedEvent in the Stock Trader Reference Implementation (Stock Trader RI). The payload is a string containing the company symbol. Notice how the implementation for this class is empty.
C# public class TickerSymbolSelectedEvent : CompositePresentationEvent<string>{}
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