7.9. Component-driven events
Seam components interact by calling each other's methods. Stateful components can even implement the observer/observable pattern. However, to enable more loosely-coupled interaction, Seam provides component-driven events.
We specify event listeners (observers) in
components.xml.
<components>
<event type="hello">
<action execute="#{helloListener.sayHelloBack}"/>
<action execute="#{logger.logHello}"/>
</event>
</components>
Here, the event type is an arbitrary string.
When an event occurs, the actions registered for that event will be called in the order they appear in
components.xml. Seam provides a built-in component to raise events.
@Name("helloWorld")
public class HelloWorld {
public void sayHello() {
FacesMessages.instance().add("Hello World!");
Events.instance().raiseEvent("hello");
}
}
You can also use an annotation, like so:
@Name("helloWorld")
public class HelloWorld {
@RaiseEvent("hello")
public void sayHello() {
FacesMessages.instance().add("Hello World!");
}
}
This event producer is not dependent upon event consumers. The event listener can now be implemented with absolutely no dependency upon the producer:
@Name("helloListener")
public class HelloListener {
public void sayHelloBack() {
FacesMessages.instance().add("Hello to you too!");
}
}
The method binding defined above in
components.xml maps the event to the consumer. If you prefer, you can also do this with annotations:
@Name("helloListener")
public class HelloListener {
@Observer("hello")
public void sayHelloBack() {
FacesMessages.instance().add("Hello to you too!");
}
}
If you are familiar with component-driven events, you may be wondering about event objects. In Seam, event objects do not need to propagate state between the event producer and listener. State is held in the Seam contexts, and shared between components. However, if you do want to pass an event object, you can do so:
@Name("helloWorld")
public class HelloWorld {
private String name;
public void sayHello() {
FacesMessages.instance().add("Hello World, my name is #0.", name);
Events.instance().raiseEvent("hello", name);
}
}@Name("helloListener")
public class HelloListener {
@Observer("hello")
public void sayHelloBack(String name) {
FacesMessages.instance().add("Hello #0!", name);
}
}
