Posts Tagged ‘bus’

Overview

Introduction

Let’s consider a application, JonathanGwtApplication, divided in three main panels

• a panel to select animal name name
• a panel to display, expand and collapse trees of the animal ancestors
• a panel of information to display many indicators (colors, ages, etc.).

An issue we encounter is: how to make the different panels communicate? In more technical terms, how to fire events from a panel to another one?

A first solution would be to declare each panel as listener to the other panels. Indeed, this principle may go further, and declare each component as listener to a list of other components…
Main drawbacks:

• the code becomes hard to read
• adding or removing a component requires to modify many parts of the code
• we don’t follow GWT 2’s “philosophy”, which is to use Handlers rather than Listeners.

Hence, these reasons incited us to provide a global EventBusHandler.

The EventBusHandler concept

The EventBusHandler is a global bus which is aware of all events that should be shared between different panels, and fires them to the right components.
The EventBusHandler is a field of JonathanGwtApplicationContext.

Intrastructure

• lalou.jonathan.application.web.gwt.animal.events.HandledEvent: generic interface for a event. Abstract method:
  [java]EventTypeEnum getEventEnum();[/java]
• lalou.jonathan.application.web.gwt.animal.handler.EventHandler: generic interface for a component able to handle an event. Abstract method:
  [java]void handleEvent(HandledEvent handledEvent);[/java]
• lalou.jonathan.application.web.gwt.animal.handler.EventHandlerBus: the actual bus. As a concrete class, it has two methods:
  [java]/**
  * Fires an event to all components declared as listening to this event
  * event type.
  *
  * @param baseEvent
  */
  public void fireEvent(HandledEvent baseEvent) {
  // …
  }

  /**
  * Adds an listener/handler for the event type given as parameter
  *
  * @param eventTypeEnum
  * @param eventHandler
  * @return The List of handlers for the key given as parameter. This list
  * contains the eventHandler that was given as second parameter
  */
  public List<EventHandler> put(EventTypeEnum eventTypeEnum,
  EventHandler eventHandler) {
  // …
  }[/java]

How to use the bus?

1. Define an event: in JonathanGwtApplication, an event is decribed by two elements:
   • a functionnal entity: eg: “animal”, “food”, “tree node”. The functionnal entity must be isomorph to a technical DTO, eg: AnimalDTO for an entity Animal.(in the scope of this turoriel we assume to have DTOs, even though the entities may ne sufficient)
   • a technical description of the event: “selection changed”, “is expanded”
2. Add an entry in the enum EventTypeEnum. Eg: “ANIMAL_SELECTION_CHANGED“
3. in lalou.jonathan.application.web.gwt.animal.events, create an event, implementing HandledEvent and its method getEventEnum(). The match between EventTypeEnum and DTO is achieved here. Eg:
   [java]public class AnimalSelectionChangedEvent extends
   SelectionChangedEvent<AnimalDTO> implements HandledEvent {

   public AnimalSelectionChangedEvent(
   SelectionProvider<AnimalDTO> provider,
   List<AnimalDTO> selection) {
   super(provider, selection);
   }

   public EventTypeEnum getEventEnum() {
   return EventTypeEnum.ANIMAL_SELECTION_CHANGED;
   }

   }[/java]