Date: Sat, 20 Dec 1997 04:30:26 +0300
To: chris@marrin.com
From: Dmitri Kondratiev <dima@paragraph.com>
Subject: Browser bean (was: Re: Updated EAI Proposal available)
Cc: vrml-eai@vrml.org
Sender: vrml-eai-approval@vrml.org

Chris Marrin wrote:

>As far as JavaBeans go, I think this layer atop the EAI, rather than
>being integrated into it.
>

Why you think that should be done one level up ? What is so special about
beans ? In fact, beans are  Java classes that follow property access
patterns and conform to particular event model. Thinking about beans as
"visual components only" is a sad and wide-spread misunderstanding :.(
Let's assume for a moment that Browser is a bean. Communication with it
becomes much easier both in one address space and from "external
environment". To say more this communication becomes *uniform*. Besides,
which is even more important, being a bean allows to be manipulated in bean
composition frameworks, such as Visual Java Script, which in turn implies
easy integration in HTML docs. Not bad, I think...

Communication is straightforward :

Those objects interested in Browser object are its "listeners". Browser
fires BrowserEvent-s. 
I hope that following below code snippet will illustrate the main idea well
enough.

//Base event class :
vrml.external.BrowserEvent extends java.util.EventObject;

//The following events may be defined :

// browser fires this event upon completion of loading of the base file.
vrml.external.InitializeEvent            extends BrowserEvent;

// browser fires this event when the scene is about to be unloaded.
vrml.external.ShutdownEvent            extends BrowserEvent;

// browser fires this event when world was just replaced
vrml.external.NewWorldEvent          extends BrowserEvent;

// browser fires this event when event group starts 
vrml.external.BeginUpdateEvent            extends BrowserEvent;

// browser fires this event when event group ends 
vrml.external.EndUpdateEvent              extends BrowserEvent;

/**
Browser also fires an event whenever it is generated from  eventOut
requested by some listener. From this EventOutChanged event listener should
be able to find out the DEF-ed node name/identifier that fired this event. 
*/

vrml.external.EventOutChanged              extends BrowserEvent;

/*** Listening for events ***/

//Listener interface
public interface vrml.external.BrowserEventListener {
    public void handleBrowserEvents(BrowserEvent evt);
}

//Somebody interested in EAI Browser :
class Foo implements vrml.external.BrowserEventListener {
  
  // register with browser
  public Foo() {
    Browser b = new Browser(..);
    b.addBrowserEventListener(this);
  }

  public void handleBrowserEvents(BrowserEvent evt) {

    if (evt instanceof InitializeEvent ) { //....// }
    else if (evt instanceof ShutdownEvent { //....// }
    else if (evt instanceof NewWorldEvent { //....// }
    else if (evt instanceof BeginUpdateEvent { //....// }
    ...
    else if (evt instanceof EventOutChanged {
       String defName = evt.getNodeDefName();
       ...
    }
  }
}

Being based on "message passing" instead of "message call" mechanism, Bean
event model has one great advantage : 
One can use it both for components in the same address space and in
different spaces connected by some protocol. Note, that going for bean
model, makes Browser interface *really independent* from protocol that
implements it while remaining *well defined*.

Dima





-----------------
Dmitri Kondratiev
dima@paragraph.com
102401.2457@compuserve.com
http://www.geocities.com/SiliconValley/Lakes/3767/
tel: 07-095-464-9241

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