The AWeb OO model

Introduction

AWeb internally uses its own object oriented model for representation of data. By means of the plugin interface, this model can be expanded by the plugin modules. This document does not explain object oriented modelling, that is something assumed to be known to the reader.

The OO system used by AWeb is similar to BOOPSI, although it is a completely separate system. The existing BOOPSI image and gadget models were not useable because of AWeb's scrolling and RastPort clipping demands. Building a new model using BOOPSI would have been possible, but I decided to build my own system instead, for the following reasons:

The way BOOPSI implements inheritance is basically very good. If you derive a class from a certain superclass, you can only access the superclasses attributes through its methods. That is perfect when deriving from a foreign class, but if you own the superclass as well, it's a bit of a hassle. One of the very few things I like about C++ is the way you can access your superclasses public attributes as if they were your own.
Building my own central dispatcher allowed me to add a trace option. Simply by specifying a couple of arguments when starting AWeb, I can make a full trace of every single message that is sent around. Filtering is possible by class, or by method, or both. A great help when debugging.

Usage

Dispatcher

Like BOOPSI, every class must have a dispatcher function that handles incoming OO messages.

Invoking methods of other objects is done by calling the Amethod[A]() function, AWebs equivalent of DoMethod[A](). This function forwards the message to the dispatcher of the class.

Inheritance

Deriving a class from a superclass is different from the way it is done in BOOPSI. The object instance data structure of the derived class must start with the data structure for the superclass. This implies that you must know the data structure of the superclass, so either it must be public, or it must be your own private class. It is not possible to inherit from other classes, you should use delegation instead. (Popularly, delegation is having a pointer to another object and forward the messages to it.)

There is no such function as DoSuperMethod(), in the AWeb OO system the central dispatcher does not know what your superclass is. To pass a message to your superclass you must use Amethodas[A](), AWebs version of CoerceMethod[A]().

Implementation

Every object implementation must have a dispatcher to process messages sent to the object.

Synopsis

   retval = Dispatch(object, message)
   D0                A0      A1

   ULONG Dispatch(struct Aobject *, struct Amessage *);

Function

The dispatcher function is called whenever a message is sent to the object. Depending on the message type (method), the object code should take appropriate actions.

Inputs

Parameter	Description
object	A pointer to the object instance that the message was sent to. This will be NULL for the AOM_NEW method.
message	A pointer to the message that was sent to the object.

Results

The value to return depends on the method invoked. See the documentation for the individual methods.