AngelScript: Using script classes

# Using script classes

When there are multiple objects controlled by the same script implementation it may be favourable to use script classes, rather than global script functions. Using script classes each instance can have it's own set of variables within the class, contrary to the global functions that needs to rely on global variables to store persistent information.

Of course, it would be possible to duplicate the script modules, so that there is one module for each object instance, but that would be impose a rather big overhead for the application. Script classes don't have that overhead, as all instances share the same module, and thus the same bytecode and function ids, etc.

## Instanciating the script class

Before instanciating the script class you need to know which class to instanciate. Exactly how this is done depends on the application, but here are some suggestions.

If the application knows the name of the class, either hardcoded or from some configuration, the class type can easily be obtained by calling the module's GetTypeIdByDecl with the name of the class. The application can also choose to identify the class through some properties of the class, e.g. if the class implements a predefined interface. Then the application can enumerate the class types implemented in the script with GetObjectTypeByIndex and then examine the type through the asIObjectType interface.

A third option, if you're using the script builder add-on, is to use the metadata to identify the class. If you choose this option, use the asIScriptModule to enumerate the declared types and then query the CScriptBuilder for their metadata.

Once the object type is known you create the instance by calling the class' factory function, passing it the necessary arguments, e.g. a pointer to the application object which the script class should be bound to. The factory function id is found by querying the asIObjectType.

    // Get the object type
    asIScriptModule *module = engine->GetModule("MyModule");
    asIObjectType *type = engine->GetObjectTypeById(module->GetTypeIdByDecl("MyClass"));

    // Get the factory function id from the object type
    int factoryId = type->GetFactoryIdByDecl("MyClass @MyClass(int)");

The factory function is called as a regular global function and returns a handle to the newly instanciated class.

## Calling a method on the script class

Calling the methods of the script classes are similar to calling global functions except that you obtain the function id from the asIObjectType, and you must set the object pointer along with the rest of the function arguments.

    // Obtain the id of the class method
    int funcId = type->GetMethodIdByDecl("void method()");

    // Prepare the context for calling the method
    ctx->Prepare(funcId);

    // Set the object pointer
    ctx->SetObject(obj);

    // Execute the call
    ctx->Execute();