How to add functions to the scripting interface

First a little bit about good API-design. The (mostly) property-based object model in InDesign is a great example how to create a well-designed easy API. Take a look at this guide (PDF, 3.2 MB).

You start with a scaffolding-like script:

Go to the scripterng plug-in directory and call
./new_api.py test
This will generate two files:

• api_test.h
• api_test.cpp

from api_example.{h,cpp} as a template.

The files are added to CMakeLists.txt and the header file is included in scripterngimpl.h

Inside these files there is a class TestAPI defined. By default the class is a child of ScriperNGImpl and has the object-name Test. So the class will be automatically available under the name ScriperNG.Test from Python or QtScript.

The class is created at start-up in scriperngimpl.cpp but you might want to change that. Look for the activeDocument Q_PROPERTY in scripterngimpl to see how to create an object on demand.

To write a function which is available for scripting you have to write a public slot method. Allowed as parameters and as return types are basic types (int, bool, ..) and Qt objects. Pointers to pointers and complicated stuff like that does not work - but would not make any sense anyway...
One caveat: If you want to return instances of QObject or QWidget you have to declare the base class QObject or QWidget as return type. The correct typecasting is done later at runtime via the Qt Meta-Object system.

That's all. You do not have to know anything about the scripting language or about any binding api to do parameter conversation and validation. This is done by ScripterNG.