Work with Non-COM Properties

The non-COM properties mechanism provides access to object properties at runtime for reading and modification in end-user applications. Unlike the legacy mechanism that uses COM technology, the non-COM properties mechanism is cross-platform. It is recommended for use in newly developed objects, although the legacy mechanism also remains supported by the ODA platform.

Overview

From the user's point of view, a property is a value associated with an object that is accessible by name at runtime.

The non-COM properties mechanism is an extension of the ODA Kernel runtime system. This mechanism allows you to describe properties of objects in a platform-independent way in order to make them accessible from end-user applications.

The non-COM properties mechanism is represented by the following set of classes and structures:

The implementation of the non-COM properties mechanism is located in the RxProperties module.

Get Object Properties

To list properties of a given object, use an iterator. To get an iterator for object properties, call the OdRxMemberQueryEngine::theEngine()->newMemberIterator() method. Each member returned by the iterator can be either an OdRxPropertyBase or OdRxMethod object.


odrxDynamicLinker()->loadModule(RxPropertiesModuleName);
OdRxMemberPtr testCat = OdRxCategory::createObject(L"test", OdRxCategory::rootCategory());
OdRxMemberIteratorPtr pIterator = OdRxMemberQueryEngine::theEngine()->newMemberIterator(testCat); 
while (!pIterator->done())
{
  OdRxMemberPtr pMember = pIterator->current();
  ...
  pIterator->next();
}

You can also find a known property by its name using the OdRxMemberQueryEngine::find() function.


OdRxMemberPtr pMember = pIterator->find(L"ClassName");

Property Value Overview

The OdRxValue class is an instance of the OdRxValueType metaclass and is registered in the class dictionary. The metaclass can contain information about the type such as the enumeration members list for enumerated types. All common C++ types such as int and double are registered automatically. You can register your own types.

For more information about the variant type system, see the following header files:

Kernel/Include/RxValueType.h
Kernel/Include/RxValueTypeUtil.h

Get and Set Property Values

The OdRxProperty object is a single value property object, and it returns OdRxValue via the getValue() method. OdRxValue is a generic variant type; it can contain any C++ type if the corresponding metaclass was declared. The contained C++ type is accessed with a free template function rxvalue_cast<>().


OdRxValue value;
if (eOk == OdRxPropertyPtr(pMember)->getValue(testCat, value))
{
  OdString s = *rxvalue_cast< OdString >(&value);
}

The value of the property can be set via the setValue() method.


OdRxPropertyPtr pProperty = OdRxMemberQueryEngine::theEngine()->find(testCat, L"LocalizedName");
pProperty->setValue(testCat, OdRxValue(L"Category"));

Collection Property Types

There are also several collection types for properties, like OdRxCollectionProperty, OdRxIndexedProperty, OdRxDictionaryProperty, etc. (see RxProperty.h for details).

To get values from collection properties, use the value iterator instead of the getValue() method.


OdDbBlockTablePtr bt = pDb->getBlockTableId().safeOpenObject();
OdRxCollectionPropertyPtr pCollectionProperty = OdRxMemberQueryEngine::theEngine()->find(bt, L"Items");
OdRxValueIteratorPtr pIterator = pCollectionProperty->newValueIterator(bt);
while (!pIterator->done())
{
  OdRxValue value = pIterator->current();
  ...
  pIterator->next();
}

Property Attributes

Additional information about a property can be provided by the attributes attached to it (OdRxMember::attributes). Attributes usage and interpretation is up to the end-user application. Attributes are identified by the type; there can be only one attribute attached of a given type.

Code sample below illustrates how to add attributes to the property of an dbTable object that determines the flow direction of table elements.


OdRxMemberPtr pProperty = OdRxMember::createObject();
pProperty->attributes().add(OdRxDescriptionAttribute::createObject(L"Determines the direction that parts of the table flow."));
pProperty->attributes().add(OdRxCOMAttribute::createObject(L"TableBreakFlowDirection"));

See the list of existing attribute types in RxAttribute.h.

Providing Non-COM Properties for Classes

There are several ways to provide non-COM properties for the class:

For a custom class, the properties should be registered in the class dictionary by passing them as the OdRxMemberCollectionConstructorPtr parameter for the newOdRxClass() class registration function.
To separate the property provider from the class implementation, you can use the odrxSetMemberConstructor() global function.
If you do not provide all the properties for the class but want to support additional custom properties, use the OdRxMemberQueryEngine::addFacetProvider() function to set the interface to extend a class with additional methods or properties.

Examples

For examples of Drawings SDK object properties, see header files in Drawing/Extensions/DbProperties.