DOM resolver subsystem

Summary: The resolver subsystem manages the resolution of URIs based on a URI's scheme and filename extensions.

Purpose

The resolver subsystem allows custom URI resolvers to be written and added to the COLLADA DOM. These resolvers can be used to load various types of external references-- for example, binary representations of mesh data--or to provide access to different data systems--for example, using a database to access elements.

Default behavior

By default, the COLLADA DOM has only two resolvers:

• The main resolver, daeLIBXMLResolver, resolves elements from COLLADA documents (.dae or .xml) with the file or http URI scheme. This can resolve elements from the same document or external document references.

• The newly added DOM raw resolver class, daeRawResolver, resolves only .raw files with the file URI scheme.

Instantiating an object of a class that inherits from daeURIResolver adds a custom resolver to the COLLADA DOM.

How it works

The base class daeURIResolver contains a static array named _KnownResolvers that contains resolvers. The constuctor for daeURIResolver adds a reference to this to this list of resolvers. When a DOM URI class attempts to resolve itself, it searches through the _KnownResolvers list for a resolver that matches its URI scheme and file extension. When an appropriate resolver is found, the resolver is then asked to resolve the current URI.

Related classes and functions

• DOM URI class: The class used to represent URIs in a COLLADA document.
• DOM resolver class: An interface to inherit from for creating URI resolvers.

DOM URI class (daeURI)

The DOM URI class, daeURI, is part of the DOM resolver subsystem. It represents a URI data type in a COLLADA document. Some internal notes about the class:

URI strings are parsed in internalSetURI.

The baseURI is constructed from the current working directory (cwd) when the daeURI( int ) constructor is called.

The daeElement container is the element that holds the daeURI as either an attribute or value.

The validate method resolves relative URI references. It is called in the following situations:

• When attempting to resolve the URI
• In the daeURI::daeURI(daeString uriString, daeBool nofrag) constructor, when noFrag is set to true
• When daeURI::makeRelativePath is called

daeURI::validate resolves relative URI references against one of (listed in priority):

• The URI string passed in as an argument
• The xml:base attribute on the root <COLLADA> element of the containing document
• The documentURI (taken from the container element if present)
• The current working directory

The DOM always operates on absolute URIs.

daeURI::resolveElement calls daeURIResolver::attemptResolveElement.

The daeURI::getState method returns various URI status values, including:

• uri_empty: A default constructed URI.
• uri_loaded: A URI reference exists, but it may still need to be resolved to a full URI.
• pending: We have a full URI, but the URI hasn't been resolved to obtain a daeElement yet.
• success, failed_unsupprted_protocol, failed_file_not_found, failed_id_not_found, failed_missing_containter, and failed_external_document: Possible states after attemptResolve.

DOM resolver class (daeURIResolver)

Some internal notes about this class:

• daeLibxmlResolver and daeRawResolver inherit from the DOM resolver base class, daeURIResolver.

• _loadExternalDocuments is a flag that an application can use to control whether URI resolution automatically loads documents.

• attemptResolveElement iterates over the list of _KnownResolvers and, if the protocol (URI scheme) and document extension are supported, it calls resolveElement on the resolver.

Note: attemptResolveURI is never called. It is dead.

Future: This is a good system but the program flow should be structured differently. See DOM future work.