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 for 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.

((EDITOR: This page needs the following improvement: Check whether might be able to move some of the following into DOM guide: Resolving URIs, or maybe some is already there and can be removed from here. ))

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) (where available) 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 normalizes a URI string. It is called when attempting to resolve the URI, in the copy constructor, when the noFrag flag is true in the constructor, when attempting to makeRelativePath, during resolveURI. ((EDITOR: This page needs the following improvement: If this is a list of 5 different things, change to bulleted list. If fewer than 5, make into list with items combined appropriately. ))

It normalizes the URI string against one of (listed in priority):

• The URI string passed in as an argument
• The documentURI (taken from the container element if present)
• The static baseURI (cwd)

The DOM always operates on fully normalized absolute URIs.

resolveElement calls daeURIResolver attemptResolveElement.

The URI state returns various URI status values, including:

• "empty": A default constructed URI.
• "loaded": A URI string exists but it hasn’t been normalized.
• "pending": The URI is normalized but hasn't yet been resolved.
• "success", "failed_unsupprted_protocol", "failed_file_not_found", "failed_id_not_found", "failed_missing_containter", and "failed_external_document": Possible states after attemptResolve.

((EDITOR: This page needs the following improvement: DOM guide: Resolving URIs mentions states like "uri_pending, uri_failed_*", and so on. Are these the same ones? If so, fix either these or those to correct values. Also, can we replace "the URI state returns" with "The daeURI::getState method returns"? ))

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 control for whether URI resolution 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.