/// <summary>
/// Loads the Object identified by the given Node as an object of the given type based on information from the Configuration Graph
/// </summary>
/// <param name="g">Configuration Graph</param>
/// <param name="objNode">Object Node</param>
/// <param name="targetType">Target Type</param>
/// <returns></returns>
/// <remarks>
/// <para>
/// Callers of this method should be careful to check that the Object returned is of a usable type to them. The Target Type parameter does not guarantee that the return value is of that type it is only used to determine which registered instances of <see cref="IObjectLoader">IObjectLoader</see> are potentially capable of creating the desired Object
/// </para>
/// <para>
/// Callers should also take care that any Objects returned from this method are disposed of when the caller no longer has a use for them as otherwise the reference kept in the cache here will cause the Object to remain in-memory consuming resources
/// </para>
/// </remarks>
public static Object LoadObject(IGraph g, INode objNode, Type targetType)
{
if (targetType == null) throw new DotNetRdfConfigurationException("Unable to load the Object identified by the Node '" + objNode.ToString() + "' as a null target type was provided - this may be due to a failure to specify a fully qualified type name with the dnr:type property for this object");
if (objNode == null) throw new DotNetRdfConfigurationException("Unable to load an Object as a null Object Node was provided");
if (objNode.NodeType == NodeType.GraphLiteral || objNode.NodeType == NodeType.Literal)
{
throw new DotNetRdfConfigurationException("Unable to load an Object as the Object Node was not a URI/Blank Node as required");
}
//Use an Object caching mechanism to avoid instantiating the same thing multiple times since this could be VERY costly
CachedObjectKey key = new CachedObjectKey(objNode, g);
if (_cache.ContainsKey(key))
{
if (_cache[key] == null)
{
//This means we've begun trying to cache the Object but haven't loaded it yet
//i.e. we've encountered an indirect circular reference or the caller failed to check
//for direct circular references with the CheckCircularReference() method
throw new DotNetRdfConfigurationException("Unable to load the Object identified by the Node '" + objNode.ToString() + "' as we have already started trying to load this Object which indicates that your Configuration Graph contains a circular reference");
}
else if (_cache[key] is UnloadableObject)
{
//We don't retry loading if we fail
throw new DotNetRdfConfigurationException("Unable to load the Object identified by the Node '" + objNode.ToString() + "' as previous attempt(s) to load the Object failed. Call ClearCache() before attempting loading if you wish to retry loading");
}
else
{
//Return from Cache
return _cache[key];
}
}
else
{
_cache.Add(key, null);
}
Object temp = null;
//Try and find an Object Loader that can load this object
try
{
foreach (IObjectFactory loader in _factories)
{
if (loader.CanLoadObject(targetType))
{
if (loader.TryLoadObject(g, objNode, targetType, out temp)) break;
}
}
}
catch (DotNetRdfConfigurationException)
{
_cache[key] = new UnloadableObject();
throw;
}
catch (Exception ex)
{
_cache[key] = new UnloadableObject();
throw new DotNetRdfConfigurationException("Unable to load the Object identified by the Node '" + objNode.ToString() + "' as an error occurred in the Object Loader which attempted to load it", ex);
}
//Error or return
if (temp == null) throw new DotNetRdfConfigurationException("Unable to load the Object identified by the Node '" + objNode.ToString() + "' as an instance of type '" + targetType.ToString() + "' since no Object Loaders are able to load this type");
_cache[key] = temp;
return temp;
}
/// <summary>
/// Gets whether the given Object has already been loaded and cached
/// </summary>
/// <param name="g">Configuration Graph</param>
/// <param name="objNode">Object Node</param>
/// <returns></returns>
/// <remarks>
/// If this returns true then loading that object again should be essentially instantaneous as it will come from the cache
/// </remarks>
public static bool IsCached(IGraph g, INode objNode)
{
CachedObjectKey key = new CachedObjectKey(objNode, g);
return _cache.ContainsKey(key);
}