Begins adding a new cache item.
key
Unique item identifier.
regionName
Optional region name.
readWrite
true if the read and write access to the URI is requested; otherwise, false if only read access is requested.
Returns URI to the path holding the data for the item.
Cache consumers should call this method only if DefaultCacheCapabilities.ExternalResources is supported by this cache (by checking the DefaultCacheCapabilities property.
The LEADTOOLS Document classes uses this method to quickly add items. It speeds up access and updating the data if the value contain large amount of data such as an image file.
After this method returns, EndAddExternalResource<T> must be called to either commit this item into the cache or discard it.