This interface represents a content node that loads images.  The interface
 exists to allow getting information on the images that the content node
 loads and to allow registration of observers for the image loads.

 Implementors of this interface should handle all the mechanics of actually
 loading an image -- getting the URI, checking with content policies and
 the security manager to see whether loading the URI is allowed, performing
 the load, firing any DOM events as needed.

 An implementation of this interface may support the concepts of a
 "current" image and a "pending" image.  If it does, a request to change
 the currently loaded image will start a "pending" request which will
 become current only when the image is loaded.  It is the responsibility of
 observers to check which request they are getting notifications for.

 Observers added in mid-load will not get any notifications they
 missed.  We should NOT freeze this interface without considering
 this issue.  (It could be that the image status on imgIRequest is
 sufficient, when combined with the imageBlockingStatus information.)
[scriptable, uuid(e036857e-3417-4812-a5f2-89668a616781)]
interface nsIImageLoadingContent : imgIDecoderObserver


 Request types.  Image loading content nodes attempt to do atomic
 image changes when the image url is changed.  This means that
 when the url changes the new image load will start, but the old
 image will remain the "current" request until the new image is
 fully loaded.  At that point, the old "current" request will be
 discarded and the "pending" request will become "current".
const long UNKNOWN_REQUEST = -1

const long CURRENT_REQUEST = 0

const long PENDING_REQUEST = 1


 Gets the URI of the current request, if available.
 Otherwise, returns the last URI that this content tried to load, or
 null if there haven't been any such attempts.
readonly attribute nsIURI currentURI
 Returns the image blocking status (@see nsIContentPolicy).  This
 will always be an nsIContentPolicy REJECT_* status for cases when
 the image was blocked.  This status always refers to the
readonly attribute short imageBlockingStatus
 loadingEnabled is used to enable and disable loading in
 situations where loading images is unwanted.  Note that enabling
 loading will *not* automatically trigger an image load.
attribute boolean loadingEnabled


 Used to register an image decoder observer.  Typically, this will
 be a proxy for a frame that wants to paint the image.
 Notifications from ongoing image loads will be passed to all
 registered observers.  Notifications for all request types,
 current and pending, will be passed through.

 @param aObserver the observer to register

void addObserver(in imgIDecoderObserver aObserver)
 Enables/disables image state forcing. When |aForce| is PR_TRUE, we force
 nsImageLoadingContent::ImageState() to return |aState|. Call again with |aForce|
 as PR_FALSE to revert ImageState() to its original behaviour.
void forceImageState(in boolean aForce, in long aState)
 forceReload forces reloading of the image pointed to by currentURI

 @throws NS_ERROR_NOT_AVAILABLE if there is no current URI to reload
void forceReload()
 Accessor to get the image requests

 @param aRequestType a value saying which request is wanted

 @return the imgIRequest object (may be null, even when no error
 is thrown)

 @throws NS_ERROR_UNEXPECTED if the request type requested is not
imgIRequest getRequest(in long aRequestType)
 Used to find out what type of request one is dealing with (eg
 which request got passed through to the imgIDecoderObserver
 interface of an observer)

 @param aRequest the request whose type we want to know

 @return an enum value saying what type this request is

 @throws NS_ERROR_UNEXPECTED if aRequest is not known
long getRequestType(in imgIRequest aRequest)
 loadImageWithChannel allows data from an existing channel to be
 used as the image data for this content node.

 @param aChannel the channel that will deliver the data

 @return a stream listener to pump the image data into

 @see imgILoader::loadImageWithChannel

 @throws NS_ERROR_NULL_POINTER if aChannel is null
nsIStreamListener loadImageWithChannel(in nsIChannel aChannel)
 Used to unregister an image decoder observer.

 @param aObserver the observer to unregister
void removeObserver(in imgIDecoderObserver aObserver)