The access handler is added when the progress notification is generated when the Deferred object is generated.
This parameter can be an array of single functions or functions. When deferred is invoked by calling the generated progress notification notify or notifyWith, progressCallbacks is accessed. Later deferred.progress() returns the Deferred object. Deferred objects have methods to link to this one. When deferred is resolved or rejected, the progress callback will no longer be called. For more information, see the document Deferred object .
An array of functions or functions that are called to deferred to generate progress notifications.