the I/O priority of the request. E.g. glib.types.PRIORITY_LOW, glib.types.PRIORITY_DEFAULT or glib.types.PRIORITY_HIGH.
optional #GCancellable object, null to ignore.
function to call back with progress information, or null if progress information is not needed.
a #GAsyncReadyCallback to call when the request is satisfied.
Loads asynchronously the file or input stream contents into the #GtkSourceBuffer. See the #GAsyncResult documentation to know how to use this function.