the I/O priority of the request. E.g. G_PRIORITY_LOW, G_PRIORITY_DEFAULT or G_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 classBuffer.
See the gio.async_result.AsyncResult documentation to know how to use this function.