callback will be called.
You can then call gOutputStreamSpliceFinish to get the
result of the operation.gOutputStreamSpliceAsync(object, source, flags = "G_OUTPUT_STREAM_SPLICE_NONE", io.priority = 0, cancellable = NULL, callback, user.data = NULL)objectsourceGInputStream.flagsGOutputStreamSpliceFlags.io.prioritycancellableGCancellable object, NULL to ignore.callbackuser.datacallback.gOutputStreamSplice.