Gst.Bus.prototype.set_sync_handler

function set_sync_handler(func:Gst.BusSyncHandler):void {
    // Gjs wrapper for gst_bus_set_sync_handler()
}

Sets the synchronous handler on the bus. The function will be called every time a new message is posted on the bus. Note that the function will be called in the same thread context as the posting object. This function is usually only called by the creator of the bus. Applications should handle messages asynchronously using the gst_bus watch and poll functions.

You cannot replace an existing sync_handler. You can pass NULL to this function, which will clear the existing handler.