The gstreamer.Promise object implements the container for values that may be available later. i.e. a Future or a Promise in <ulink url="https://en.wikipedia.org/wiki/Futures_and_promises">https://en.wikipedia.org/wiki/Futures_and_promises</ulink> As with all Future/Promise-like functionality, there is the concept of the producer of the value and the consumer of the value.
Undocumented in source.