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.
A gstreamer.Promise is created with Promise.new by the consumer and passed to the producer to avoid thread safety issues with the change callback. A gstreamer.Promise can be replied to with a value (or an error) by the producer with Promise.reply. Promise.interrupt is for the consumer to indicate to the producer that the value is not needed anymore and producing that value can stop. The GST_PROMISE_RESULT_EXPIRED state set by a call to Promise.expire indicates to the consumer that a value will never be produced and is intended to be called by a third party that implements some notion of message handling such as gstreamer.Bus A callback can also be installed at gstreamer.Promise creation for result changes with Promise.newWithChangeFunc. The change callback can be used to chain gstreamer.Promises's together as in the following example.
1 2 const GstStructure *reply; 3 GstPromise *p; 4 if (gst_promise_wait (promise) != GST_PROMISE_RESULT_REPLIED) 5 return; // interrupted or expired value 6 reply = gst_promise_get_reply (promise); 7 if (error in reply) 8 return; // propagate error 9 p = gst_promise_new_with_change_func (another_promise_change_func, user_data, notify); 10 pass p to promise-using API
In order to support multi-threaded code, Promise.reply, Promise.interrupt and Promise.expire may all be from different threads with some restrictions and the final result of the promise is whichever call is made first. There are two restrictions on ordering:
The change function set with Promise.newWithChangeFunc is called directly from either the Promise.reply, Promise.interrupt or Promise.expire and can be called from an arbitrary thread. gstreamer.Promise using APIs can restrict this to a single thread or a subset of threads but that is entirely up to the API that uses gstreamer.Promise
Sets our main struct and passes it to the parent class.
func will be called exactly once when transitioning out of GST_PROMISE_RESULT_PENDING into any of the other GstPromiseResult states.
Expire a promise. This will wake up any waiters with GST_PROMISE_RESULT_EXPIRED. Called by a message loop when the parent message is handled and/or destroyed (possibly unanswered).
Get the main Gtk struct
Retrieve the reply set on promise. promise must be in GST_PROMISE_RESULT_REPLIED and the returned structure is owned by promise
the main Gtk struct as a void*
Interrupt waiting for a promise. This will wake up any waiters with GST_PROMISE_RESULT_INTERRUPTED. Called when the consumer does not want the value produced anymore.
Set a reply on promise. This will wake up any waiters with GST_PROMISE_RESULT_REPLIED. Called by the producer of the value to indicate success (or failure).
Wait for promise to move out of the GST_PROMISE_RESULT_PENDING state. If promise is not in GST_PROMISE_RESULT_PENDING then it will return immediately with the current result.
the main Gtk struct