Promise

The Promise object implements the container for values that may be available later.

i.e. a Future or a Promise in <https://en.wikipedia.org/wiki/Futures_and_promises>. As with all Future/Promise-like functionality, there is the concept of the producer of the value and the consumer of the value.

A Promise is created with Promise by the consumer and passed to the producer to avoid thread safety issues with the change callback. A Promise can be replied to with a value (or an error) by the producer with reply. The exact value returned is defined by the API contract of the producer and null may be a valid reply. 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 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 Bus. A callback can also be installed at Promise creation for result changes with Promise.with_change_func. The change callback can be used to chain Promises's together as in the following example.

const GstStructure *reply;
GstPromise *p;
if (gst_promise_wait (promise) != GST_PROMISE_RESULT_REPLIED)
	return; // interrupted or expired value
reply = gst_promise_get_reply (promise);
if (error in reply)
	return; // propagate error
p = gst_promise_new_with_change_func (another_promise_change_func, user_data, notify);
pass p to promise-using API

Each Promise starts out with a PromiseResult of PENDING and only ever transitions once into one of the other PromiseResult's.

In order to support multi-threaded code, reply, interrupt and 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:

1. That reply and interrupt cannot be called after expire 2. That reply and interrupt cannot be called twice.

The change function set with Promise.with_change_func is called directly from either the reply, interrupt or expire and can be called from an arbitrary thread. 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 Promise.