Class Promise

A #GstPromise is created with gst_promise_new() by the consumer and passed to the producer to avoid thread safety issues with the change callback. A #GstPromise can be replied to with a value (or an error) by the producer with gst_promise_reply(). The exact value returned is defined by the API contract of the producer and %NULL may be a valid reply. gst_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 gst_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 #GstBus. A callback can also be installed at #GstPromise creation for result changes with gst_promise_new_with_change_func(). The change callback can be used to chain #GstPromises'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 #GstPromise starts out with a #GstPromiseResult of %GST_PROMISE_RESULT_PENDING and only ever transitions once into one of the other #GstPromiseResult's.

In order to support multi-threaded code, gst_promise_reply(), gst_promise_interrupt() and gst_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:

1. That gst_promise_reply() and gst_promise_interrupt() cannot be called after gst_promise_expire()
2. That gst_promise_reply() and gst_promise_interrupt() cannot be called twice.

The change function set with gst_promise_new_with_change_func() is called directly from either the gst_promise_reply(), gst_promise_interrupt() or gst_promise_expire() and can be called from an arbitrary thread. #GstPromise using APIs can restrict this to a single thread or a subset of threads but that is entirely up to the API that uses #GstPromise.