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The #GstPromise 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 #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.

record

Hierarchy

  • Promise

Index

Constructors

Properties

parent: Gst.MiniObject

parent #GstMiniObject

field
name: string

Methods

  • expire(): void
  • 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).

    Returns void

  • Retrieve the reply set on promise. promise must be in %GST_PROMISE_RESULT_REPLIED and the returned structure is owned by promise

    Returns Gst.Structure

  • interrupt(): 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.

    Returns void

  • 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).

    If promise has already been interrupted by the consumer, then this reply is not visible to the consumer.

    Parameters

    • s: Gst.Structure

      a #GstStructure with the the reply contents

    Returns void

  • unref(): void
  • Decreases the refcount of the promise. If the refcount reaches 0, the promise will be freed.

    Returns void

  • 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.

    Returns PromiseResult

  • func will be called exactly once when transitioning out of %GST_PROMISE_RESULT_PENDING into any of the other #GstPromiseResult states.

    Parameters

    Returns Gst.Promise

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