Struct gstreamer::Promise[]

pub struct Promise(_);

The Promise object implements the container for values that may be available later. i.e. a Future or a Promise in</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 Promise is created with Promise::new 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 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 PromiseResult::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 Bus. A callback can also be installed at Promise creation for result changes with 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 Promise starts out with a PromiseResult of PromiseResult::Pending and only ever transitions once into one of the other PromiseResult's.

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:

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

The change function set with Promise::new_with_change_func is called directly from either the Promise::reply, Promise::interrupt or Promise::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.

Feature: v1_14


impl Promise

Feature: v1_14


a new Promise

func will be called exactly once when transitioning out of PromiseResult::Pending into any of the other PromiseResult states.

Feature: v1_14


a GstPromiseChangeFunc to call


argument to call func with


notification function that user_data is no longer needed


a new Promise

Expire a self. This will wake up any waiters with PromiseResult::Expired. Called by a message loop when the parent message is handled and/or destroyed (possibly unanswered).

Feature: v1_14

Retrieve the reply set on self. self must be in PromiseResult::Replied and the returned structure is owned by self

Feature: v1_14


The reply set on self

Interrupt waiting for a self. This will wake up any waiters with PromiseResult::Interrupted. Called when the consumer does not want the value produced anymore.

Feature: v1_14

Set a reply on self. This will wake up any waiters with PromiseResult::Replied. Called by the producer of the value to indicate success (or failure).

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

Feature: v1_14


a Structure with the the reply contents

Wait for self to move out of the PromiseResult::Pending state. If self is not in PromiseResult::Pending then it will return immediately with the current result.

Feature: v1_14


the result of the promise

Trait Implementations

impl Clone for Promise

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl StaticType for Promise

Returns the type identifier of Self.

impl Default for Promise

Returns the "default value" for a type. Read more

impl Send for Promise

impl Sync for Promise