Struct gstreamer::promise::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 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::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. The exact value returned is defined by the API contract of the producer and None may be a valid 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

Implementations

impl Promise

pub fn new() -> Promise

Feature: v1_14

Returns

a new Promise

pub fn with_change_func<F>(func: F) -> Promise where
    F: FnOnce(Result<Option<&StructureRef>, PromiseError>) + Send + 'static, 

pub fn new_future<'a>() -> (Self, PromiseFuture<'a>)

pub fn expire(&self)

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

pub fn get_reply(&self) -> Option<&StructureRef>

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

Feature: v1_14

Returns

The reply set on self

pub fn interrupt(&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

pub fn reply(&self, s: Option<Structure>)

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

s

a Structure with the the reply contents

pub fn wait(&self) -> PromiseResult

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

Returns

the result of the promise

Trait Implementations

impl Clone for Promise

fn clone(&self) -> Promise

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Promise

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Promise

fn default() -> Self

Returns the "default value" for a type.

impl Eq for Promise

impl Hash for Promise

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl Ord for Promise

fn cmp(&self, other: &Promise) -> Ordering

This method returns an Ordering between self and other.

#[must_use]fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

#[must_use]fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

#[must_use]fn clamp(self, min: Self, max: Self) -> Self

🔬 This is a nightly-only experimental API. (clamp)

Restrict a value to a certain interval.

impl PartialEq<Promise> for Promise

fn eq(&self, other: &Promise) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Promise) -> bool

This method tests for !=.

impl PartialOrd<Promise> for Promise

fn partial_cmp(&self, other: &Promise) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Promise) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Promise) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Promise) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Promise) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl Send for Promise

impl StaticType for Promise

fn static_type() -> Type

Returns the type identifier of Self.

impl StructuralEq for Promise

impl StructuralPartialEq for Promise

impl Sync for Promise