Struct mozjs::jsapi::ReadableStreamUnderlyingSource

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#[repr(C)]
pub struct ReadableStreamUnderlyingSource { pub vtable_: *const ReadableStreamUnderlyingSource__bindgen_vtable, }
Expand description

Abstract base class for external underlying sources.

The term “underlying source” is defined in the Streams spec: https://streams.spec.whatwg.org/#underlying-source

A ReadableStreamUnderlyingSource is an underlying source that is implemented in C++ rather than JS. It can be passed to JS::NewReadableExternalSourceStreamObject to create a custom, embedding-defined ReadableStream.

There are several API difference between this class and the standard API for underlying sources implemented in JS:

  • JS underlying sources can be either byte sources or non-byte sources. External underlying source are always byte sources.

  • The C++ API does not bother with controller objects. Instead of using controller methods, the underlying source directly calls API functions like JS::ReadableStream{UpdateDataAvailableFromSource,Close,Error}.

  • External readable streams are optimized to allow the embedding to interact with them with a minimum of overhead: chunks aren’t enqueued as individual typed arrays; instead, the embedding only updates the amount of data available using JS::ReadableStreamUpdateDataAvailableFromSource. When JS requests data from a reader, writeIntoReadRequestBuffer is invoked, asking the embedding to write data directly into the buffer we’re about to hand to JS.

  • The C++ API provides extra callbacks onClosed() and onErrored().

  • This class has a finalize() method, because C++ cares about lifetimes.

Additionally, ReadableStreamGetExternalUnderlyingSource can be used to get the pointer to the underlying source. This locks the stream until it is released again using JS::ReadableStreamReleaseExternalUnderlyingSource.

Embeddings can use this to optimize away the JS ReadableStream overhead when an embedding-defined C++ stream is passed to an embedding-defined C++ consumer. For example, consider a ServiceWorker piping a fetch Response body to a TextDecoder. Instead of copying chunks of data into JS typed array buffers and creating a Promise per chunk, only to immediately resolve the Promises and read the data out again, the embedding can directly feed the incoming data to the TextDecoder.

Compartment safety: All methods (except finalize) receive cx and stream arguments. SpiderMonkey enters the realm of the stream object before invoking these methods, so stream is never a wrapper. Other arguments may be wrappers.

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§vtable_: *const ReadableStreamUnderlyingSource__bindgen_vtable

Trait Implementations§

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impl Debug for ReadableStreamUnderlyingSource

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

Formats the value using the given formatter. Read more
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impl PartialEq for ReadableStreamUnderlyingSource

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

This method tests for self and other values to be equal, and is used by ==.
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fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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impl StructuralPartialEq for ReadableStreamUnderlyingSource

Auto Trait Implementations§

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impl<T> Any for T
where T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for T
where T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for T
where T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> Filterable for T

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fn filterable( self, filter_name: &'static str, ) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>

Creates a filterable data provider with the given name for debugging. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for T
where U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T> IntoEither for T

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fn into_either(self, into_left: bool) -> Either<Self, Self>

Converts self into a Left variant of Either<Self, Self> if into_left is true. Converts self into a Right variant of Either<Self, Self> otherwise. Read more
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fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
where F: FnOnce(&Self) -> bool,

Converts self into a Left variant of Either<Self, Self> if into_left(&self) returns true. Converts self into a Right variant of Either<Self, Self> otherwise. Read more
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impl<T, U> TryFrom<U> for T
where U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
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impl<T> ErasedDestructor for T
where T: 'static,

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impl<T> MaybeSendSync for T