Struct mozjs::jsapi::ReadableStreamUnderlyingSource

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

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.

Fields

vtable_: *const ReadableStreamUnderlyingSource__bindgen_vtable

Trait Implementations

impl Debug for ReadableStreamUnderlyingSource

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

Formats the value using the given formatter.

impl PartialEq for ReadableStreamUnderlyingSource

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

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

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.

impl StructuralPartialEq for ReadableStreamUnderlyingSource