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###### websocket::stream::async_read_some (1 of 2 overloads)

Read part of a message asynchronously.

Synopsis

    template<
        class DynamicBuffer,
        class ReadHandler>
    DEDUCED
    async_read_some(
        DynamicBuffer& buffer,
        std::size_t limit,
        ReadHandler&& handler);

Description

This function is used to asynchronously read part of a message from the stream. The function call always returns immediately. The asynchronous operation will continue until one of the following is true:

- Some or all of the message is received.
- A close frame is received. In this case the error indicated by the function will be `websocket::closed`.
- An error occurs on the stream.

This operation is implemented in terms of one or more calls to the next layer's `async_read_some` and `async_write_some` functions, and is known as a composed operation. The program must ensure that the stream performs no other reads until this operation completes.

Received message data, if any, is appended to the input area of the buffer. The functions `websocket::stream::got_binary` and `websocket::stream::got_text` may be used to query the stream and determine the type of the last received message. The function `websocket::stream::is_message_done` may be called to determine if the message received by the last read operation is complete.

While this operation is active, the implementation will read incoming control frames and handle them automatically as follows:

- The `websocket::stream::control_callback` will be invoked for each control frame.
- For each received ping frame, a pong frame will be automatically sent.
- If a close frame is received, the WebSocket close procedure is performed. In this case, when the function returns, the error `websocket::closed` will be indicated.

Because of the need to handle control frames, asynchronous read operations can cause writes to take place. These writes are managed transparently; callers can still have one active asynchronous read and asynchronous write operation pending simultaneously (a user initiated call to `websocket::stream::async_close` counts as a write).

Parameters

Name | Description
--- | ---
`buffer` | A dynamic buffer to hold the message data after any masking or decompression has been applied. This object must remain valid until the handler is called.
`limit` | An upper limit on the number of bytes this function will append into the buffer. If this value is zero, then a reasonable size will be chosen automatically.
`handler` | Invoked when the operation completes. The handler may be moved or copied as needed. The equivalent function signature of the handler must be:
    void handler(
        error_code const& ec,       // Result of operation
        std::size_t bytes_written   // Number of bytes appended to buffer
    );
Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `boost::asio::io_context::post`.