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##### websocket::stream::async_write_some

Write some message data asynchronously.

###### Synopsis

    template<
        class ConstBufferSequence,
        class WriteHandler = net::default_completion_token_t<                executor_type>>
    DEDUCED
    async_write_some(
        bool fin,
        ConstBufferSequence const& buffers,
        WriteHandler&& handler = net::default_completion_token_t< executor_type >{});

###### Description

This function is used to asynchronously write part of a message.

This call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- The message data is written.
- An error occurs.

The algorithm, known as a composed asynchronous operation, is implemented in terms of calls to the next layer's `async_write_some` function. The program must ensure that no other calls to `write`, `write_some`, `async_write`, or `async_write_some` are performed until this operation completes.

If this is the beginning of a new message, the message opcode will be set to text or binary based on the current setting of the `binary` (or `text`) option. The actual payload sent may be transformed as per the WebSocket protocol settings.

This function always writes a complete WebSocket frame (not WebSocket message) upon successful completion, so it is well defined to perform ping, pong, and close operations in parallel to this operation.

###### Parameters

Name | Description
--- | ---
`fin` | `true` if this is the last part of the message.
`buffers` | The buffers containing the message part to send. The implementation will make copies of this object as needed, but ownership of the underlying memory is not transferred. The caller is responsible for ensuring that the memory locations pointed to by buffers remains valid until the completion handler is called.
`handler` | The completion handler to invoke when the operation completes. The implementation takes ownership of the handler by performing a decay-copy. The equivalent function signature of the handler must be:
    void handler(
        error_code const & ec,           // Result of operation
        std::size_t bytes_transferred   // Number of bytes sent from the
                                        // buffers. If an error occurred,
                                        // this will be less than the buffer_size.
    );
If the handler has an associated immediate executor, an immediate completion will be dispatched to it. Otherwise, the handler will not be invoked from within this function. Invocation of the handler will be performed by dispatching to the immediate executor. If no immediate executor is specified, this is equivalent to using `net::post`.

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following net::cancellation_type values:

- `net::cancellation_type::terminal`
- `net::cancellation_type::total`

`total`cancellation succeeds if the operation is suspended due to ongoing control operations such as a ping/pong.

`terminal` cancellation succeeds when supported by the underlying stream.

`terminal` cancellation leaves the stream in an undefined state, so that only closing it is guaranteed to succeed.