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Read part of a message asynchronously.
template< class DynamicBuffer, class ReadHandler> void-or-deduced async_read_some( DynamicBuffer& buffer, std::size_t limit, ReadHandler&& handler);
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:
websocket::closed.
              
            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:
websocket::stream::control_callback will be
                invoked for each control frame.
              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).
          
| Name | Description | 
|---|---|
| 
                       | 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. | 
| 
                       | 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. | 
| 
                       | The handler to be called when the read operation completes. Copies will be made of the handler as required. 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  |