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            WebSocket is an underlying network protocol that enables you to build other standard
            protocols on top of it.

            The WebSocket Protocol enables full duplex communication between a client and
            a remote server over the Web, and supports transmission of binary data and text strings.
            The protocol consists of an opening handshake followed by basic message framing, and is
            layered over TCP.


            WebSocket programming follows an asynchronous programming model, which
            means that as long as a WebSocket connection is open, your application simply listens
            for events. Your client does not need to actively poll the server for more information. To
            start listening for the events, you simply add callback functions to the WebSocket object.
            Alternatively, you can use the addEventListener() DOM method to add event listeners
            to your WebSocket objects.
            A WebSocket object dispatches four different events:
            · Open
            · Message
            · Error
            · Close
            As with all web APIs, you can listen for these events using on<eventname> handler
            properties, as well as using the addEventListener(); method.

            ws.onopen
            ws.onmessage
            ws.onerror
            ws.onclose

            e.data instanceof Blob
            ws.binaryType = "blob"

            WebSocket objects have two methods: send() and close()


            Listing 2-12. Waiting for the Open Event Before Sending a Message
            // Wait until the open event before calling send().
            var ws = new WebSocket("ws://echo.websocket.org")
            ws.onopen = function(e) {
            ws.send("Initial data");
            }

            Listing 2-13. Checking the readyState Property for an Open WebSocket
            // Handle outgoing data. Send on a WebSocket if that socket is open.
            function myEventHandler(data) {
            if (ws.readyState === WebSocket.OPEN) {
            // The socket is open, so it is ok to send the data.
            ws.send(data);
            } else {
            // Do something else in this case.
            //Possibly ignore the data or enqueue it.
            }
            }


            // Send a Blob
            var blob = new Blob("blob contents");
            ws.send(blob);
            // Send an ArrayBuffer
            var a = new Uint8Array([8,6,7,5,3,0,9]);
            ws.send(a.buffer);


            Listing 2-15. Calling the close() Method
            // Close the WebSocket connection
            ws.close();
            You can optionally pass two arguments to the close() method: code (a numerical
            status code) and reason (a text string). Passing these arguments transmits information
            to the server about why the client closed the connection. We will discuss the status
            codes and reasons in greater detail in Chapter 3, when we cover the WebSocket closing
            handshake. Listing 2-16 shows an example of calling the close() method with an
            argument.
            Listing 2-16. Calling the close() Method with a Reason
            // Close the WebSocket connection because the session has ended successfully
            ws.close(1000, "Closing normally");
            Listing 2-16 uses code 1000, which means, as it states in the code, that the
            connection is closing normally.


            WebSocket Object Attributes
            There are several WebSocket Object attributes you can use to provide more information
            about the WebSocket object: readyState, bufferedAmount, and protocol.

            Table 2-1. readyState Attributes, Values, and Status Descriptions
            Attribute Constant     Value     Status
            WebSocket.CONNECTING   0   The connection is in progress but has not been established.
            WebSocket.OPEN   1   The connection has been established. Messages can flow between the client and server.
            WebSocket.CLOSING   2   The connection is going through the closing handshake.
            WebSocket.CLOSED   3   The connection has been closed or could not be opened.

            WebSocket Object Attribute: protocol
            The protocol attribute is the empty string before the
            opening handshake completes and remains an empty string if the server does not choose
            one of the protocols offered by the client.

             

            posted on 2016-02-22 17:02 Enic 閱讀(273) 評論(0)  編輯 收藏 引用 所屬分類: websocket
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